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← Sewerage and Sewage Treatment

V. DESIGN OF SEWERAGE SYSTEMS…

Part 2 of 7 · Babbitt, Harold E. (Harold Eaton) · Public domain text

About this edition

Source: Project Gutenberg e-text #61773, public domain in the United States · text not rewritten or summarized · How RedLectern is built

 

Contents (44)
  1. Opening under 1 min
  2. Preface about 2 min
  3. I. INTRODUCTION under 1 min
  4. II. WORK PRELIMINARY TO DESIGN under 1 min
  5. III. QUANTITY OF SEWAGE under 1 min
  6. IV. HYDRAULICS OF SEWERS under 1 min
  7. V. DESIGN OF SEWERAGE SYSTEMS under 1 min
  8. VI. APPURTENANCES under 1 min
  9. VII. PUMPS AND PUMPING STATIONS under 1 min
  10. VIII. MATERIALS FOR SEWERS under 1 min
  11. IX. DESIGN OF THE SEWER RING under 1 min
  12. X. CONTRACTS AND SPECIFICATIONS under 1 min
  13. XI. CONSTRUCTION about 1 min
  14. XII. MAINTENANCE OF SEWERS under 1 min
  15. XIII. COMPOSITION AND PROPERTIES OF SEWAGE under 1 min
  16. XIV. DISPOSAL BY DILUTION under 1 min
  17. XV. SCREENING AND SEDIMENTATION under 1 min
  18. XVI. SEPTICIZATION under 1 min
  19. XVII. FILTRATION AND IRRIGATION under 1 min
  20. XVIII. ACTIVATED SLUDGE under 1 min
  21. XIX. ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION under 1 min
  22. XX. SLUDGE under 1 min
  23. XXI. AUTOMATIC DOSING DEVICES under 1 min
  24. I. INTRODUCTION about 10 min
  25. II. WORK PRELIMINARY TO DESIGN about 17 min
  26. III. QUANTITY OF SEWAGE about 41 min
  27. IV. THE HYDRAULICS OF SEWERS about 31 min
  28. V. DESIGN OF SEWERAGE SYSTEMS about 47 min
  29. VI. APPURTENANCES about 34 min
  30. VII. PUMPS AND PUMPING STATIONS about 50 min
  31. VIII. MATERIALS FOR SEWERS about 47 min
  32. IX. DESIGN OF THE SEWER RING about 23 min
  33. X. CONTRACTS AND SPECIFICATIONS about 35 min
  34. XI. CONSTRUCTION about 2 h 19 min
  35. XII. MAINTENANCE OF SEWERS about 26 min
  36. XIII. COMPOSITION AND PROPERTIES OF SEWAGE about 31 min
  37. XIV. DISPOSAL BY DILUTION about 16 min
  38. XV. SCREENING AND SEDIMENTATION about 35 min
  39. XVI. SEPTICIZATION about 30 min
  40. XVII. FILTRATION AND IRRIGATION about 50 min
  41. XVIII. ACTIVATED SLUDGE about 26 min
  42. XIX. ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION about 19 min
  43. XX. SLUDGE about 12 min
  44. XXI. AUTOMATIC DOSING DEVICES about 44 min
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V. DESIGN OF SEWERAGE SYSTEMS

=41. The Plan.=—Good practice demands that a comprehensive plan for a sewerage system be provided for the needs of a community for the entire extent of its probable future growth, and that sewers be constructed as needed in accordance with this plan.

Sewerage systems may be laid out on any one of three systems: separate, storm, or combined. A separate system of sewers is one in which only sanitary sewage or industrial wastes or both are allowed to flow. Storm sewers carry only surface drainage, exclusive of sanitary sewage. Combined sewers carry both sanitary and storm sewage. The use of a combined or a separate system of sewerage is a question of expediency. Portions of the same system may be either separate, combined, or storm sewers.

Some conditions favorable to the adoption of the separate system are where:

a. The sanitary sewage must be concentrated at one outlet, such as at a treatment plant, and other outlets are available for the storm drainage.

b. The topography is flat necessitating deep excavation and steeper grades for the larger combined sewers.

c. The sanitary sewers must be placed materially deeper than the necessary depth for the storm-water drains.

d. The sewers are to be laid in rock, necessitating more difficult excavation for the larger combined sewers.

e. An existing sewerage system can be used to convey the dry weather flow, but is not large enough for the storm sewage.

f. The city finances are such that the greater cost of the combined system cannot be met and sanitary drainage is imperative.

g. The district to be sewered is an old residential section where property values are not increasing and the assessment must be kept down.

Some additional points given in a report by Alvord and Burdick to the city of Billings, Montana, are:

The separate system of sewerage should be used, where:

1st. Storm water does not require extensive underground removal, or where it can be concentrated in a few shallow underground channels.

2nd. Drainage areas are short and steep facilitating rapid flow of water over street surfaces to the natural water courses.

3rd. The sanitary sewage must be pumped.

4th. Sewers are being built in advance of the city’s development to encourage its growth.

5th. The existing sewer is laid at grades unsuitable for sanitary sewage, it can be used as a storm sewer.

A combined system must be relatively larger than a separate storm sewer as the latter may overflow on exceptional occasions, but the former never.

A combined system of sewerage should be used where:

1st. It is evident that storm and sanitary sewerage must be provided soon.

2nd. Both sanitary and storm sewage must be pumped.

3rd. The district is densely built up.

=42. Preliminary Map.=—The first step in the design of a sewerage system is the preparation of a map of the district to be served within the limits of its probable growth. The map should be on a scale of at least 200 feet to the inch in the built up sections or other areas where it is anticipated that sewers may be built, and where much detail is to be shown a scale as large as 40 feet to the inch may have to be used. The adoption of so large a scale will usually necessitate the division of the city or sewer district into sections. A key map should be drawn to such a scale that the various sections represented by separate drawings can all be shown upon it. In preparing the enlarged portions of the map it is not necessary to include these portions of the city in which it is improbable that sewers will be constructed, such as parks and cemeteries.

The contour interval should depend on the character of the district and the slope of the land. In those sections drawn to a scale of 200 feet to the inch for slopes over 5 per cent, the contour interval need not be closer than 10 feet. For slopes between 1 and 5 per cent the contour interval should be 5 feet. For flatter slopes the interval should not exceed 2 feet, and a one foot interval is sometimes desirable. In general the horizontal distances between contours should not exceed 400 feet and they should be close enough to show important features of the natural drainage. Elevations should also be given at street intersections, and at abrupt changes in grade. For portions of the map on a smaller scale the contours need be sufficiently close to show only the drainage lines and the general slope of the land.

The following may be shown on the preliminary map: the elevation of lots and cellars; the character of the built up districts, whether cheap frame residences, flat-roof buildings, manufacturing plants, etc.; property lines; width of streets between property lines and between curb lines; the width and character of the sidewalks and pavements; street car and railroad tracks; existing underground structures such as sewers, water pipes, telephone conduits, etc.; the location of important structures which may have a bearing on the design of the sewers such as bridges, railroad tunnels, deep cuts, culverts, etc.; and the location of possible sewer outlets and the sites for sewage disposal plants.

Fig. 24 shows a preliminary map for a section of a city, on which the necessary information has been entered. The map is made from survey notes. All streets are paved with brick. The alleys are unpaved. The entire section is built up with high-class detached residences averaging one to each lot. The lots vary from 1 to 3 feet above the elevation of the street.

=43. Layout of the Separate System.=—Upon completion of the preliminary map a tentative plan of the system is laid out. The lines of the sewer pipe are drawn in pencil, usually along the center line of the street or alley in such a manner that a sewer will be provided within 50 feet or less of every lot. The location of the sewers should be such as to give the most desirable combination of low cost, short house connections, proper depth for cellar drainage, and avoidance of paved streets. Some dispute arises among engineers as to the advisability of placing pipes in alleys, although there is less opposition to so placing sewers than any other utility conduit. The principal advantage in placing sewers in alleys is to avoid disturbing the pavement of the street, but if both street and alley are paved it is usually more economical to place the sewer in the street as the house connections will be shorter. On boulevards and other wide streets such as Meridian Avenue in Fig. 24, the sewers are placed in the parking on each side of the street, rather than to disturb the pavement and lay long house connections to the center of the street.

All pipes should be made to slope, where possible, in the direction of the natural slope of the ground. The preliminary layout of the system is shown in Fig. 24. The lowest point in the portion of the system shown is in the alley between Alabama and Tennessee Streets. The flow in all pipes is towards this point, and only one pipe drains away from any junction, except that more than one pipe may drain from a terminal manhole on a summit.

=44. Location and Numbering of Manholes.=—Manholes are next located on the pipes of this tentative layout. Good practice calls for the location of a manhole at every change in direction, grade, elevation, or size of pipe, except in sewers 60 inches in diameter or larger. The manholes should not be more than 300 to 500 feet apart, and preferably as close as 200 to 300 feet. In sewers too small for a man to enter the distance is fixed by the length of sewer rods which can be worked successfully. In the larger sewers the distances are sometimes made greater but inadvisedly so, since quick means of escape should be provided for workmen from a sudden rise of water in the sewer, or the effect of an asphyxiating gas. In the preliminary layout the manholes are located at pipe intersections, changes in direction, and not over 300 to 500 feet apart on long straight runs at convenient points such as opposite street intersections where other sewers may enter.

No standard system of manhole numbering has been adopted. A system which avoids confusion and is subject to unlimited extension is to number the manholes consecutively upwards from the outlet, beginning a new series of numbers prefixed by some index number or letter for each branch or lateral. This system has been followed with the manholes on Fig. 24.

=45. Drainage Areas.=—The quantity of dry weather sewage is determined by the population rather than the topography. Lot lines and street intersections or other artificial lines marking the boundaries between districts are therefore taken as watershed lines for sanitary sewers. The quantity of sewage to be carried and the available slope are the determining factors in fixing the diameter of the sewer. Since there may be no change in diameter or slope between manholes the quantity of sewage delivered by a sewer into any manhole will determine the diameter of the sewer between it and the next manhole above. In order to determine the additional amount contributed between manholes a line is drawn around the drainage area tributary to each manhole. This line generally follows property lines and the center lines of streets or alleys, its position being such that it includes all the area draining into one manhole, and excludes all areas draining elsewhere. An entire lot is usually assumed to lie within the drainage area into which the building on the lot drains. In laying out these areas it is best to commence at the upper end of a lateral and work down to a junction. Then start again at the upper end of another lateral entering this junction, and continue thus until the map has been covered.

The areas are given the same numbers as the manholes into which they drain. The dividing lines for the drainage areas on Fig. 24 are shown as dot and dash lines, and the areas enclosed are appropriately numbered. If more than one sewer drains into the same manhole the area should be subdivided so that each subdivision encloses only the area contributing through one sewer. Such a condition is shown at manhole _C_2. The areas are designated by subletters or symbols corresponding to the symbol used for the sewer into which they drain. For example, the two areas contributing to manhole _C_2 are lettered _C_2_{K} and _C_2_{D}. The sewer from manhole _C_3 to _C_2 receives no addition, it being assumed that all the lots adjacent to it drain into the sewer on the alley. There is therefore no area _C_2. Likewise there is no area _A_1_{C}.

=46. Quantity of Sewage.=—The remaining work in the computation of the quantity of sewage is best kept in order by a tabulation. Table 19 shows the computations for the sewers discharging from the east into manhole No. 142. The computation should begin at the upper end of a lateral, continue to a junction, and then start again at the upper end of another lateral entering this junction. Each line in the table should be filled in completely from left to right before proceeding with the computations on the next line. In the illustrative solution in Table 19, computations for quantity have not been made between manholes where it was apparent that there would be an insufficient additional quantity to necessitate a change in the size of the pipe.

In making these computations the assumptions of quantity and other factors given below indicate the sort of assumptions which must be made, based on such studies as are given in Chapter III. The density of population was taken as 20 persons per acre, the assumption being based on the census and the character of the district. The average sanitary sewage flow was taken as 100 gallons per capita per day. The per cent which the maximum dry weather flow is of the average was taken as M = 500⁄P^⅕, in which P is the population in thousands. The per cent is not to exceed 500 nor to be less than 150. The rate of infiltration of ground water was assumed as 50,000 gallons per mile of pipe per day.

In the first line of Table 19, the entries in columns (1) to (6) are self-explanatory. There are no entries in columns (7) to (10), as no additional sewage is contributed between manholes 3.5 and 3.4. In column (11), 2250 persons are recorded as the number tributary to manhole No. 3.5 in the district to the north and west. These people contribute an average of 100 gallons per person per day, or a total of 0.346 second foot. This quantity is entered in column (13). The figure in column (14) is obtained from the expression M = (500)⁄P^⅕. Column (15) is .01 of the product of columns (13) and (14). Column (16) is the product of the length of pipe between manholes 3.5 and 3.4, and the ground water unit reduced to cubic feet per second. Column (17) is the sum of column (16), and all of the ground water tributary to manhole 3.5, which is not recorded in the table. Column (18) is the sum of columns (15) and (17).

No new principle is represented in the second and third lines.

In the fourth line the first 10 columns need no further explanation. The (11th) column is the sum of the (10th) column, and the (11th) column in the third line. It represents the total number of persons tributary to manhole 3.4 on lateral No. 8. Column (13) in the fourth line is the sum of column (13) in the third line and the (12th) column in the fourth line, and the (15th) column in the fourth line is the product of the 2 preceding columns in the fourth line. Note that in no case is the figure in column (15) the sum of any previous figures in column (15). With this introduction the student should be able to check the remaining figures in the table, and should compute the quantity of sewage entering manhole No. 142 from the west, making reasonable assumptions for the tributary quantities from beyond the limits of the map.

TABLE 19

COMPUTATIONS FOR QUANTITY OF SEWAGE FOR A SEPARATE SEWERAGE SYSTEM

──────────┬──────────┬──────────┬───────┬───────┬──────┬───────┬───── On Street │ From │To Street │ From │ To │Length│Mark of│Area, │ Street │ │Manhole│Manhole│ Feet │ Added │Acres │ │ │ │ │ │ Areas │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼───── Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ │ St. │ │ Grant │ │ │ │ │ │ │ St. │ │ │ │ │ Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 8.2│ 2.7 of Grant│ │ Missouri│ │ │ │ │ St. │ │ St. │ │ │ │ │ Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 8.1│ 3.41 of Grant│ Missouri│ Kansas │ │ │ │ │ St. │ St. │ St. │ │ │ │ │ Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│3.4_{8}│ 2.68 of Grant│ Kansas │ St. │ │ │ │ │ St. │ St. │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ │ St. │ of Grant│ of │ │ │ │ │ │ St. │ Meridian│ │ │ │ │ │ │ │ │ │ │ 7.1│ Alley S. │Railroad │Nebraska │ 7.2│ 3.3│ 800│3.3_{7}│ 7.14 of │ │ St. │ │ │ │ │ Meridian│ │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ │ St. │ of │ of Smith│ │ │ │ │ │ Meridian│ Av. │ │ │ │ │ │ │ │ │ │ │ 6.1│ Alley S. │Railroad │Nebraska │ 6.2│ 3.2│ 609│3.2_{6}│ 3.82 of Smith│ │ St. │ │ │ │ │ Ave. │ │ │ │ │ │ │ Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ │ St. │ of Smith│ Cordovez│ │ │ │ │ │ Ave. │ St. │ │ │ │ │ S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│3.1_{4}│ 3.10 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│3.1_{5}│ 2.69 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ │ St. │ Cordovez│ │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 148│ 1.53 │ │ St. │ │ │ │ │ Long St. │Nebraska │N. │ 148│ 147│ 492│ │ │ St. │ Carolina│ │ │ │ │ │ │ St. │ │ │ │ │ Long St. │N. │Georgia │ 147│ 146│ 430│ │ │ Carolina│ St. │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 146│ 0.81 │ St. │ │ │ │ │ │ │ │ │ │ │ │ 2.1│ Long St. │Harris St.│Tennessee │ 145│ 143│ 725│143–145│ 6.6 │ │ St. │ │ │ │ │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8) (1) │ │ │ │ │ │ │ ──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴─────

──────────┬──────────┬──────────┬──────────┬───────┬─────────┬───────── On Street │ From │To Street │Population│Number │ Total │ Avg. │ Street │ │ per Acre │ of │ Persons │Sanitary │ │ │ │Persons│Tributary│ Flow, │ │ │ │ │ │ C.F.S. │ │ │ │ │ │ ──────────┼──────────┼──────────┼──────────┼───────┼─────────┼───────── Nebraska │Map margin│Alley S. │ │ │ 2250│ 0.0000 St. │ │ Grant │ │ │ │ │ │ St. │ │ │ │ Alley S. │Railroad │E. of │ 20│ 54│ 54│ .0084 of Grant│ │ Missouri│ │ │ │ St. │ │ St. │ │ │ │ Alley S. │E. of │E. of │ 20│ 68│ 122│ .0106 of Grant│ Missouri│ Kansas │ │ │ │ St. │ St. │ St. │ │ │ │ Alley S. │E. of │Nebraska │ 20│ 54│ 176│ .0084 of Grant│ Kansas │ St. │ │ │ │ St. │ St. │ │ │ │ │ Nebraska │Alley S. │Alley S. │ │ │ 2428│ .0000 St. │ of Grant│ of │ │ │ │ │ St. │ Meridian│ │ │ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ 20│ 142│ 142│ .0221 of │ │ St. │ │ │ │ Meridian│ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ │ │ 2568│ .0000 St. │ of │ of Smith│ │ │ │ │ Meridian│ Av. │ │ │ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ 20│ 76│ 76│ .0119 of Smith│ │ St. │ │ │ │ Ave. │ │ │ │ │ │ Nebraska │Alley S. │S. of │ │ │ 2644│ .0000 St. │ of Smith│ Cordovez│ │ │ │ │ Ave. │ St. │ │ │ │ S. of │Railroad │Nebraska │ 20│ 62│ 62│ .0096 Cordovez│ │ St. │ │ │ │ St. │ │ │ │ │ │ S. of │Map margin│Nebraska │ 20│ 54│ 54│ .0084 Cordovez│ │ St. │ │ │ │ St. │ │ │ │ │ │ Nebraska │S. of │Long St. │ │ │ 2760│ .0000 St. │ Cordovez│ │ │ │ │ │ St. │ │ │ │ │ Long St. │Map margin│Nebraska │ 20│ 31│ 31│ .0048 │ │ St. │ │ │ │ Long St. │Nebraska │N. │ │ │ 2791│ .0000 │ St. │ Carolina│ │ │ │ │ │ St. │ │ │ │ Long St. │N. │Georgia │ │ │ 2791│1.000[33] │ Carolina│ St. │ │ │ │ │ St. │ │ │ │ │ Long St. │Georgia │Harris St.│ 20│ 16│ 2807│ .0025 │ St. │ │ │ │ │ │ │ │ │ │ │ Long St. │Harris St.│Tennessee │ 20│ 132│ 2936│ .0205 │ │ St. │ │ │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12) (1) │ │ │ │ │ │ ──────────┴──────────┴──────────┴──────────┴───────┴─────────┴─────────

──────────┬──────────┬──────────┬──────────┬────────┬───────── On Street │ From │To Street │Cumulative│Per cent│ Total │ Street │ │ Avg. │ Max. │ Max. │ │ │ Sanitary │Sanitary│Sanitary, │ │ │ Flow, │ is of │ C.F.S. │ │ │ C.F.S. │Average │ ──────────┼──────────┼──────────┼──────────┼────────┼───────── Nebraska │Map margin│Alley S. │ 0.346│ 425│ 1.47 St. │ │ Grant │ │ │ │ │ St. │ │ │ Alley S. │Railroad │E. of │ .0084│ 500│ 0.041 of Grant│ │ Missouri│ │ │ St. │ │ St. │ │ │ Alley S. │E. of │E. of │ .0190│ 500│ 0.095 of Grant│ Missouri│ Kansas │ │ │ St. │ St. │ St. │ │ │ Alley S. │E. of │Nebraska │ .0274│ 500│ 0.137 of Grant│ Kansas │ St. │ │ │ St. │ St. │ │ │ │ Nebraska │Alley S. │Alley S. │ .373│ 423│ 1.58 St. │ of Grant│ of │ │ │ │ St. │ Meridian│ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ .0221│ 500│ 0.111 of │ │ St. │ │ │ Meridian│ │ │ │ │ Nebraska │Alley S. │Alley S. │ .395│ 414│ 1.63 St. │ of │ of Smith│ │ │ │ Meridian│ Av. │ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ .0119│ 500│ 0.060 of Smith│ │ St. │ │ │ Ave. │ │ │ │ │ Nebraska │Alley S. │S. of │ .407│ 414│ 1.68 St. │ of Smith│ Cordovez│ │ │ │ Ave. │ St. │ │ │ S. of │Railroad │Nebraska │ .0096│ 500│ 0.048 Cordovez│ │ St. │ │ │ St. │ │ │ │ │ S. of │Map margin│Nebraska │ .0084│ 500│ 0.042 Cordovez│ │ St. │ │ │ St. │ │ │ │ │ Nebraska │S. of │Long St. │ .425│ 409│ 1.74 St. │ Cordovez│ │ │ │ │ St. │ │ │ │ Long St. │Map margin│Nebraska │ .0048│ 500│ 0.024 │ │ St. │ │ │ Long St. │Nebraska │N. │ .430│ 409│ 1.76 │ St. │ Carolina│ │ │ │ │ St. │ │ │ Long St. │N. │Georgia │ .430│ 409│ 1.76 │ Carolina│ St. │ │ │ │ St. │ │ │ │ Long St. │Georgia │Harris St.│ .433│ 407│ 1.76 │ St. │ │ │ │ │ │ │ │ │ Long St. │Harris St.│Tennessee │ .454│ 403│ 1.83 │ │ St. │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (13) │ (14) │ (15) (1) │ │ │ │ │ ──────────┴──────────┴──────────┴──────────┴────────┴─────────

──────────┬──────────┬──────────┬─────────┬──────────┬──────┬────── On Street │ From │To Street │Increment│Cumulative│Total │ Line │ Street │ │of Ground│ Ground │Flow, │Number │ │ │ Water, │ Water, │C.F.S.│ │ │ │ C.F.S. │ C.F.S. │ │ │ │ │ │ │ │ ──────────┼──────────┼──────────┼─────────┼──────────┼──────┼────── Nebraska │Map margin│Alley S. │ 0.005│ 0.0187│ 1.66│ 1 St. │ │ Grant │ │ │ │ │ │ St. │ │ │ │ Alley S. │Railroad │E. of │ .0048│ .0048│ 0.046│ 2 of Grant│ │ Missouri│ │ │ │ St. │ │ St. │ │ │ │ Alley S. │E. of │E. of │ .0052│ .010│ 0.105│ 3 of Grant│ Missouri│ Kansas │ │ │ │ St. │ St. │ St. │ │ │ │ Alley S. │E. of │Nebraska │ .0050│ .015│ 0.152│ 4 of Grant│ Kansas │ St. │ │ │ │ St. │ St. │ │ │ │ │ Nebraska │Alley S. │Alley S. │ .0058│ .208│ 1.79│ 5 St. │ of Grant│ of │ │ │ │ │ St. │ Meridian│ │ │ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ .0117│ .0117│ 0.123│ 6 of │ │ St. │ │ │ │ Meridian│ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ .0045│ .224│ 1.85│ 7 St. │ of │ of Smith│ │ │ │ │ Meridian│ Av. │ │ │ │ │ │ │ │ │ │ Alley S. │Railroad │Nebraska │ .0089│ .0089│ 0.069│ 8 of Smith│ │ St. │ │ │ │ Ave. │ │ │ │ │ │ Nebraska │Alley S. │S. of │ .0044│ .237│ 1.92│ 9 St. │ of Smith│ Cordovez│ │ │ │ │ Ave. │ St. │ │ │ │ S. of │Railroad │Nebraska │ .006│ .006│ 0.054│ 10 Cordovez│ │ St. │ │ │ │ St. │ │ │ │ │ │ S. of │Map margin│Nebraska │ .0056│ .0056│ 0.048│ 11 Cordovez│ │ St. │ │ │ │ St. │ │ │ │ │ │ Nebraska │S. of │Long St. │ .0025│ .251│ 1.99│ 12 St. │ Cordovez│ │ │ │ │ │ St. │ │ │ │ │ Long St. │Map margin│Nebraska │ .0056│ .0056│ 0.030│ 13 │ │ St. │ │ │ │ Long St. │Nebraska │N. │ .0072│ .264│ 2.02│ 14 │ St. │ Carolina│ │ │ │ │ │ St. │ │ │ │ Long St. │N. │Georgia │ .0064│ 1.27│ 3.03│ 15 │ Carolina│ St. │ │ │ │ │ St. │ │ │ │ │ Long St. │Georgia │Harris St.│ .0061│ 1.28│ 3.04│ 16 │ St. │ │ │ │ │ │ │ │ │ │ │ Long St. │Harris St.│Tennessee │ .024│ 1.30│ 3.13│ 17 │ │ St. │ │ │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (16) │ (17) │ (18) │ (1) │ │ │ │ │ │ ──────────┴──────────┴──────────┴─────────┴──────────┴──────┴────── TABLE 20

COMPUTATIONS FOR SLOPE AND DIAMETER OF PIPES FOR A SEPARATE SEWERAGE SYSTEM

──────────┬──────────┬──────────┬───────┬───────┬──────┬─────────────── On Street │ From │To Street │ From │ To │Length│El. of Surface │ Street │ │Manhole│Manhole│ Feet │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┬─────── │ │ │ │ │ │ Upper │ Lower │ │ │ │ │ │Manhole│Manhole ──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼─────── Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ 105.8│ 102.4 St. │ │ Grant │ │ │ │ │ │ │ St. │ │ │ │ │ Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 113.5│ 112.0 of Grant│ │ Missouri│ │ │ │ │ St. │ │ St. │ │ │ │ │ Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 112.0│ 107.7 of Grant│ Missouri│ Kansas │ │ │ │ │ St. │ St. │ St. │ │ │ │ │ Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│ 107.7│ 102.4 of Grant│ Kansas │ St. │ │ │ │ │ St. │ St. │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ 102.4│ 100.7 St. │ of Grant│ of │ │ │ │ │ │ St. │ Meridian│ │ │ │ │ Alley S. │Railroad │Kansas St.│ 7.2│ 7.1│ 400│ 111.8│ 107.0 of │ │ │ │ │ │ │ Meridian│ │ │ │ │ │ │ Alley S. │Kansas St.│Nebraska │ 7.1│ 3.3│ 400│ 107.0│ 100.7 of │ │ St. │ │ │ │ │ Meridian│ │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ 100.7│ 99.3 St. │ of │ of Smith│ │ │ │ │ │ Meridian│ Av. │ │ │ │ │ Alley S. │Railroad │East of │ 6.2│ 6.1│ 305│ 109.3│ 105.3 of Smith│ │ Kansas │ │ │ │ │ Ave. │ │ St. │ │ │ │ │ Alley S. │East of │Nebraska │ 6.1│ 3.2│ 304│ 105.3│ 99.3 of Smith│ Kansas │ St. │ │ │ │ │ Ave. │ St. │ │ │ │ │ │ Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ 99.3│ 101.1 St. │ of Smith│ Cordovez│ │ │ │ │ │ Ave. │ St. │ │ │ │ │ S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│ 100.8│ 101.1 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│ 104.6│ 101.1 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ 101.1│ 98.7 St. │ Cordovez│ │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 103.8│ 98.7 │ │ St. │ │ │ │ │ Long St. │Nebraska │N. │ 148│ 147│ 492│ 98.7│ 103.8 │ St. │ Carolina│ │ │ │ │ │ │ St. │ │ │ │ │ Long St. │N. │Georgia │ 147│ 146│ 430│ 103.8│ 99.1 │ Carolina│ St. │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 99.1│ 96.9 │ St. │ │ │ │ │ │ Alley S. │End of │Harris St.│ 2.2│ 2.1│ 350│ 105.2│ 98.1 of Janis│ Janis │ │ │ │ │ │ St. │ St. │ │ │ │ │ │ Harris St.│Alley N. │Long St. │ 2.1│ 145│ 135│ 98.1│ 96.9 │ of Janis│ │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Harris St.│Kentucky │ 145│ 144│ 258│ 96.9│ 94.4 │ │ St. │ │ │ │ │ Long St. │Kentucky │Tennessee │ 144│ 143│ 282│ 94.4│ 93.6 │ St. │ St. │ │ │ │ │ Tarbell │Harris St.│Long St. │ 1.1│ 143│ 417│ 98.7│ 92.6 Ave. │ │ │ │ │ │ │ Long St. │Tennessee │Alley W. │ 143│ 142│ 185│ 92.6│ 92.3 │ St. │ of Tenn.│ │ │ │ │ │ │ St. │ │ │ │ │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8) (1) │ │ │ │ │ │ │ ──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴───────

──────────┬──────────┬──────────┬──────┬──────┬──────┬────────┬──────── On Street │ From │To Street │Total │Slope │ Dia. │Velocity│Capacity │ Street │ │Flow, │ │ of │ when │ when │ │ │C.F.S.│ │Pipe, │ Full, │ Full, │ │ │ │ │Inches│Ft. per │ Second │ │ │ │ │ │ Second │ Feet ──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼──────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ ──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼──────── Nebraska │Map margin│Alley S. │ 1.66│0.0108│ 10│ 3.25│ 1.78 St. │ │ Grant │ │ │ │ │ │ │ St. │ │ │ │ │ Alley S. │Railroad │E. of │ 0.046│.00575│ 8│ 2.00│ 0.71 of Grant│ │ Missouri│ │ │ │ │ St. │ │ St. │ │ │ │ │ Alley S. │E. of │E. of │ 0.105│ .0110│ 8│ 2.78│ 0.98 of Grant│ Missouri│ Kansas │ │ │ │ │ St. │ St. │ St. │ │ │ │ │ Alley S. │E. of │Nebraska │ 0.152│ .0156│ 8│ 3.27│ 1.18 of Grant│ Kansas │ St. │ │ │ │ │ St. │ St. │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 1.79│.00385│ 12│ 2.28│ 1.79 St. │ of Grant│ of │ │ │ │ │ │ St. │ Meridian│ │ │ │ │ Alley S. │Railroad │Kansas St.│ │ .0120│ 8│ 2.90│ 1.03 of │ │ │ │ │ │ │ Meridian│ │ │ │ │ │ │ Alley S. │Kansas St.│Nebraska │ 0.123│ .0157│ 8│ 3.28│ 1.18 of │ │ St. │ │ │ │ │ Meridian│ │ │ │ │ │ │ Nebraska │Alley S. │Alley S. │ 1.85│ .0042│ 12│ 2.36│ 1.85 St. │ of │ of Smith│ │ │ │ │ │ Meridian│ Av. │ │ │ │ │ Alley S. │Railroad │East of │ │ .0131│ 8│ 3.00│ 1.08 of Smith│ │ Kansas │ │ │ │ │ Ave. │ │ St. │ │ │ │ │ Alley S. │East of │Nebraska │ 0.069│ .0197│ 8│ 3.70│ 1.32 of Smith│ Kansas │ St. │ │ │ │ │ Ave. │ St. │ │ │ │ │ │ Nebraska │Alley S. │S. of │ 1.92│.00213│ 15│ 2.00│ 2.45 St. │ of Smith│ Cordovez│ │ │ │ │ │ Ave. │ St. │ │ │ │ │ S. of │Railroad │Nebraska │ │.00574│ 8│ 2.00│ 0.71 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ S. of │Map margin│Nebraska │ 0.054│.00854│ 8│ 2.46│ 0.87 Cordovez│ │ St. │ │ │ │ │ St. │ │ │ │ │ │ │ Nebraska │S. of │Long St. │ 1.99│.00213│ 15│ 2.00│ 2.45 St. │ Cordovez│ │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Map margin│Nebraska │ 0.030│ .0134│ 8│ 3.04│ 1.08 │ │ St. │ │ │ │ │ Long St. │Nebraska │N. │ 2.02│.00213│ 15│ 2.00│ 2.45 │ St. │ Carolina│ │ │ │ │ │ │ St. │ │ │ │ │ Long St. │N. │Georgia │ 3.03│ .0016│ 18│ 2.00│ 3.50 │ Carolina│ St. │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Georgia │Harris St.│ 3.04│ .0016│ 18│ 2.00│ 3.50 │ St. │ │ │ │ │ │ Alley S. │End of │Harris St.│ │ .0203│ 8│ 3.78│ 1.35 of Janis│ Janis │ │ │ │ │ │ St. │ St. │ │ │ │ │ │ Harris St.│Alley N. │Long St. │ │ .0088│ 8│ 2.53│ 0.89 │ of Janis│ │ │ │ │ │ │ St. │ │ │ │ │ │ Long St. │Harris St.│Kentucky │ │.00353│ 18│ 2.98│ 5.20 │ │ St. │ │ │ │ │ Long St. │Kentucky │Tennessee │ │.00635│ 18│ 4.00│ 7.00 │ St. │ St. │ │ │ │ │ Tarbell │Harris St.│Long St. │ │ .0146│ 8│ 3.18│ 1.14 Ave. │ │ │ │ │ │ │ Long St. │Tennessee │Alley W. │ 3.13│ .0016│ 18│ 2.00│ 3.50 │ St. │ of Tenn.│ │ │ │ │ │ │ St. │ │ │ │ │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12) │ (13) (1) │ │ │ │ │ │ │ ──────────┴──────────┴──────────┴──────┴──────┴──────┴────────┴────────

──────────┬──────────┬──────────┬───────────────┬────── On Street │ From │To Street │ El. of Invert │ Line │ Street │ │ │Number │ │ │ │ │ │ │ │ │ │ │ │ ──────────┼──────────┼──────────┼───────┬───────┼────── │ │ │ Upper │ Lower │ │ │ │Manhole│Manhole│ ──────────┼──────────┼──────────┼───────┼───────┼────── Nebraska │Map margin│Alley S. │ 97.80│ 94.40│ 1 St. │ │ Grant │ │ │ │ │ St. │ │ │ Alley S. │Railroad │E. of │ 105.50│ 103.62│ 2 of Grant│ │ Missouri│ │ │ St. │ │ St. │ │ │ Alley S. │E. of │E. of │ 103.61│ 99.70│ 3 of Grant│ Missouri│ Kansas │ │ │ St. │ St. │ St. │ │ │ Alley S. │E. of │Nebraska │ 99.69│ 94.40│ 4 of Grant│ Kansas │ St. │ │ │ St. │ St. │ │ │ │ Nebraska │Alley S. │Alley S. │ 94.07│ 92.61│ 5 St. │ of Grant│ of │ │ │ │ St. │ Meridian│ │ │ Alley S. │Railroad │Kansas St.│ 103.80│ 99.00│ 6 of │ │ │ │ │ Meridian│ │ │ │ │ Alley S. │Kansas St.│Nebraska │ 98.99│ 92.70│ 7 of │ │ St. │ │ │ Meridian│ │ │ │ │ Nebraska │Alley S. │Alley S. │ 92.37│ 91.09│ 8 St. │ of │ of Smith│ │ │ │ Meridian│ Av. │ │ │ Alley S. │Railroad │East of │ 101.30│ 97.30│ 9 of Smith│ │ Kansas │ │ │ Ave. │ │ St. │ │ │ Alley S. │East of │Nebraska │ 97.29│ 91.30│ 10 of Smith│ Kansas │ St. │ │ │ Ave. │ St. │ │ │ │ Nebraska │Alley S. │S. of │ 90.84│ 90.20│ 11 St. │ of Smith│ Cordovez│ │ │ │ Ave. │ St. │ │ │ S. of │Railroad │Nebraska │ 92.80│ 90.62│ 12 Cordovez│ │ St. │ │ │ St. │ │ │ │ │ S. of │Map margin│Nebraska │ 96.60│ 93.10│ 13 Cordovez│ │ St. │ │ │ St. │ │ │ │ │ Nebraska │S. of │Long St. │ 90.04│ 89.87│ 14 St. │ Cordovez│ │ │ │ │ St. │ │ │ │ Long St. │Map margin│Nebraska │ 95.80│ 90.70│ 15 │ │ St. │ │ │ Long St. │Nebraska │N. │ 89.86│ 88.94│ 16 │ St. │ Carolina│ │ │ │ │ St. │ │ │ Long St. │N. │Georgia │ 88.69│ 88.00│ 17 │ Carolina│ St. │ │ │ │ St. │ │ │ │ Long St. │Georgia │Harris St.│ 87.99│ 87.32│ 18 │ St. │ │ │ │ Alley S. │End of │Harris St.│ 97.20│ 90.10│ 19 of Janis│ Janis │ │ │ │ St. │ St. │ │ │ │ Harris St.│Alley N. │Long St. │ 90.09│ 88.90│ 20 │ of Janis│ │ │ │ │ St. │ │ │ │ Long St. │Harris St.│Kentucky │ 87.31│ 86.40│ 21 │ │ St. │ │ │ Long St. │Kentucky │Tennessee │ 86.39│ 84.60│ 22 │ St. │ St. │ │ │ Tarbell │Harris St.│Long St. │ 90.70│ 84.60│ 23 Ave. │ │ │ │ │ Long St. │Tennessee │Alley W. │ 83.77│ 83.47│ 24 │ St. │ of Tenn.│ │ │ │ │ St. │ │ │ │ │ │ │ │ Column No.│ (2) │ (3) │ (14) │ (15) │ (1) │ │ │ │ │ ──────────┴──────────┴──────────┴───────┴───────┴──────

=47. Surface Profile.=—A profile of the surface of the ground along the proposed lines of the sewers should be drawn after the completion of the computations for quantity. An example of a profile is shown in Fig. 26 for the line between manholes No. 3.5 and No. 147. The vertical scale should be at least 10 times the horizontal. A horizontal scale of 1 inch to 200 feet can be used where not much detail is to be shown, but a scale of one 1 to 100 feet is more common and more satisfactory and even one inch to 10 feet has been used. The information to be given and the method of showing it are illustrated on Fig. 26. The profile should show the character of the material to be passed through and the location of underground obstacles which may be encountered. The method of obtaining this information is taken up in Chapter II. The collection of the information should be completed as far as possible previous to design, and borings and other investigations made as soon as the tentative routes for the sewers have been selected.

=48. Slope and Diameter of Sewers.=—After the quantity of sewage to be carried has been determined, and the profile of the ground surface has been drawn, it is possible to determine the slope and diameter of the sewer. A table such as No. 20 is made up somewhat similar to No. 19, or which may be an extension of Table 19 since the first 6 columns in both tables are the same. The elevation of the surface at the upper and lower manholes is read from the profile.

The depth of the sewer below the ground surface is first determined. Sewers should be sufficiently deep to drain cellars of ordinary depth. In residential districts cellars are seldom more than 5 feet below the ground surface. To this depth must be added the drop necessary for the grade of the house sewer. Six-inch pipe laid on a minimum grade of 1.67 per cent is a common size and slope restriction for house drains or sewers. An additional 12 inches should be allowed for the bends in the pipe and the depth of the pipe under the cellar floor. Where the elevation of the street and lots is about the same, and the street is not over 80 feet in width between property lines, a minimum depth of 8 feet to the invert of sewers, 24 inches or less in diameter is satisfactory. This is on the assumption that the axes of the house drain and the sewer intersect. For larger pipes the depth should be increased so that when the street sewer is flowing full, sewage will not back up into the cellars or for any great distance into the tributary pipes.

The grade or slope at which a sewer shall be may be fixed by: the slope of the ground surface; the minimum permissible self-cleansing velocity; a combination of diameter, velocity, and quantity; or the maximum permissible velocity of flow. Sewers are laid either parallel to the ground surface where the slope is sufficient or where possible without coming too near the surface they are laid on a flatter grade to avoid unnecessary excavation. The minimum permissible slope is fixed by the minimum permissible velocity.

The velocity of flow in a sewer should be sufficient to prevent the sedimentation of sludge and light mineral matter. Such a velocity is in the neighborhood of 1 foot per second. Since sewers seldom flow full this velocity should be available under ordinary conditions of dry weather flow. The minimum velocity when full should therefore be about 2 feet per second. Under this condition, the velocity of 1 foot per second is not reached until the sewer is less than 18 per cent full. The velocity in small sewers should be made somewhat faster than in large sewers since the velocity of flow for small depths in small pipes is less than for the same proportionate depth in large pipes. The maximum permissible velocity of flow is fixed at about 10 feet per second in order to avoid excessive erosion of the invert. If the sewer is carefully laid this limit may be exceeded in sanitary sewers.

The method for determining the grade and diameter of sewers is best explained through an illustrative problem which is worked out in Table 20 for the profile shown on Fig. 26. The figures are inserted in the table from left to right in each line, one line being completed before the next one is commenced. The headings in the first 6 columns are self-explanatory. The elevations of the surface at the upper and lower manholes are read from the profile. The total flow is read from column (18) in Table 19. The slope of the ground surface is then computed, and with the quantity, slope, and coefficient of roughness, the diameter of the pipe and the velocity of flow are read from Fig. 15.

The following conditions may arise:

(1) The diameter required is less than 8 inches. Use a diameter of 8 inches as experience has shown that the use of smaller diameters is unsatisfactory.

(2) The velocity of flow when the sewer is full is less than 2 feet per second. Increase the slope until the velocity when full is 2 feet per second.

(3) The diameter of the pipe required is not one of the commercial sizes shown in Fig. 15. Use the next largest commercial size.

(4) The slope of the ground surface is steeper than necessary to maintain the required minimum velocity and the upper end of the sewer is deeper than the required minimum depth. Place the sewer on the minimum permissible grade, or upon such a grade that its lower end will be at the minimum permissible depth.

(5) The slope of the ground surface is so steep as to make the velocity of flow greater than the maximum rate permissible. Reduce the grade by deepening the sewer at the upper manhole and using a drop manhole at this point.

It is not permissible to use a pipe larger than that called for by the above conditions. This is attempted sometimes in order to reduce the grade and thereby save excavation, under the rule of a minimum velocity of 2 feet per second when full. It is better to use the smaller pipe on the flat grade as the quantity of sewage is insufficient to fill the larger sewer and the minimum permissible velocity is more quickly reached.

Having determined the slope, the diameter, and the capacity of the pipe to be used, these values are entered in the table. The elevations of the invert of the pipe at the upper and lower manholes are next computed and entered in the table. This method is followed until all of the diameters, slopes, and elevations have been determined.

The slopes are computed from center to center of manholes, but an extra allowance of 0.01 of a foot is allowed by some designers for the increased loss in head in passing through the manhole. When it becomes necessary to increase the diameter of the sewer the top of the outgoing sewer is placed at the same elevation or below the top of the lowest incoming sewer. No extra allowance is made to compensate for loss in head in the manhole in this case. This case is illustrated in columns (14) and (15) in lines (16) and (17) of Table 20. All of the conditions listed above are illustrated in Table 20, except the condition for a velocity greater than 10 feet per second.

The first condition is met at the head of practically every lateral, and is illustrated in the second line.

The second condition is also illustrated in the second line. The slope of the ground surface is 0.0046, which gives a velocity of only 1.8 feet per second in an 8–inch pipe. The slope is therefore increased to 0.00575, on which the full velocity is 2 feet per second.

The third condition is met in the first line. The diameter called for to carry 1.66 cubic feet per second on a slope of 0.0108 is slightly less than 10 inches. A 10–inch pipe is therefore used and its full capacity and velocity are recorded.

The fourth condition is illustrated in the fourteenth line. The cut at manhole No. 3.1 is 11.1 feet. The slope of the ground is 0.014, much steeper than is necessary to maintain the minimum velocity in a 15–inch pipe. The pipe is therefore placed on the minimum permissible slope, and excavation is saved. The student should check the figures in Table 20 and be sure that they are understood before an attempt is made to make a design independently.

=49. The Sewer Profile.=—The profile is next completed as shown in Fig. 26, the pipe line being drawn in as the computations are made. The cut is recorded to the nearest ⅒th of a foot at each manhole, or change in grade. It should not be given elsewhere as it invites controversy with the contractor. The cut is the difference of the elevation of the invert of the lowest pipe in the trench at the point in question, and the surface of the ground.

The stationing should be shown to the nearest ⅒th of a foot. It should commence at 0 + 00 at the outlet and increase up the sewer. The station of any point on the sewer may show the distance from it to the outlet, or a new system of stationing may be commenced at important junctions or at each junction.

Elevations of the surface of the ground should be shown to the nearest ⅒th of a foot, and the invert elevation to the nearest 1/100th of a foot.

Only the main line sewer is shown in profile in Fig. 26. The profiles of the laterals computed in Table 20, have not been shown. The approximate location of all house inlets are shown on the profile and located exactly, and are made a matter of record during construction.

DESIGN OF A STORM WATER SEWER SYSTEM

=50. Planning the System.=—Storm sewer systems are seldom as extensive as separate or combined sewer systems, since storm sewage can be discharged into the nearest suitable point in a flowing stream or other drainage channel, whereas dry weather or combined sewage must be conducted to some point where its discharge will be inoffensive. The need of a comprehensive general plan of a storm sewer system is quite as great, however, as for a separate system. The haphazard construction of sewers at the points most needed for the moment results in the duplication of forgotten drains, expense in increasing the capacity of inadequate sewers, and difficult construction due to underground structures thoughtlessly located. A comprehensive plan permits the construction of sewers where they are needed as they are required, and enables all probable future needs to be cared for at a minimum of expense.

The same preliminary survey, map, and underground information are necessary for the design of a storm sewer system as for a separate sewer system. The map shown on Fig. 25 has been used for the design of a storm-water sewer system.

The steps in the design of a storm-water sewer system are:

1st. Note the most advantageous points to locate the inlets and lay out the system to drain these inlets. 2nd. Determine the required capacity of the sewers by a study of the run-off from the different drainage areas. 3rd. Draw the profile and compute the diameter and slope of the pipes required.

=51. Location of Street Inlets.=—The location of storm sewers is determined mainly by the desirable location of the street inlets. The inlets must therefore be located before the system can be planned. In general the inlets should be located so that no water will flow across a street or sidewalk, in order to reach the sewer. This requires that inlets be placed on the high corners at street intersections, in depressions between street intersections, and at sufficiently frequent intervals that the gutters may not be overloaded. City blocks are seldom so long as to necessitate the location of inlets between crossings solely on account of inadequate gutter capacity. The capacity of a gutter can be computed approximately by the application of Kutter’s formula. Inlet capacities are discussed in Chapter VI. When the area drained is sufficiently large to tax the capacity of the gutter or inlet, an inlet should be installed regardless of the location of the street intersections.

The street inlets are located on the map as shown in Fig. 25. The sewer lines are then located so as to make the length of pipe to pass near to all inlets a minimum. Storm sewers are seldom placed near the center of a street because of the frequent crowded condition on this line.

=52. Drainage Areas.=—The outline of a drainage area is drawn so that all water falling within the area outlined will enter the same inlet, and water falling on any point beyond the outline will enter some other inlet. This requires that the outline follow true drainage lines rather than the artificial land divisions used in locating the drainage lines in the design of sanitary sewers. The drainage lines are determined by pavement slopes, location of downspouts, paved or unpaved yards, grading of lawns and the many other features of the natural drainage which are altered by the building up of a city. The location of the drainage lines is fixed as the result of a study of local conditions.

The watershed or drainage lines are shown on Fig. 25 by means of dot and dash lines. A drainage line passes down the middle of each street because the crown of the street throws the water to either side and directs it to different inlets. A watershed line is drawn about 50 feet west of such streets as Kentucky St., Florida St., etc., because the downspouts from the houses on those streets discharge or will discharge into the street on which they face. The location of any watershed line within 20 feet more or less is, in most cases, a matter of judgment rather than exactness. Each area is given an identifying number or mark which is useful only in design. It usually corresponds to the inlet number.

=53. Computation of Flood Flow by McMath Formula.=—McMath’s Formula is used as an example of the method pursued when an empirical formula is adopted for the computation of run-off, and because of its frequent use in practice. Other formulas may be more satisfactory under favorable conditions.

Computations should be kept in order by a tabulation such as is shown in Table 21, in which the quantity of storm flow discharged from the sewer at the foot of Tennessee St., on Fig. 25, has been computed by means of the McMath Formula, using the constants suggested for St. Louis conditions, i = 2.75, and c = 0.75. The solutions of the formula have been made by means of Fig. 11. The column headings in the Table are explanatory of the figures as recorded. The computation should begin at the upper end of a lateral, proceed to the first junction and then return to the head of another lateral tributary to this junction. They should be continued in the same manner until all tributary areas have been covered. Special computations will be necessary for the determination of the maximum quantity of storm water entering each inlet to avoid the flooding of an inlet or gutter. These computations have not been shown as they are so easily made by the application of McMath’s Formula to each area concerned.

The determination of the average slope ratio is a matter of judgment, based on the average natural slope of the surface of the ground and an estimate of the probable future conditions.

=54. Computation of Flood Flow by Rational Method.=—The rational method for the computation of storm-water run-off is described in Chapter III. An example of its application to storm sewer design is given here for the district shown in Fig. 25.[34] The computations are shown in Table 21. As in the preceding designs the table has been filled in from left to right and line by line. Computations have started at the upper end of laterals tributary to each junction. The column headed I represents the imperviousness factor in the expression Q = AIR. It is based on judgment guided by the constants given in Chapter III concerning imperviousness. The column headed “Equivalent 100 per cent I acres” is the product of the two preceding columns. It reduces all areas to the same terms so that they can be added for entry in the column headed “Total 100 per cent I acres.” It may be necessary to record the values for this column on several lines where the imperviousnesses of the tributary areas are different. This condition is illustrated in the last line of the table, for the length of sewer nearest the outlet. In the preceding lines the imperviousness recorded represents an average for all the tributary areas.

TABLE 21

COMPUTATIONS FOR THE QUANTITY OF STORM SEWAGE AT THE FOOT OF TENNESSEE STREET ON FIGURE 25

─────────┬──────────┬──────────┬───────────┬───────────────────────────────── On Street│ From │To Street │Identifying│ By McMath’s Formula │ Street │ │ Number of │ │ │ │ Acres │ │ │ │ Drained │ ─────────┼──────────┼──────────┼───────────┼──────────┬───────┬───────┬────── │ │ │ │Additional│ Total │ Slope │ Run │ │ │ │ Acres │ Acres │ of │Off in │ │ │ │ Drained │Drained│Surface│C.F.S. │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼──────────┼──────────┼───────────┼──────────┼───────┼───────┼────── State │N. │S. │ 91 and 92 │2.35 │ 2.35│ 0.005│ 5.5 │ Carolina│ Carolina│ │ │ │ │ State │S. │Georgia │88, 89 and │3.0 │ 5.35│ .005│ 10.8 │ Carolina│ │ 90 │ │ │ │ State │Georgia │Florida │85, 86 and │3.0 │ 8.35│ .007│ 16.5 │ │ │ 87 │ │ │ │ State │Florida │Kentucky │81, 83 and │3.0 │ 11.35│ .009│ 22.0 │ │ │ 84 │ │ │ │ State │Kentucky │Tennessee │79, 80 and │3.0 │ 14.35│ .010│ 28.0 │ │ │ 82 │ │ │ │ State │Texas │Louisiana │ 76 and │3.8 │ 3.8│ .005│ 8.3 │ │ │ others │ │ │ │ State │Louisiana │Alabama │73, 74 and │3.7 │ 7.5│ .007│ 15.0 │ │ │ 75 │ │ │ │ State │Alabama │Tennessee │70, 71 and │3.0 │ 10.5│ .006│ 19.0 │ │ │ 72 │ │ │ │ Tennessee│State │Talon │68, 69, 77 │4.3 │ 29.15│ .15│ 52 │ │ │ and 78 │ │ │ │ Talon │Albemarle │Tennessee │65, 66 and │2.8 │ 2.8│ .018│ 8.4 │ │ │ 67 │ │ │ │ Tennessee│Talon │Burnside │ 64 and │0.7 │ 29.85│ .15│ 55 │ │ │ 64_a_ │ │ │ │ Burnside │N. │S. │57, 58 and │2.84 │ 2.84│ .008│ 7.2 │ Carolina│ Carolina│ 59 │ │ │ │ Burnside │S. │Georgia │54, 55 and │3.88 │ 6.72│ .010│ 14.9 │ Carolina│ │ 56 │ │ │ │ Burnside │Georgia │Florida │50, 52 and │3.88 │ 10.60│ .012│ 22 │ │ │ 53 │ │ │ │ Burnside │Florida │Kentucky │47, 48 and │3.88 │ 14.48│ .013│ 30 │ │ │ 51 │ │ │ │ Burnside │Kentucky │Tennessee │44, 45 and │3.88 │ 18.36│ .013│ 36 │ │ │ 46 │ │ │ │ Tennessee│Burnside │Elm │ 42 and 43 │2.84 │ 51.05│ .015│ 82 Elm │Above │Chetwood │ Included in next line below │ │ │ Chetwood│ │ │ │ Elm │Chetwood │Albemarle │31, 32 and │2.75 │ 2.75│ .007│ 7.0 │ │ │ 33 │ │ │ │ Elm │Albemarle │Tennessee │27, 28, 29 │5.75 │ 8.50│ .016│ 20 │ │ │ and 30 │ │ │ │ Tennessee│Elm │Varennes │25, 26 and │2.62 │ 62.17│ .017│ 100 │ │ │ 41 │ │ │ │ Varennes │S. │Georgia │17, 18 and │3.17 │ 3.17│ .010│ 8.3 │ Carolina│ │ 19 │ │ │ │ Varennes │Georgia │Florida │14, 15 and │3.17 │ 6.34│ .011│ 14.5 │ │ │ 16 │ │ │ │ Varennes │Florida │Kentucky │11, 12 and │3.17 │ 9.51│ .013│ 21 │ │ │ 13 │ │ │ │ Varennes │Kentucky │Tennessee │8, 9 and 10│3.17 │ 12.68│ .013│ 26 Tennessee│Varennes │Boulevard │ 6 and 7 │2.32 │ 77.17│ .017│ 120 Tennessee│Boulevard │Outlet │1, 2, 3, 4,│4.72 │ 81.89│ .017│ 122 │ │ │ and 5 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┴──────────┴──────────┴───────────┴──────────┴───────┴───────┴──────

─────────┬──────────┬──────────┬─────────────────────────────────────────── On Street│ From │To Street │ By Rational Method │ Street │ │ │ │ │ │ │ │ ─────────┼──────────┼──────────┼─────┬─────┬──────────┬─────┬────────────── │ │ │Area,│ I │Equivalent│Total│ Time of │ │ │Acres│ │ 100 Per │ 100 │Concentration, │ │ │ │ │ Cent I │ Per │ Minutes │ │ │ │ │ Acres │Cent │ │ │ │ │ │ │ I │ │ │ │ │ │ │Acres│ ─────────┼──────────┼──────────┼─────┼─────┼──────────┼─────┼────────────── State │N. │S. │ 2.35│ 0.50│ 1.17│ 1.17│ 7.0 │ Carolina│ Carolina│ │ │ │ │ State │S. │Georgia │ 3.00│ .50│ 1.50│ 2.67│ 8.1 │ Carolina│ │ │ │ │ │ State │Georgia │Florida │ 3.00│ .50│ 1.50│ 4.17│ 9.0 │ │ │ │ │ │ │ State │Florida │Kentucky │ 3.00│ .50│ 1.50│ 5.67│ 9.9 │ │ │ │ │ │ │ State │Kentucky │Tennessee │ 3.00│ .50│ 1.50│ 7.17│ 10.7 │ │ │ │ │ │ │ State │Texas │Louisiana │ 3.80│ .35│ 1.33│ 1.33│ 10.0 │ │ │ │ │ │ │ State │Louisiana │Alabama │ 3.70│ .40│ 1.48│ 2.81│ 11.9 │ │ │ │ │ │ │ State │Alabama │Tennessee │ 3.00│ .45│ 1.35│ 4.16│ 12.9 │ │ │ │ │ │ │ Tennessee│State │Talon │ 4.30│ .50│ 2.15│13.48│ 14.5 │ │ │ │ │ │ │ Talon │Albemarle │Tennessee │ 2.80│ .40│ 1.12│ 1.12│ 8.0 │ │ │ │ │ │ │ Tennessee│Talon │Burnside │ 0.70│ .20│ 0.14│14.74│ 15.3 │ │ │ │ │ │ │ Burnside │N. │S. │ 2.84│ .55│ 1.56│ 1.56│ 10.0 │ Carolina│ Carolina│ │ │ │ │ Burnside │S. │Georgia │ 3.88│ .55│ 2.13│ 3.69│ 11.1 │ Carolina│ │ │ │ │ │ Burnside │Georgia │Florida │ 3.88│ .55│ 2.13│ 5.82│ 12.2 │ │ │ │ │ │ │ Burnside │Florida │Kentucky │ 3.88│ .55│ 2.13│ 7.95│ 13.1 │ │ │ │ │ │ │ Burnside │Kentucky │Tennessee │ 3.88│ .55│ 2.13│10.08│ 13.8 │ │ │ │ │ │ │ Tennessee│Burnside │Elm │ 2.84│ .45│ 2.28│26.10│ 15.7 Elm │Above │Chetwood │ │ │ │ │ │ Chetwood│ │ │ │ │ │ Elm │Chetwood │Albemarle │ 2.75│ .40│ 1.10│ 1.10│ 8.0 │ │ │ │ │ │ │ Elm │Albemarle │Tennessee │ 5.75│ .45│ 2.59│ 3.69│ 9.5 │ │ │ │ │ │ │ Tennessee│Elm │Varennes │ 2.62│ .50│ 1.31│30.00│ 16.2 │ │ │ │ │ │ │ Varennes │S. │Georgia │ 3.17│ .55│ 1.74│ 1.74│ 9.0 │ Carolina│ │ │ │ │ │ Varennes │Georgia │Florida │ 3.17│ .55│ 1.74│ 3.48│ 9.9 │ │ │ │ │ │ │ Varennes │Florida │Kentucky │ 3.17│ .55│ 1.74│ 5.22│ 10.8 │ │ │ │ │ │ │ Varennes │Kentucky │Tennessee │ 3.17│ .55│ 1.74│ 6.96│ 11.4 Tennessee│Varennes │Boulevard │ 2.32│ .55│ 1.28│32.84│ 16.5 Tennessee│Boulevard │Outlet │ 0.18│ .80│ 0.14│ Area No. 1 │ │ │ │ │ │ │ │ │ 1.38│ .50│ 0.69│ Area No. 2 │ │ │ 2.80│ .55│ 1.54│ Areas No. 3 and 4 │ │ │ 0.36│ .75│ 0.27│35.48│ 16.9 │ │ │ │ │ │ │ ─────────┴──────────┴──────────┴─────┴─────┴──────────┴─────┴──────────────

─────────┬──────────┬──────────┬─────────────────────────────────┬────── On Street│ From │To Street │ By Rational Method │ Line │ Street │ │ │Number │ │ │ │ │ │ │ │ ─────────┼──────────┼──────────┼───┬────┬─────┬────┬───────┬─────┼────── │ │ │R│Q │ S │V │ Sewer │Time │ │ │ │ │ │ │ │Length,│ in │ │ │ │ │ │ │ │ Feet │Sewer│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼──────────┼──────────┼───┼────┼─────┼────┼───────┼─────┼────── State │N. │S. │4.8│ 5.6│0.011│ 4.6│ 300│ 1.1│ 1 │ Carolina│ Carolina│ │ │ │ │ │ │ State │S. │Georgia │4.6│12.2│ .010│ 5.5│ 300│ 0.9│ 2 │ Carolina│ │ │ │ │ │ │ │ State │Georgia │Florida │4.4│18.3│ .012│ 5.8│ 300│ 0.9│ 3 │ │ │ │ │ │ │ │ │ State │Florida │Kentucky │4.2│23.9│ .009│ 6.0│ 300│ 0.8│ 4 │ │ │ │ │ │ │ │ │ State │Kentucky │Tennessee │4.1│29.3│ .009│ 6.2│ 300│ 0.8│ 5 │ │ │ │ │ │ │ │ │ State │Texas │Louisiana │4.2│ 5.6│ .009│ 3.2│ 370│ 1.9│ 6 │ │ │ │ │ │ │ │ │ State │Louisiana │Alabama │3.9│11.0│ .011│ 5.2│ 300│ 1.0│ 7 │ │ │ │ │ │ │ │ │ State │Alabama │Tennessee │3.8│15.8│ .002│ 3.2│ 300│ 1.6│ 8 │ │ │ │ │ │ │ │ │ Tennessee│State │Talon │3.6│48.5│ .019│ 9.8│ 450│ 0.8│ 9 │ │ │ │ │ │ │ │ │ Talon │Albemarle │Tennessee │4.6│ 5.2│ .004│ 3.0│ 210│ 1.2│ 10 │ │ │ │ │ │ │ │ │ Tennessee│Talon │Burnside │3.5│51.5│ .006│ 5.0│ 120│ 0.4│ 11 │ │ │ │ │ │ │ │ │ Burnside │N. │S. │4.2│ 6.5│ .008│ 4.5│ 300│ 1.1│ 12 │ Carolina│ Carolina│ │ │ │ │ │ │ Burnside │S. │Georgia │4.0│14.8│ .007│ 4.7│ 300│ 1.1│ 13 │ Carolina│ │ │ │ │ │ │ │ Burnside │Georgia │Florida │3.9│22.7│ .011│ 5.8│ 300│ 0.9│ 14 │ │ │ │ │ │ │ │ │ Burnside │Florida │Kentucky │3.7│29.4│ .016│ 7.5│ 300│ 0.7│ 15 │ │ │ │ │ │ │ │ │ Burnside │Kentucky │Tennessee │3.7│37.3│ .019│ 9.2│ 300│ 0.5│ 16 │ │ │ │ │ │ │ │ │ Tennessee│Burnside │Elm │3.4│88.8│ .015│10.2│ 280.│ 0.5│ 17 Elm │Above │Chetwood │ │ │ │ │ │ │ 18 │ Chetwood│ │ │ │ │ │ │ │ Elm │Chetwood │Albemarle │4.6│ 5.1│ .020│ 5.3│ 480│ 1.5│ 19 │ │ │ │ │ │ │ │ │ Elm │Albemarle │Tennessee │4.3│15.8│ .012│ 6.1│ 410│ 1.1│ 20 │ │ │ │ │ │ │ │ │ Tennessee│Elm │Varennes │3.4│ 102│ .012│10.2│ 180│ 0.3│ 21 │ │ │ │ │ │ │ │ │ Varennes │S. │Georgia │4.4│ 7.7│ .012│ 5.2│ 270│ 0.9│ 22 │ Carolina│ │ │ │ │ │ │ │ Varennes │Georgia │Florida │4.2│14.5│ .010│ 5.7│ 300│ 0.9│ 23 │ │ │ │ │ │ │ │ │ Varennes │Florida │Kentucky │4.1│21.4│ .017│ 7.7│ 300│ 0.6│ 24 │ │ │ │ │ │ │ │ │ Varennes │Kentucky │Tennessee │4.0│27.8│ .015│ 7.8│ 300│ 0.6│ 25 Tennessee│Varennes │Boulevard │3.3│ 108│ .012│10.2│ 230│ 0.4│ 26 Tennessee│Boulevard │Outlet │ │ │ │ │ │ │ 27 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ 28 │ │ │ │ │ │ │ │ 29 │ │ │3.3│ 117│Areas No. 1–5 inclusive │ 30 │ │ │ │ │ │ │ │ │ ─────────┴──────────┴──────────┴───┴────┴─────┴────┴───────┴─────┴──────

The time of concentration in minutes is assumed by judgment for the first area. For all subsequent areas it is the sum of the time of concentration for the area or areas tributary to the inlet next above and the time of flow in the sewer from the inlet next above to the inlet in question. For example, in line 2 the time 8.1 minutes is the sum of 7.0 minutes time of concentration to the inlet at the corner of State and North Carolina St., and the time of flow of 1.1 minute in the sewer on State St. from North Carolina St. to South Carolina St. Where two sewers are converging as at the corner of Varennes Road and Tennessee St. the longest time is taken. For example, the time of concentration down Varennes Road to Tennessee St. is shown in line 25 as 11.4 + 0.6 = 12.0 minutes. The time to the same point down Tennessee St. is shown in line 21 as 16.2 + 0.3 = 16.5 minutes. This time is therefore used in line 26.

R, the rate of rainfall in inches per hour is determined by Talbot’s formula.

Q, is in cubic feet per second and is the product of the 8th and 10th columns. Since the 8th column is the sum of the products of the 5th and the 6th columns for the lines representing tributary areas, then the 11th column is the product of A, I, and R.

S, is the slope on which it is assumed that the sewer will be laid. It is usually assumed as parallel to the ground surface unless the velocity for this slope becomes less than 2 feet per second. In such a case the slope is taken as one which will cause this velocity.

V, the velocity in feet per second, is computed from diagrams for the solution of Kutter’s formula. The length in feet is scaled from the map as the distance between inlets or groups of inlets, and the time is the length in feet divided by the velocity in feet per minute.

Having computed the quantity of flow to be carried in the sewer, the design is completed by drawing the profile and computing the diameters and slopes by the same method as used in the design of separate sewers.

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VI. APPURTENANCES

=55. General.=—The appurtenances to a sewerage system are those devices which, in addition to the pipes and conduits, are essential to or are of assistance in the operation of the system. Under this heading are included such structures and devices as: manholes, lampholes, flush-tanks, catch-basins, street inlets, regulators, siphons, junctions, outlets, grease traps, foundations and underdrains.

=56. Manholes.=—A manhole is an opening constructed in a sewer, of sufficient size to permit a man to gain access to the sewer. Manholes are the most common appurtenances to sewerage systems and are used to permit inspection and the removal of obstructions from the pipes. The details of the Baltimore standard manholes are shown in Fig. 27 and a manhole on a large sewer in Omaha is shown in Fig. 28. The features of these designs which should be noted are the size of the opening and working space, and the strength of the structure. Manhole openings are seldom made less than 20 inches in diameter and openings 24 inches in diameter are preferable. A man can pass through any opening that he can get his hips through provided he can bend his knees and twist his shoulders immediately on passing the hole. For this reason the manhole should widen out rapidly immediately below the opening, as shown in Fig. 27 and 38.

The walls of the manhole may be built either of brick or of concrete. Brick is more commonly used, as the forms necessary for concrete make the work more expensive unless they can be used a number of times. The walls of the manhole should be at least 8 inches thick. Greater thicknesses are used in treacherous soils and for deep manholes, or to exclude moisture. A rough expression for the thickness of the walls of a brick manhole more than 12 feet deep in ordinary firm material is t = d⁄2 + 2, in which t is the thickness in inches and d is the depth in feet. The thickness of brick walls may be changed every 5 to 10 feet or so. Concrete walls may be built thinner than brick walls.

The bottoms of brick manholes are frequently made of concrete as shown in Fig. 27. The floor slopes towards the center and is constructed so that the sewage flows in a half round or U-shaped channel of greater capacity than the tributary sewers. The sides of the channel should be high enough to prevent the overflow of sewage onto the sloping floor, which should have a pitch of about one vertical to 10 or 12 horizontal. In manholes where two or more sewers join at approximately the same level the channels in the bottom should join with smooth easy curves. Where the inlet and outlet pipes are not of the same diameter the tops of the pipes should ordinarily be placed at the same elevation to prevent back flow in the smaller pipes when the larger pipes are flowing full.

The dimensions of the manhole should not be less than 3 feet wide by 4 feet long for a height of at least 4 feet, when built in the form of an ellipse, or 4 feet in diameter when built circular. No standard method for the reduction of the diameter of the manhole near the top is observed, the rate being more or less dependent on the depth of the manhole. The use of sloping sides above the frost line is desirable as such a form is more resistant to heaving by frost action.

For sewers up to 48 inches in diameter the manhole is usually centered over the intersection of the pipes and has a special foundation. For larger sewers the manhole walls spring from the walls of the sewer as shown in Fig. 28.

In the case of a decided drop in the elevation of a sewer, or of a tributary sewer appreciably higher than an outlet in any manhole, the sewage is allowed to drop vertically at the manhole, hence the name drop manhole. The Baltimore standard drop manhole is shown in Fig. 27. A well hole is an unusually deep drop manhole in which the force of the vertical drop of sewage is broken by a series of baffle plates, or by a sump at the bottom of the well hole. Fig. 28 shows a well hole at St. Paul, Minn. The use of drop manholes can be avoided in large sewers by the construction of a flight of steps or flight sewer as shown in Fig. 29, which allows the use of a steep grade and serves to break the velocity of the sewage.

The specifications of the Sanitary District of Chicago, covering the construction of manhole covers and frames are:

All castings shall be of tough, close grained, gray iron, free from blow holes, shrinkage and cold shuts, and sound, smooth, clean and free from blisters and all defects.

All castings shall be made accurately to dimensions to be furnished and shall be planed where marked or where otherwise necessary to secure perfectly flat and true surfaces. Allowance shall be made in the patterns so that the specified thickness shall not be reduced.

All castings shall be thoroughly cleaned and painted before rusting begins and before leaving the shop with two coats of high grade asphaltum or any other varnish that the Engineer may direct. After the castings have been placed in a satisfactory manner, all foreign adhering substances shall be removed and the castings given one additional coat of asphaltum. No castings shall be accepted the weight of which shall be less than that due to its dimensions by more than 5 per cent.

The weights of frames and covers in use vary from 200 to 600 pounds, the weight of the frame being about 5 times that of the cover. The lightest weights are used where no traffic other than an occasional pedestrian will pass over the manhole. Frames and covers weighing about 400 pounds are commonly used on residential streets, whereas 600 pound frames and covers are desirable in streets on which the traffic is heavy. The frames should be so designed that the pavement will rest firmly against it and wear at the same rate as the surrounding street surface. Experience has shown that vertical sides should be used for the outside of the frame to approach this condition, and that the frame should not be less than 8 inches high. The cover should be roughened in some desirable pattern as shown in Fig. 30. Smooth covers become dangerously slippery. Where the ventilation of the sewers is not satisfactory the manhole covers are sometimes perforated. This is undesirable from other points of view as the rising odors and vapors are obnoxious at the surface and the entering dirt and water are detrimental to the operation of the sewer. The stealing and destruction of manhole covers and the unauthorized entering of sewers has occasionally required the locking of the covers to the frame when in place. The locks commonly used consist of a tumbler which falls into place when the manhole is closed, and which can be opened only by a special wrench or hook. Adjustable frames are sometimes used where the street grade is settling, or may be raised in order that the elevation of the top of the cover may be made to conform to that of the street surface, without reconstructing the top of the manhole. One type of adjustable cover is shown in Fig. 31. Manhole covers should be so marked that the sanitary sewer can be distinguished from the storm-water sewer, and both from the telephone conduit, etc.

Iron steps are set into the walls of the manhole about 15 inches apart vertically to allow entrance and exit to and from the manhole. Galvanized iron is preferable to unprotected metal as the action of rust is particularly rapid in the moist air of the sewer. One type of these manhole steps is shown in Fig. 27. The steps should have a firm grip in the wall as a loose step is a source of danger.

=57. Lampholes.=—A lamphole is an opening from the surface of the ground into a sewer, large enough to permit the lowering of a lantern into the sewer. Lampholes are used in the place of manholes to permit the inspection or the flushing of sewers, and to avoid the expense of a manhole. They are located from 300 to 400 feet from the nearest manhole in such a manner that a lamp lowered in the lamp hole can be seen from the two nearest manholes.

Lampholes should be constructed of 8– to 12–inch tile or cast-iron pipe. The lower section should be a cast-iron T on a firm foundation, but if constructed of tile it should be reinforced with concrete to take up the weight of the shaft. The details of the Baltimore standard lamphole are shown in Fig. 32. Lampholes are not commonly used on sewerage systems on account of their lack of real usefulness and the troubles encountered by breaking of the pipe below the shaft.

=58. Street Inlets.=—A street inlet is an opening in the gutter through which storm water gains access to the sewer. The types used in different cities vary widely. Details of an inlet in successful use are shown in Fig. 33. The figure shows also a common form of connection to the sewer. A water-seal trap is sometimes used to prevent the escape of odors from the sewer. Care must be taken in design that such traps do not freeze in winter nor dry out in summer, although it is not always possible to prevent these contingencies.

The important features to be observed in the design of a street inlet are: height and length of opening, character of grating, and location. The general location of inlets is discussed in Chapter V. The clear height of opening commonly used is from 5 to 6 inches, with a clear length of 24 to 30 inches or longer. Inlets of this size have given satisfaction on paved streets with moderate slopes, where the drainage area is not greater than 10,000 to 12,000 square feet of pavement. W. W. Horner states:[35]

The St. Louis type of inlet “old” style was a vertical opening in the curb 8 inches high and 4 feet in length with a horizontal bar making the net opening about 5 inches. It has a broad sill extending under the sidewalk. The “new” style inlet is 4½ feet long with a clear opening of 6 inches and no bar. The sill is done away with and the opening drops down directly from the curb. Tests were made of the capacity of this inlet on pavements on different slopes with sumps of depths varying from 0 to 6 inches in front of the inlet, extending out 3 feet from the gutter, and returning to the elevation of the gutter at a slope of 3 inches to the foot. The results of these tests are shown in Table 22. The capacity of the inlet is expressed as the amount of water entering just before some water begins to lap past. If a large amount of water is allowed to flow past much more water will enter the inlet thus furnishing a factor of safety for large storms. It was noted that by beginning the rise in the pavement about opposite the middle of the inlet the capacity of the inlet was increased from 20 to 50 per cent.

TABLE 22

CAPACITIES OF ST. LOUIS STREET INLETS

From tests by W. W. Horner. Cubic feet per second ─────────┬───────────────────┬─────────────────── Slope in │ 0.42 │ 1.5 Per Ct. │ │ ─────────┼────┬────┬────┬────┼────┬────┬────┬──── Depth of │ 0.0│ 2│ 4│ 6│ 0│ 2│ 4│ 6 Sump, │ │ │ │ │ │ │ │ Inches │ │ │ │ │ │ │ │ Capacity,│ │ │1.27│ │ │ │ │ old │ │ │ │ │ │ │ │ style │ │ │ │ │ │ │ │ Capacity,│ 0.1│ 0.5│ 1.5│ 2.5│0.08│ 0.4│ 1.1│ 2.1 new │ │ │ │ │ │ │ │ style │ │ │ │ │ │ │ │ ─────────┴────┴────┴────┴────┴────┴────┴────┴────

─────────┬───────────────────┬─────────────────── Slope in │ 2.85 │ 4.5 Per Ct. │ │ ─────────┼────┬────┬────┬────┼────┬────┬────┬──── Depth of │ 0│ 2│ 4│ 6│ 0│ 2│ 4│ 6 Sump, │ │ │ │ │ │ │ │ Inches │ │ │ │ │ │ │ │ Capacity,│0.03│0.25│0.78│1.49│ │ │ │ old │ │ │ │ │ │ │ │ style │ │ │ │ │ │ │ │ Capacity,│0.03│0.28│0.87│1.62│0.02│0.15│0.45│ 1.0 new │ │ │ │ │ │ │ │ style │ │ │ │ │ │ │ │ ─────────┴────┴────┴────┴────┴────┴────┴────┴────

Gratings with horizontal bars will admit more water than gratings with vertical bars, but they will also admit more rubbish such as sticks, papers, leaves, etc., which serve to clog the sewers. Vertical barred gratings and gratings in the bottom of the gutter clog more quickly than other types. In the selection of the type of grating to be used a decision must be made as to whether it is more desirable to clean the sewer or catch-basin, or to flood the street as a result of clogged inlets. Where catch-basins are used or the sewers are large, horizontal bars are more satisfactory. The openings between bars should be small enough to prevent the entrance of a horse’s hoof or objects of sufficient size to clog the sewer. Four inches in the clear for vertical openings and 6 inches for horizontal openings are reasonable sizes.

The location of the inlets at the intersection of the two curb lines at a corner results in a lower first cost but on heavily traveled streets this may result in a higher maintenance cost than for other locations because of the concentration of traffic at street corners, hammering the inlet casting out of shape or position. Vehicles making short turns will tend to climb the curb and will intensify the wear upon the inlet. These objections can be overcome by the use of two inlets at each corner, set back far enough from the curb intersection to avoid interference with the cross-walks. This also makes it possible to raise the cross-walks without the use of gutters under them.

The size of the pipe from the inlet to the catch-basin or sewer should be large enough to care for all of the water which may enter the inlet. As the capacity of the inlet is seldom known with accuracy and the capacity of the pipe is difficult of determination, it has become customary to use a 10–inch or a 12–inch connecting pipe for each ordinary independent inlet.

=59. Catch-basins.=—Catch-basins are used to interrupt the velocity of sewage before entering the sewer, causing the deposition of suspended grit and sludge and the detention of floating rubbish which might enter and clog the sewer. A separate catch-basin may be used for each inlet, or to save expense, the pipes from several inlets may discharge into one catch-basin.

The types in successful use are extremely varied, but the distinguishing feature of all is an outlet located above the floor of the basin. A common form of catch-basin is shown in Fig. 34. It is constructed similar to a manhole with a diameter of about 4 or 4½ feet and a depth of retained water from 3 to 4 feet. Catch-basins of this size will care for the sewage from the inlets at the four corners of a street intersection, each draining a city block. In unusual situations it may be necessary to install a larger basin, but too large a catch-basin is less desirable than one which is too small, as the former stinks and the latter is useless. Traps are sometimes used to prevent the escape of odors from the sewer into the street, but odors are often created in the catch-basins themselves. Some engineers arrange the trap so that it can be opened for observation down the sewer as in Fig. 34, thus combining the advantages of a manhole with the catch-basin.

The use of catch-basins is objectionable because: they furnish a breeding place for mosquitoes and other flying insects; the septic action in them produces offensive odors; if on a combined sewer they permit the escape of offensive odors in dry weather when the water seal in the trap has evaporated; and the freezing of the water seal in the trap prevents the entrance of water to the sewer. The sole advantage lies in the prevention of the clogging of the sewers, but this may be sufficient to overbalance all of the disadvantages. In general catch-basins should be provided on paved streets which are cleaned by flushing the material into the sewers, or where the drainage is from an unimproved or macadamized street, sandy country, or into sewers in which the velocity of flow is less than 2 feet per second.

=60. Grease Traps.=—The presence of grease in sewers results in the formation of incrustations which are difficult to remove and which cause a material loss in the capacity of the sewer. The presence of oil and gasoline has resulted in violent and destructive explosions as is described in Chapter XII. A type of grease trap used on the drains from hotels, restaurants, or other large grease producing industries is shown in Fig. 35. The trap is similar to a catch-basin except that it is too small for a man to enter, and the outlet is necessarily trapped in order to prevent the escape of grease. The details of a gasoline and oil separator approved by the New York City Fire Department are shown in Fig. 36.[36]

=61. Flush-Tanks.=—These are devices to hold water used in flushing sewers. They are required only on sanitary and combined sewers. Their use tends to prevent the clogging of sewers laid on flat grades and permits flatter grades in construction than could otherwise be adopted. Flush-tanks may be operated either by hand or automatically. Automatic operation is more common than hand operation. The hand-operated tanks are similar to manholes so arranged that the inlet and outlet sewers can be plugged while the manhole or tank is being filled with water either from a hose or a special service connection. When sufficient water has been accumulated the outlet is opened and the sewer is flushed by the rush of water. A sluice gate, flap valve, or a specially fitted board is sufficient to fit over the mouth of the inlet and outlet during the filling of the tank. Such an arrangement has the advantage of being cheap, simple, and satisfactory, though somewhat crude. In some cases water is run into the tank at the same rate that it is discharged through the open outlet, maintaining a depth of 4 or 5 feet in the tank until the water passing the manhole below runs clean. The volume of water required by this method is large. Flushing water under a relatively high head is sometimes obtained by the use of tank wagons which are quickly emptied into the sewer through a canvas pipe dropped down a manhole. In all such cases if not well constructed the manhole is subject to caving due to the rush of water around the outlet. Precautions should be taken to minimize this danger by limiting the depth of water which may be accumulated. This can be done by constructing an overflow at a height of 4 or 5 feet above the bottom of the manhole, discharging into the sewer through an outside drain.

Automatic flush-tanks are constructed similar to a manhole, but special care should be taken to make them water-tight. The apparatus for providing the automatic discharge may operate either with or without moving parts, the latter being preferable as they require less attention and are not so liable to get out of order. An automatic flush-tank of the Miller type is shown in Fig. 37. It is a patented device manufactured by the Pacific Flush Tank Company. The small pipe at the left is a service connection to the water main. Water is allowed to flow continuously into the tank at such a rate as to fill it in the required interval between discharges. The tanks are discharged as nearly once a day as it is practicable to regulate them. The rate of flow into the tank is determined by trial and varies to some extent with the water pressure. The regulator shown in the figure is desirable as the continuous flow through the ordinary cock soon wears it away. Some waters will cause deposits to form in the small passages of the cocks or regulators, thus cutting off the flow.

The tank operates as follows: when the water rising in the tank reaches the bottom of the bell, air is trapped in the bell and prevented from escaping through the main trap by the water at A. As the water continues to rise in the tank the air in the bell is compressed, the water level at A is driven down and water trickles from the siphon at C. The height of the water in the tank above the level of the water in the bell is equal at all times to the height of C above the lowering position of A. When A reaches the position of B a small amount of air is released through the short leg of the trap and a corresponding volume of water enters the bell. The head of water above the bell then becomes greater than the head of water in the short leg of the trap, which results in the discharge of all of the air in the long leg of the trap and the rapid discharge of the water in the tank through the siphon. The discharge is continued until the siphonic action is broken by the admission of air when the water level in the tank is lowered to the bottom of the bell. The size of the siphons is fixed by the diameter of the leg of the siphon. Table 23 shows the capacity and size of sewers for which the different sizes of siphons are recommended by the manufacturers.[37]

TABLE 23

SIZES OF SIPHONS TO BE USED WITH AUTOMATIC FLUSH-TANKS

──────────┬───────────┬───────────┬───────────┬───────────┬──────────── │ │ │ │ │ Height of │Diameter of│ Total │ │ │ the │Tank at the│ Discharge │ Average │ │ Discharge Diameter │ Discharge │ for One │ Rate of │Diameter of│ Line above of Siphon │ Line in │ Flush in │ Discharge │ Sewer in │the Edge of in Inches │ Feet │ Gallons │in Sec.-ft.│ Inches │ the Bell ──────────┼───────────┼───────────┼───────────┼───────────┼──────────── 4│ 3│ 60│ 0.35│ 4 to 6│1 ft. 2 in. 5│ 3│ 100│ 0.73│ 6 to 8│1 ft. 11 in. 6│ 4│ 240│ 1.06│ 8 to 10│2 ft. 6 in. 8│ 4│ 280│ 2.12│ 12 to 15│2 ft. 11 in. ──────────┴───────────┴───────────┴───────────┴───────────┴────────────

When flush-tanks are placed at the upper end of laterals provision should be made for inspecting and cleaning the sewer by the construction of a separate manhole, or by combining the features of a manhole and a flush-tank in the same structure. Such a combination is shown in Fig. 38 from a design by Alexander Potter.

Except under unusual conditions flush-tanks are used only on separate sewers. They should be placed at the upper end of laterals in which the velocity of flow when full is less than 2 to 4 feet per second. The capacity of the tank or the volume of the dose is dependent on the diameter and slope of the sewer. The most effective flush is obtained by a volume of water traveling at a high velocity and completely filling the sewer. A large volume allowed to run slowly through the sewer will have but little if any flushing action. Data on the quantity of flushing water needed are given in Table 24.[38] As the result of a series of experiments conducted by Prof. H. N. Ogden on the flushing of sewers,[39] the conclusion was reached that the effect of a flush of about 300 gallons in an 8–inch sewer on a grade less than 1 per cent would not be effective beyond 800 to 1,000 feet, but that on steeper grades much smaller quantities of water would produce equally good results.

TABLE 24

GALLONS OF WATER NEEDED FOR FLUSHING SEWERS

─────────────────┬───────────────────────────────────────────────────── Slope │ Diameter of Sewer in Inches ─────────────────┼─────────────────┬─────────────────┬───────────────── │ 8│ 10│ 12 ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.005 │ 80│ 90│ 100 .0075 │ 55│ 65│ 80 .01 │ 45│ 55│ 70 .02 │ 20│ 30│ 35 .03 │ 15│ 20│ 24 ─────────────────┴─────────────────┴─────────────────┴─────────────────

Engineers do not agree upon the advisability of the use of automatic flush-tanks, some believing that they are a needless expense that can be avoided by hand flushing, and others feeling that a flush-tank should be placed at the upper end of every lateral. These diverse opinions are the result of different experiences in different cities.

=62. Siphons.=—There are two forms of siphons used in sewerage practice, a true siphon and an inverted siphon. A true siphon is a bent tube through which liquid will flow at a pressure less than atmospheric, first upwards and then downwards, entering and leaving at atmospheric pressure. An inverted siphon is a bent tube through which liquid will flow at a pressure greater than atmospheric first downwards and then upwards, entering and leaving at atmospheric pressure.

In sewerage practice the word siphon refers to an inverted siphon unless otherwise qualified. Siphons, both true and inverted, are used in sewerage systems to pass above or below obstacles. True siphons are seldom used as they must be kept constantly filled with liquid.[40] Accumulated gas must be removed in order to prevent the breaking of the siphon which results in the cessation of flow. By the breaking of a true siphon is meant the stoppage of siphonic action due to the accumulation of air or gas at the peak of the siphon. Since the rate of flow of sewage fluctuates widely it is extremely difficult to control the flow so that a true siphon may be completely filled with liquid at all times.

In the design of inverted siphons care must be taken to prevent sedimentation, and to permit inspection and cleaning. Sedimentation is prevented by maintaining a velocity greater than a fixed minimum, usually taken at about 2 feet per second. This minimum is attained by providing a number of channels. The smallest channel is designed to convey the least expected flow at the minimum velocity. Each of the other channels is made as small as possible, within the limits of economy and simplicity, in order that the minimum velocity shall be exceeded quickly after flow has commenced in them. The last channel or channels to be filled are made somewhat larger, because the sewage conveyed in them contains less settleable matter than is contained in the more concentrated dry weather flow. The type of siphon used in New York to pass under the subway is shown in Fig. 39. Note should be taken of the clean-out manhole provided on the 14–inch pipe. The other pipes are large enough for a man to enter and clean.

The computations involved in the design of a siphon are illustrated in the following example, in which it is desired to construct a siphon to pass under the railway cut shown in Fig. 40. The first step is to determine the limiting diameter and slope of the smallest pipe in the siphon. One-sixth of the capacity of the 6–foot approach sewer or 19 cubic feet per second will be assumed as the minimum flow. The diameter of the pipe necessary to carry 19 cubic feet per second at a velocity of 2 feet per second is 42 inches. The hydraulic gradient should have a slope of 0.0005 if the material used has a roughness coefficient of .015. This is the minimum permissible slope of the siphon. The selection of a steeper slope will necessitate the laying of the sewer at a greater depth, and will permit the use of smaller pipes for the siphon. The selection of the exact slope must then be based on judgment with the minimum limitation above placed. The slope will be arbitrarily selected as 0.001, the same as that of the approach sewer. The diameter of the dry weather pipe will therefore be 36 inches, with a capacity of 18 second-feet, which is approximately the assumed dry weather flow. The velocity of flow will be 2.5 feet per second. The length of flow along the siphon is 150 feet.

The next step should be the determination of the elevation at the lower end of the 36–inch pipe. This is done by multiplying the assumed grade by the equivalent length of straight pipe, and subtracting the product from the elevation at the upper end. The length of straight pipe which will give the same loss of head as the siphon is called the equivalent pipe. It is determined as follows:

First, determine the head loss at entrance. This will vary between nothing and one velocity head, dependent on the arrangement at the entrance.[41] The length of straight pipe which will give this same loss can be computed from the expression l = h⁄S, using for S the assumed slope of the hydraulic gradient.

Second, determine the head loss due to the bends, This is determined from the expression

h = (fl)⁄(d) (V^2)⁄(2_g_)

in which h = the head loss in the bend;

l = the length of the bend;

d = the diameter of the pipe;

v = the average velocity of flow;

g = the acceleration due to gravity;

f = a factor dependent on the radius (R) of the bend and d.

The relation between f, R, and d, for 90° bends is shown as follows:[42]

R⁄d 24 16 10 6 4 2.4

f 0.036 0.037 0.047 0.060 0.062 0.072

After the head loss has been determined, the equivalent length of straight pipe is determined as above.

Third. The equivalent length of pipe will be the sum of the actual length of pipe and the equivalent lengths as found above.

In the problem in hand the head lost at the entrance will be assumed as one-third of a velocity head, or 0.0324 foot. With the assumed slope of 0.001 this is equivalent to 32 feet of pipe. The radius of the bend is about 20 feet and the length for a 45° central angle is about 16 feet. The head loss for this angle will probably be a little more than one-half that for a 90° angle. The expression will therefore be taken as about 0.2(V^2)⁄(2_g_) and for two bends is equivalent to about 40 feet of pipe. The equivalent length of pipe is therefore 150 + 32 + 40 = 222 feet. The elevation at the lower end should therefore be: the elevation at the upper end, 92.07 − 222 × .001 = 91.85.

The diameters of the remaining pipes in the siphon are determined so that the sewage in the approach sewer is backed up as little as is consistent with good judgment before each pipe comes into action. This is done satisfactorily by a method of cut and try. Let it be assumed that the siphon will be composed of three pipes: the dry weather pipe taking 18 second-feet, the second pipe taking 28 second-feet, and the third pipe taking the remaining 70 second-feet. The diameters of the two larger pipes on the assumed slope of 0.001 will therefore be 42 inches and 60 inches respectively. Other combinations might be used which would be equally satisfactory. There are many methods by which the sewage can be diverted into the different channels of the siphon. For example, the openings into the different pipes may be placed at the same elevation, and the sewage allowed to enter them in turn through automatically or hand-controlled gates, or in another method of control the openings may be placed at such elevations that when the capacity of one pipe has been exceeded the sewage will flow into the next largest pipe as shown in Fig. 40. The outlets from the different pipes are ordinarily placed at the same elevation, thus leaving each pipe standing full of sewage. Stop planks should be provided at the outlet in order that the pipes may be pumped out for cleaning. The objection to this arrangement is that the larger pipes may operate at a velocity less than 2 feet per second, and they will be standing full of sewage which might become septic. However, as they will take nothing but the storm flow near the top of the sewer no difficulty should be encountered from sedimentation in them, and all are large enough for a man to enter for inspection or cleaning.

=63. Regulators.=—Regulators are commonly used to divert the direction of flow of sewage in order to prevent the overcharging of a sewer or to regulate the flow to a treatment plant. Sewer regulators are of two types, those with moving parts and those without moving parts. An example of the moving part type is shown in Fig. 41. In this type as the sewage rises the float closes the gate to the inlet sewer, thus preventing the entrance of sewage under head from the larger sewer. There are many variations in the details of float controlled regulators, but the principle of operation is similar in all. These regulators can be adjusted to fix the maximum rate of flow to a relief channel or sewage treatment plant, or during times of storm to cut off the outlet to the dry weather channel. Another form of the moving part type is shown in Fig. 42.[43] It has been used extensively by the Milwaukee Sewerage Commission. In its operation the dry weather flow is diverted by the dam into the intercepter. It passes under the movable gate on its way to the treatment plant. As the flow increases the dam is overtopped and flood waters are discharged down the storm channel. The movable gate is hung on a pivot placed below center. As the water rises in the intercepter, the pressure against the upper portion of the gate becomes greater than that against the lower portion, and the gate closes. An opening is left at the bottom to allow an amount of sewage equal to the dry weather flow to escape beneath the gate to prevent clogging or sedimentation in the intercepter channel.

Objections to all moving part regulators are their need of attention and liability to become clogged.

The overflow weir and the leaping weir have no moving parts and are used for the regulation of the flow in sewers. A leaping weir is formed by a gap in the invert of a sewer through which the dry weather flow will fall and over which a portion or all of the storm flow will leap. One form of leaping weir is shown in Fig. 43. An overflow weir is formed by an opening in the side of a sewer high enough to permit the discharge of excess flow into a relief channel. A weir at San Francisco is shown in Fig. 44. A series of tests were run on leaping weirs and overflow weirs in the hydraulic laboratory of the University of Illinois. The type of leaping weir tested was formed by the smooth spigot end of a standard vitrified sewer pipe. The overflow weirs were formed by a steel knife edge in the side of the pipe parallel to its axis as shown in Fig. 45. Tests were made in 18–inch and 24–inch pipes on various slopes from 0.018 to 0.005, for both leaping weirs and overflow weirs. The overflow weirs were varied in length from 16 inches to 42 inches and were placed at various heights from 25 per cent to 50 per cent of the diameter above the invert of the sewer. As the result of the observations the following formulas were developed. For the leaping weir the expressions for the coordinates of the curve of the surfaces of the falling stream, are:

For the outside surface x = 0.53_V_^⅔ + y^{4⁄7}

For the inside surface x = 0.30_V_^{4⁄7} + y^¾

in which x and y are the coordinates. The origin is in the upper surface of the stream vertically above the end of the invert of the pipe. The ordinate y is measured vertically downwards. V is the velocity of approach in feet per second. These expressions are applicable to any diameter of sewer up to 10 or 15 feet. They should not be used for depths of flow greater than about 14 inches; nor for slopes of more than 25 per 1,000; nor for velocities less than 1 foot per second nor more than 10 feet per second. The expression for the ordinate of the inside curve is not good for less than 6 inches nor more than 5 feet. The expression for the ordinate of the outside curve is limited to values between the origin and 5 feet below it.

The expression for the length of an overflow weir of the type shown in Fig. 45, necessary to discharge a given quantity, is in the form,

l = 2.3_Vd_ log _h__{1}⁄_h__{2}

in which l = the length of the weir in feet;

V = the velocity of approach in feet per second;

d = the diameter of the pipe in feet;

_h__{1} = the head of water on the upper end of the weir;

_h__{2} = the head of water on the lower end of the weir.

In the design of an overflow weir by this formula the height of the weir above the invert of the sewer and the flow over the weir should be determined arbitrarily. The height should be subtracted from the computed depth of water above the weir to determine the value of _h__{1}. The difference between the flow over the weir and the flow above the weir will represent the rate of flow in the sewer below the weir. The value of _h__{2} can then be computed as the difference in the depth of flow below the weir and the height of the weir above the invert. The value of V is computed from Kutter’s formula. The length of the weir is determined by substituting these values in the formula.

=64. Junctions.=—At the junction of two or more sewers the elevation of the inverts should be such that the normal flow lines are at the same elevation in all sewers. The sewers should approach the junction on a steep grade to prevent sewage backing up in one when the other is flowing full. The velocity of flow at the junction should not be decreased and turbulence should be avoided in order to prevent sedimentation and loss of head. The junction should be effected on smooth easy curves with radii at least five times the diameter of the sewer where possible. Curves with short radii cause backing up of sewage thus reducing the capacity of the sewers.

The terms bellmouth or trumpet arch are sometimes applied to the junction of sewers large enough to be entered by a man. In small sewers the Y branches and special junctions are manufactured so that the center lines of the pipes intersect, and the junctions of mains and laterals are made in manholes. In the construction of a bellmouth the arch is carried over all the sewers. A manhole should be constructed at these junctions as clogging frequently occurs there, due to swirling and back eddies which cannot be avoided.

=65. Outlets.=—The outlets to a sewerage system discharging into a swiftly running stream must be protected against wash and floating debris. In a stream or other body of water subject to wide variations in elevation the backing up of the sewage during high water should be avoided. Where tidal flats or low ground about the outlet may be alternately submerged and uncovered the discharge should always be into swiftly running water. In quiescent bodies of water such as lakes and harbors, and in rivers where the dilution is low, and in many other cases, the sewer outlet should be submerged.

Outlets are protected against wash and the impact of debris by the construction of deep foundations and heavy protecting walls. Although the construction of an outlet in a slow current or a back eddy would avoid danger from wash and debris, the discharge of the sewage into the most rapid current possible aids in the prevention of a local nuisance. A row of batter piles on the upstream or exposed side of the sewer is desirable, or it may be necessary to construct a break-water to prevent the wash of the current from dislodging the pipe. These break-waters are low dams of wood or broken stone, more or less loosely thrown together. The backing up of water into the sewer can be prevented by constructing the sewer above the outlet on a steep grade. Where this is not possible the use of tide gates will be helpful. A tide gate, one form of which is shown in Fig. 46, is a special form of check valve placed on the end of the sewer.

Sewer outlets are sometimes constructed on long trestles in order to reach deep or running water. Such a trestle is shown in Fig. 47. In Boston the outlet sewers are submerged under the harbor and discharge through outlets well out in the tidal currents. The sewage is discharged under pressure and the pumps are operated at some of the stations only at such times as the tidal currents will carry the sewage away from the harbor. It is not always necessary in a combined sewerage system to carry the storm flow to a distant submerged outlet. A double outlet can be constructed as shown in Fig. 48 in which the dry weather flow is carried to the channel in a submerged sewer and the storm flow is discharged on the bank.[44] Cast-iron pipe should be used for submerged outlets as the sewer is subject to disturbance by the currents, anchors, ice, and other causes.

=66. Foundations.=—Sewers constructed in firm dry soil require no special foundation to distribute the weight over the supporting medium. In soft materials the lower half of the sewer ring may be spread as shown in Fig. 22, and in rock the pressures on sewer pipes are evenly distributed by a cushion of sand. In wet ground such as quicksand, mud, swamp land, etc., a foundation must be constructed if the water cannot be drained off.

The permissible intensities of pressure on foundations in various classes of material allowed by the building codes in different cities are given in Table 25. These figures are based on the assumption that the material is restrained laterally, which is generally the condition in sewer construction. In the softer materials it becomes necessary to spread the foundations not only to reduce the intensity of pressure, but also to care for the thrust of the sewer arch. The arch thrust may be found by one of the methods of arch analysis, and the haunches spread to care for this, or the sewer invert may be transversally reinforced to assist in caring for this action. Some sewer sections in hard and soft material are shown in Fig. 22 and 23.

TABLE 25

ALLOWABLE BEARING VALUE ON SOILS IN VARIOUS CITIES

From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906

─────────────────────────┬───────────────────────────────────────────── Quicksand and alluvial │½ to 1 ton per sq. ft. for Providence, R. I., soil │ ½ ton per sq. ft. for 6 cities ─────────────────────────┼───────────────────────────────────────────── Soft clay │1 ton per sq. ft. for 27 cities, ¾ ton per │ sq. ft. for New Orleans, 2 to 3 tons for │ Providence, R. I. ─────────────────────────┼───────────────────────────────────────────── Moderately dry clay and │2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½ fine sand, clean and │ tons for Louisville, 2 to 4 tons for dry │ Providence, 3 tons for Grand Rapids and Los │ Angeles ─────────────────────────┼───────────────────────────────────────────── Clay and sand in │2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3 alternate layers │ to 5 tons for Providence ─────────────────────────┼───────────────────────────────────────────── Firm and dry loam or │3 tons for 24 cities, 2½ tons for 2 cities, 2 clay, or hard dry clay │ to 3 tons for Atlanta, 3½ tons for or fine sand │ Philadelphia, 4 tons for 6 cities ─────────────────────────┼───────────────────────────────────────────── Compact coarse sand, │4 tons for 25 cities, 3½ tons for Buffalo, 3 stiff gravel or natural│ to 4 tons for Atlanta, 4 to 5 tons for earth │ Cincinnati, 5 tons for Denver, 4 to 6 tons │ for 3 cities, 6 tons for Rochester, N. Y. ─────────────────────────┼───────────────────────────────────────────── Coarse gravel, stratified│6 tons for 3 cities, 5 tons for 2 cities, 8 stone and clay, or rock│ tons for 1 city inferior to best brick │ masonry │ ─────────────────────────┼───────────────────────────────────────────── Gravel and sand well │8 tons for 5 cities, 6 tons for 2 cities, 8 cemented │ to 10 tons for 1 city ─────────────────────────┼───────────────────────────────────────────── Good hard pan or hard │10 tons for 4 cities, 6 tons for 2 cities, 8 shale │ tons for 1 city ─────────────────────────┼───────────────────────────────────────────── Good hard pan or hard │8 tons for 1 city, 10 to 15 tons for 1 city, shale unexposed to air,│ 12 to 18 tons for 1 city frost or water │ ─────────────────────────┼───────────────────────────────────────────── Very hard native bed rock│20 tons for 5 cities, 15 tons for 1 city, 10 │ tons for 1 city, 25 to 50 tons for 1 city ─────────────────────────┼───────────────────────────────────────────── Rock under caisson │24 tons for Baltimore, 25 tons for Cleveland ─────────────────────────┴─────────────────────────────────────────────

On soft foundations such as swamps or for outfalls on the muck bottom of rivers the sewer may be carried on a platform. For small sewers 2–inch planks, 2 to 4 feet longer than the diameter of the pipe are laid across the trench, and the sewer rests directly upon them. For large sewers imposing a heavy concentrated load, a pile foundation should be constructed. The foundation may consist of piles alone, pile bents, or a wooden platform supported on pile bents. The load which can be carried by a pile is determined with accuracy only by driving a test pile and placing a load on it. Where piles do not penetrate to a firm stratum the load they will support can be determined by any one of the various formulas, either theoretical or empirical, which have been devised. Probably the best known of these formulas are the so-called Engineering News formulas one of which is:

P = 2_Wh_⁄(S + 1) for a pile driven by a drop hammer,

in which P = the safe load on the pile in pounds. The factor of safety is 6;

W = the weight of the hammer in pounds;

h = the fall of the hammer in feet;

S = the penetration of the pile in inches at the last driving blow. The blow is assumed to be driven on sound wood without rebound of the hammer.

Reference should be made to engineering handbooks for other forms of pile formulas. The accuracy of all of these formulas is not of a high degree.

The piles are driven at about 2 to 4 feet centers, to a depth of from 8 to 20 feet, unless hard bottom is struck at a lesser depth. The butt diameter of the piles used for the smallest sewers is about 6 to 8 inches. If bents are used, 2 or 3 piles are driven in a row across the line of the sewer and are capped with a timber. For brick, block, pipe, and some concrete sewers, a wooden platform must be constructed between the pile bents for the support of the sewer.

=67. Underdrains.=—The construction of special foundations can sometimes be avoided by laying drains under the sewers, thus removing the water held in the soil. The laying of the underdrains facilitates the construction of the sewer and reduces the amount of ground water entering the sewer. The underdrains usually consist of 6– or 8–inch vitrified tile laid with open joints from 1 to 2 feet below the bottom of the sewer as shown in Fig. 1. If the sewers are large, parallel lines of drains may be laid beneath them. An observation hole should be constructed from the bottom of the manhole to each underdrain. This hole usually consists of a 6– or 8–inch pipe, embedded in concrete, connected to the drain and open at the top. It is too small to permit effective cleaning of the underdrains, which are usually neglected after construction, and which as a result clog and cease to function. Since the principal period of usefulness of the drains is during construction, their stoppage after the work is completed is not serious. The hollow tile used in vitrified block sewers serve as underdrains after construction, but are of little or no assistance to the draining of the trench during construction.

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VII. PUMPS AND PUMPING STATIONS

=68. Need.=—In the design of a sewerage system it is occasionally necessary to concentrate the sewage of a low-lying district at some convenient point from which it must be lifted by pumps. In the construction of sewers in flat topography the grade required to cause proper velocity of sewage flow necessitates deep excavation. It is sometimes less expensive to raise the sewage by pumping than to continue the construction of the sewers with deep excavation.

In the operation of a sewage-treatment plant a certain amount of head is necessary. If the sewage is delivered to the plant at a depth too great to make possible the utilization of gravity for the required head, pumps must be installed to lift the sewage. In the construction of large office buildings, business blocks, etc., the sub-basements are frequently constructed below the sewer level. The sewage and other drainage from the low portion of the building must therefore be removed by pumping. Because pumps are often an essential part of a sewerage system, their details should be understood by the engineer who must write the specifications under which they are purchased and installed.

=69. Reliability.=—If the only outlet from a sewerage system is through a pumping station, the inability of the pumps to handle all of the sewage delivered to them may so back up the sewage as to flood streets and basements, endangering lives and health and destroying property. Such an occurrence should be guarded against by providing sufficient pumping capacity and machinery of the greatest reliability.

=70. Equipment.=—The equipment of a sewage pumping station, in addition to pumping machinery, may include a grit chamber, a screen, and a receiving well. The grit chamber and screen are necessary to protect the pumps from wear and clogging. Grit chambers are not necessary in sewage devoid of gritty matter, such as the average domestic sewage, unless reciprocating pumps are used. Sufficient gritty matter is found in average domestic sewage to have an undesirable effect on reciprocating pumps. Receiving wells are used in small pumping stations where the capacity of the pumps is greater than the average rate of sewage flow. The pumps are then operated intermittently, the pumps standing idle during the time that the receiving well is filling.

Except for a few types of pumps of which the valve openings are unsuitable, any machine capable of pumping water is capable of pumping sewage which has been properly screened. The principles of sewage pumps are then similar to principles of water pumps. The conditions under which these principles are applied differ but slightly in the character of the liquid, and a smaller range of discharge pressures. Pumps with large passages, discharging under low heads are more commonly found among sewage pumps.

=71. The Building.=—The pumping station should, if possible, be of pleasing design and should be surrounded by attractive grounds. The Calumet Sewage Pumping Station in Chicago is shown in Fig. 49. Its architecture is pleasing particularly in contrast with its location and immediate surroundings. Such structures tend to remove the popular prejudice from sewerage and to arouse interest in sewerage questions. Service to the public is of value. It can be rendered more easily by arousing public interest and cooperation by the erection of attractive structures, than by feeding popular prejudice by the construction of miserable eyesores.

=72. Capacity of Pumps.=—The capacity of the pumping equipment should be sufficient to care for the maximum quantity of sewage delivered to it, with the largest pumping unit shut down, and the provision of such additional capacity as, in the opinion of the designer, will provide the necessary factor of safety.

Pumps can usually be operated under more or less overload. Power pumps and centrifugal pumps driven by constant speed electric motors have no overload capacity. A power pump or a centrifugal pump may be overloaded up to the maximum horse-power of any variable speed motor or steam engine driving it, provided the pump has been designed to permit it. Direct-acting steam pumps which are designed for proper piston speed and valve action at normal loads, can carry a 50 per cent overload for short periods, although the strain on the pump is great. They will carry a 20 to 25 per cent overload for about eight hours with less vibration and strain. The use of pumps capable of working at an appreciable overload is somewhat of an additional factor of safety, but the overload factor should not be taken into consideration in determining the capacity of the pumping equipment.

The load on a pumping station consists of the quantity of sewage to be pumped and the height it must be lifted. Variations in the quantity are discussed in Chapter III. The head against which the pumps must operate fluctuates with the level in the tributary sewer or pump well, and in the discharge conduit. For a free discharge or discharge into a short force main the greater the rate of sewage flow the smaller the lift, as the depth of flow in the tributary sewer increases more rapidly than that in the discharge conduit. If the discharge is into a large body of water or under other conditions where the discharge head is approximately constant, the fluctuations in total head should not exceed the diameter of the tributary sewer. Such fluctuations are of minor importance in the operation of direct-acting steam pumps, but may be of great importance in the operation of centrifugal pumps, as is brought out in Art. 78.

=73. Capacity of Receiving Well.=—The use of receiving wells is restricted to small installations which require, in addition to the standby unit, only one pump, the capacity of which is equal to the maximum rate of sewage flow. When the receiving well has been pumped dry the pump stops, allowing the well to fill again. Although the use of a large receiving well, or an equalizing reservoir, for a large pumping station would permit the operation of the pumps under more economical conditions, the storage of sewage for the length of time required would not be feasible. The sewage would probably become septic, creating odors and corroding the pumps. The extra cost of the reservoir might not compensate for the saving in the capacity and operation of the pumps.

The capacity of the receiving well should be so designed that the pump when operating will be working at its maximum capacity, and the period of rest during conditions of average rate of flow should be in the neighborhood of 15 to 20 minutes. For example, assume an average rate of flow of 2 cubic feet per second, with a maximum rate of double this amount. The pump should have a capacity of 4 cubic feet per second, and if the receiving well is to be filled in 15 minutes by the average rate of sewage flow its capacity should be 15 × 5 × 60 × 7.5 or 14,000 gallons. Under these circumstances the pump will operate 15 minutes and rest 15 minutes, during average conditions of flow.

=74. Types of Pumping Machinery.=—The two principal types of pumping machines for lifting sewage are centrifugal pumps and reciprocating pumps. A centrifugal pump is, in general, any pump which raises a liquid by the centrifugal force created by a wheel, called the impeller, revolving in a tight casing, as shown in Fig. 50. A reciprocating pump is one in which there is a periodic reversal of motion of the parts of the pump.

Centrifugal pumps are sometimes classified as volute pumps and turbine pumps. A volute pump is a centrifugal pump with a spiral casing into which the water is discharged from the impeller with the same velocity at all points around the circumference, as shown in Fig. 51. A turbine pump is a centrifugal pump in which the water is discharged from the impeller through guide passages into a collecting chamber, in such a manner as to prevent loss of energy in changing from kinetic head to pressure head. A turbine pump is shown in section in Fig. 51. Centrifugal pumps are sometimes classified as single stage and multi-stage. A centrifugal pump from which the water is discharged at the pressure created by a single impeller is called a single-stage pump. If the water in the pump is discharged from one impeller into the suction of another impeller the pump is known as a multi-stage pump. The number of impellers operating at different pressures determines the number of stages of the pump. A three-stage pump is shown in Fig. 52.

375 Lubricating ring.

380 Oil hole cap.

383 Oil drain tube.

404 Shaft sleeve lock nut.

440 Driving coupling.

441 Driven coupling.

443 Coupling check nut.

450 Coupling bolt.

451 Coupling bolt nut.

452 Coupling rubber.

453 Coupling rubber steel tube.

500 Pump case.

550 Bearing bracket cap.

551 Bearing.

552 Shaft.

553 Shaft sleeve, right hand thread.

PW Impeller.

554 Shaft sleeve, left hand thread.

555 Shaft stop collar, inner.

555–1 Shaft stop collar, outer.

556 Guide ring.

560 Packing gland.

563 Bearing.

567R Impeller protecting ring, right hand thread.

567L Impeller protecting ring, left hand thread.

583 Pump case protecting ring.

567 Labyrinth packing.

583 Labyrinth packing.

600 Pump case cover.

692 Impeller key.

815 Bearing bracket, outer.

815–1 Bearing bracket, inner.

Reciprocating pumps are generally driven by steam and are either direct-acting, or of the crank-and-fly-wheel type. Power pumps are reciprocating machines which may be driven by any form of motor, to which they are connected by belt, chain or shaft. A Deming triplex power pump is shown in Fig. 53. Power pumps can be used only where the character of the sewage will not clog the valves nor corrode the pump. A direct-acting steam pump is one in which the steam and water cylinders are in the same straight line and the steam is used at full boiler pressure throughout the full length of the stroke. The crank-and-fly-wheel type of pumping engine permits the use of steam expansively during a part of the stroke, the energy stored in the flywheel carrying the machine through the remainder of the stroke. Reciprocating pumps are sometimes classified as plunger pumps and piston pumps. In the action of a plunger pump the water is expelled from the water cylinder, by the action of a plunger which only partly fills the water cylinder, as shown in Figs. 54 and 55. In a piston pump the water is expelled from the water cylinder by the action of a piston which completely fills the water cylinder, as shown in Fig. 63, which illustrates a direct-acting piston pump.

Plungers are better than pistons for pumping sewage as the wear between the pistons and the inside face of the cylinder soon reduces the efficiency of the pump. Outside packed plungers are better than the inside packed type because the packing can be taken up without stopping the pump and the leakage from the pump is visible at all times. Outside packed pumps are more expensive in first cost, but are easier to maintain and have a longer life than piston pumps.

In selecting a pump to perform certain work the size of the water cylinder and the speed of the travel of the piston should be investigated to insure proper capacity. The average linear travel of the piston for slow speed pumps is estimated at about 100 feet per minute, dependent on the length of stroke and the valve area. For short strokes and small valve areas the speed may be as low as 40 feet per minute, and for long stroke fire pumps with large valves the piston can be operated at a speed of 200 feet per minute.[45] Vertical, triple-expansion, crank-and-fly-wheel, outside packed plunger pumps with flap valves can be operated at speeds of 200 feet per minute when lifting sewage, and when equipped with mechanically operated valves and lifting water they can be run at speeds of 400 to 500 feet per minute. The speed of travel multiplied by the volume of piston or plunger displacement, with proper allowance for slippage, will give the capacity of the pump. The slippage allowance may be from 3 to 8 per cent for the best pumps, and for pumps in poor conditions it may be a high as 30 to 40 per cent.

There are two types of ejector pumps used for lifting sewage. One of these depends on the vacuum created by the velocity of a stream of water or steam passing through a small nozzle. The operation of this pump is described in Art. 139 and it is illustrated in Fig. 97. The other type of ejector pump is known as the compressed-air ejector. It is operated by means of compressed air which is turned into a receptacle containing sewage. The details of this type are explained in Art. 83 and are illustrated in Fig. 68.

=75. Sizes and Description of Pumps.=—The size of a centrifugal pump is expressed as the diameter of the discharge pipe in inches. It has nothing to do with the head for which the pump is suited. On the assumption of a velocity of flow of 10 feet per second through the discharge pipe the capacity of the pump can be approximated.

The size of a reciprocating pump involves the expression of the diameters of the steam cylinders, the water cylinder, and the length of the stroke in inches, in the order named, beginning with the steam cylinder with the highest pressure. A complete description of a steam pumping engine might be; compound, duplex, horizontal, condensing, crank-and-fly-wheel, outside-center-packed, 12″ × 24″ × 18″ × 24″ pump. The word compound means that there are a high-pressure and a low-pressure steam cylinder; the word duplex means that there are two of each of these cylinders; the word horizontal means that the axes of these cylinders are in a horizontal plane; the word condensing means that the steam is discharged from the low-pressure cylinders into a condenser; the name crank-and-fly-wheel is self-explanatory; the name outside-center-packed means that the water cylinder is divided into two portions between which the plunger is exposed to the atmosphere, and that the packing rings are on the outside of the two portions of the cylinder as shown in Fig. 55; the figures shown mean that the high-pressure steam cylinder is 12 inches in diameter, the low-pressure 24 inches in diameter, the water cylinder is 18 inches in diameter, and the stroke of the pump is 24 inches.

=76. Definitions of Duty and Efficiency.=—The duty of a pump is the number of foot-pounds of work done by the pump per million B.T.U., per thousand pounds of steam, or per hundred pounds of coal, consumed in performing the work. These units are only approximately equal as 100 pounds of coal or 1,000 pounds of steam do not always contain the same number of B.T.U. and may only approximately equal 1,000,000 B.T.U.

Since 1,000,000 B.T.U. are equal to 778,000,000 foot-pounds of work, a pump with a duty of 778,000,000 will have an efficiency of 100 per cent. The efficiency of a pump is therefore its duty based on B.T.U. divided by 778,000,000. The efficiencies or duties of various types of pumps are given in Table 26.[46]

TABLE 26

APPROXIMATE DUTIES OF STEAM PUMPS

Small duplex, non-condensing 10,000,000 Large duplex, non-condensing 25,000,000 Small simple, flywheel, condensing 50,000,000 Large simple, flywheel, condensing 65,000,000 Small compound, flywheel, condensing 65,000,000 Large compound, flywheel, condensing 120,000,000 Small triple, flywheel, condensing 150,000,000 Large triple, flywheel, condensing 165,000,000

=77. Details of Centrifugal Pumps.=—A section of a centrifugal pump with the names of the parts marked thereon is shown in Fig. 50. Among the important parts which require the attention of the purchaser are: the impeller (PW), the impeller packing rings (567 R & L), the bearings (551, 563), the thrust bearings (555–1), the shaft (552), and the shaft coupling (440).

The impeller should be of bronze, gun metal, or other alloy, because there is no rusting or roughening of the surface, and the efficiency does not fall with age. Normal fresh sewage is not corrosive, but septic sewage and sludge are usually so corrosive that iron parts cannot be used with success in contact with them. The impeller should be machined and polished to reduce the friction with the liquid. Impellers are made either closed or open, i.e., either with or without plates on the sides connecting the blades to avoid the friction of the liquid against the side of the casing. The closed type of impeller is shown in Fig. 50. Closed impellers are slightly more expensive, but generally give better service and higher efficiencies than the open type. Single impeller pumps should have an inlet on each side of the impeller to aid in balancing the machine, unless the plane of the impeller is to be horizontal when operating. Multi-impeller pumps usually have single inlet openings for each impeller. Vibration in the pump is sometimes caused by an unbalanced impeller. The moving parts may be balanced at one speed and unbalanced at another. To determine if the moving parts are balanced the pump should be run free at different speeds and the amount of vibration observed. If the impeller is removed from the pump its balance when at rest can be studied by resting it on horizontal knife edges. If there is a tendency to rotate in any direction from any position the impeller is not perfectly balanced.

Packing rings are used to prevent the escape of water from the discharge chamber back into the suction chamber. These rings should be made of the same material as the impeller. Labyrinth type rings, as shown in Fig. 50, are sometimes used as the long tortuous passages are efficient in preventing leakage.

The bearings must be carefully made because of the high speed of the pump. They are usually made of cast iron with babbitt lining. They should be placed on the ends of the shaft on the outside of the pump casing, as shown in Fig. 50, and should be split horizontally so as to be easily renewed. Exterior bearings are oil lubricated by means of ring or chain oilers with deep oil wells. Where interior bearings are necessary, because of the length of the shaft, they should be made of hard brass and should be water lubricated.

Thrust bearings or thrust balancing devices are used to take up the end thrust which occurs in even the best designed pumps. To overcome this pumps are designed with double suction, opposed impellers, or two pumps with their impellers opposed may be placed on the same shaft. Due to inequalities in wear, workmanship or other conditions, end thrust will occur and must be cared for. Various types of thrust bearings are in successful use, such as: the piston, ball, roller or marine types. The marine type thrust bearing is shown in Fig. 56. The piston type of hydraulic balancing device is shown in Fig. 57. In the figure A represents the impeller, and B a piston fixed to the shaft and revolving with it. There is a passage for water through the openings (1), (2), and (3) leading from the impeller chamber to the atmosphere or to the suction of the pump. If the impeller tends to move to the right opening (1) is closed resulting in pressure on the right of the impeller forcing it to the left. If the impeller moves to the left (1) is opened thus transmitting pressure to the piston B forcing the impeller to the right. The flange C is not essential, but is advantageous in pumps handling gritty matter. As the channel (2) wears larger the pressure against the piston decreases allowing it to move to the left. This partially closes (3) building up the pressure again.

Flexible shaft couplings should be used if the shaft of the driving motor and the pump are in the same line, as direct alignment is difficult to obtain or to maintain. Where connected to steam turbines, reduction gearing and rigid couplings are usually used on sewage pumps to obtain slow speed and permit large passages. Flexible couplings are of various types, one of which is shown in Fig. 50. A rigid coupling would be formed by bolting the flanges firmly together. Shaft couplings are usually not necessary where the pump is driven by belt connection to the engine or motor, or where the pump and pulley rest on only two bearings.

The stuffing box shown in Fig. 50 is packed loosely with two layers of hemp between which is a lantern gland, in order to permit a small amount of leakage. A drip box is placed below this gland to catch the leakage and return it to the pump. The leakage is permitted as it aids in lubrication and the tightening of the gland will cause binding of the shaft. The gland on the suction side of the pump should be connected by a small pipe to the discharge chamber in order to keep a constant supply of water for lubrication and to prevent the entrance of air to the suction end of the pump.

=78. Centrifugal Pump Characteristics.=—The capacity of a centrifugal pump is fixed by the size and type of its impeller and by the speed of revolution. Roughly, the capacity of a pump, for maximum efficiency, varies directly as the speed of revolution, the discharge pressure varies as the square of the speed, and the power varies as the cube of the speed. These relations are found not to hold exactly in tests because of internal hydraulic friction in the pump.

The characteristic curves for a centrifugal pump, or the so-called pump characteristics, are represented graphically by the relation between quantity and efficiency, quantity and power necessary to drive, and quantity and head, all at the same speed. The quantities are plotted as abscissas in every case. The curve whose ordinates are head and whose abscissas are quantities is known as “the characteristic.” The curve showing the relation between quantities and speeds is sometimes included among the characteristics. Characteristics of pumps with different styles of impellers are shown in Fig. 58. Fig. 59 shows the characteristics of a pump run at different speeds, the efficiencies at these speeds when pumping at different rates, and the maximum efficiency at different speeds. It is to be noted that the information given in this figure is more extensive than that in Fig. 58. The operating conditions under any head, rate of discharge, and speed are given. The curves of constant speed are parallel, and their distances apart vary as the square of the speed. The line of maximum efficiency is approximately a parabola.

A study of the characteristics of any particular pump should be made with a view to its selection for the load and conditions under which it is to be used. Among the important things to be considered in the selection of a centrifugal pump for the expected conditions of load are: the capacity required, the maximum and minimum total head to be pumped against, the maximum variations in suction and discharge heads, and the nature of the drive. For example, the pump, whose characteristics are shown in Fig. 59, should be operated at about 800 revolutions per minute. Under total heads between 40 and 50 feet, the discharge for the best efficiency will vary between 600 and 670 gallons per minute.

The efficiencies of centrifugal pumps increase with their capacities as is shown approximately in Fig. 60.

=79. Setting of Centrifugal Pumps.=—In setting a centrifugal pump, care should be taken to provide a firm foundation to hold the shafts of the pump and the electric motor or the reduction gearing in good alignment, or to prevent the pump from being displaced by the pull of a belt. It is desirable that the foundation be level. Centrifugal pumps should be set submerged for small pumping stations automatically controlled. Sludge pumps must be set submerged as otherwise they will not prime successfully. Provision should be made by which the pump can be lifted from the sewage, or sludge, for inspection and repair. In many cases the pump can be made self-priming by setting it in a dry, water-tight vault below the low level of sewage flow. Where possible it is desirable not to set the pump submerged as it will receive better care when easily accessible.

The suction pipe should be free from vertical bends where air might collect and should be straight for at least 18 to 24 inches from the pump casing. An elbow on the suction pipe, attached directly to the casing of the pump gives a lower efficiency than a suction pipe with a short straight run. Centrifugal pumps will operate with as high a suction lift as reciprocating pumps, but at the start they must be primed and some provision must be made for priming them. The suction pipe should be equipped with foot valves to hold the priming, or some method may be provided for exhausting the air from the suction pipe. The foot valves should be so installed as to form no appreciable obstruction to the flow of water. They should have an area of opening at least 50 per cent greater than the cross-section of the suction pipe. A strainer on the suction pipe is undesirable as it becomes clogged and is usually in an inaccessible position for cleaning. A screen should be placed at the entrance to the suction well to prevent the entrance of objects that are likely to clog the pump. A gate-valve and a check-valve should be provided on the discharge pipe, the former to assist in controlling the rate of discharge and the latter to prevent back flow into the pump when it is not operating.

Centrifugal pumps are well adapted to service in either large or small units. An installation in a manhole at Park Point, Duluth, is shown in Fig. 61. This station is controlled by an automatic electric device which is operated by a float in the suction pit. Such automatic control is an added advantage of the use of electrically driven centrifugal pumps. The Calumet Pumping Station in Chicago, shown in Fig. 49, has a capacity of approximately 1,000 cubic feet per second. The simplicity of the layout of this station is shown in Fig. 62.

=80. Steam Pumps and Pumping Engines.=—The direct-acting steam pump, one type of which is shown in Fig. 63, is adapted to pumping sewage the character of which will not corrode or clog the valves. In this form of pump it is necessary to utilize the steam at full pressure throughout the entire length of the stroke, which results in high steam consumption. A flywheel permits the use of steam expansively during a part of the stroke, thus increasing the economy of operation. Other devices used for the same purpose are known as compensators. They are not in general use.

Steam engines are classified in many different ways, for example; according to the type of valve gear, as, plain slide valve, Corliss, Lentz, etc.; or according to the number of steam expansions, as, simple, compound, triple-expansion, etc.; or according to the efficiency of the machine as low duty or high duty; or as

STEAM END

2 Steam cylinder and housing combined.

8 Steam piston head.

9 Steam piston follower.

10 Steam piston inside ring.

11 Steam piston outside ring (2).

12 Steam cylinder head.

14 Steam chest.

16 Steam chest cover.

17 Steam slide valve.

18 Steam valve rod.

20 Steam valve rod, pin and nut.

22 Steam valve rod, collar and set screw.

23 Steam valve rod, stuffing box.

24 Steam valve rod, stuffing box, nut and gland.

38 Piston rod.

47 Piston rod stuffing box.

48 Piston rod, stuffing box, nut and gland.

49 Valve gear stand.

51 Long valve crank and shaft.

52 Short valve crank and shaft.

PUMP END

115 Pump body.

127 Brass liner.

129 Water piston head.

130 Water piston follower.

137 Cylinder head.

139 Valve plate.

140 Cap.

152 Suction flange.

161 Discharge flange.

162 Valve seat, suction or discharge.

163 Valve, suction or discharge.

164 Suction valve spring.

167 Discharge valve spring.

168 Valve plate, suction or discharge.

169 Valve stem, suction or discharge.

STEAM END

55 Crank pin.

56 Valve rod link.

61 Long rocker arm.

62 Short rocker arm.

63 Rocker arm wiper.

69 Cross head.

condensing or non-condensing, etc. Throttling engines or automatic engines refer to the method of control of the steam by the governor. In throttling engines the governor controls the amount of opening of the throttle valve, in automatic engines the governor controls the position of the cut-off.

The simple slide valve, low-duty, non-condensing, throttling engine, is the lowest in first cost and the most expensive in the consumption of fuel. The triple-expansion Corliss, or the non-releasing Corliss, high-duty pumping engine is the most expensive in first cost but consumes less steam for the power delivered than any other form of reciprocating engine. For pumps of very small capacity the cost of fuel is not so important an item as the first cost of the machine. For this reason and because of the lower cost of attendance low-duty pumps are more frequently found in small pumping stations.

TABLE 27

WATER RATES OF PRIME MOVERS AT FULL AND PART LOADS

───────────────────────────────┬──────┬─────────────────────────┬────── Type of Engine │ │ │Boiler │Power,│ │Press. │ K.W. │ Per Cent of Full Load │ Lbs. ───────────────────────────────┼──────┼────┬────┬────┬─────┬────┼────── │ │ 25 │ 50 │ 75 │ 100 │125 │ ───────────────────────────────┼──────┼────┼────┼────┼─────┼────┼────── Single cylinder, high speed, │ │ │ │ │ │ │100 to non-condensing │ 25│ 33│ 27│26.3│ 27.0│27.5│ 150 │ 250│ 42│37.5│ 35│ 34.0│34.0│ │ │ │ │ │ │ │ Automatic, flat four valve, │ │ │ │ │ │ │100 to high speed │ 150│ │ 32│ 30│ 26.5│29.0│ 125 │ 250│ │ 33│ 31│ 28│30.0│ │ │ │ │ │ │ │ Tandem compound condensing, │ │ │ │ │ │ │100 to high speed │ 125│ │ 23│ 19│ 17│ 18│ 150 │ │ │ 25│ 20│ 19.5│ 21│ │ │ │ │ │ │ │ Cross compound, condensing, │ │ │ │ │ │ │ 125 high speed │ │ 30│ 26│ 24│ 23│23.5│ │ │ │ │ │ │ │ Cross compound, non-condensing,│ │ │ │ │ │ │ 125 high speed │ │ 39│ 31│ 27│ 26│27.5│ │ │ │ │ │ │ │ Single cylinder Corliss, │ │ │ │ │ │ │ 100 condensing │ 120│23.7│20.4│ 19│ 18.5│19.0│ │ 500│26.3│22.8│21.3│ 20.8│21.3│ 125 │ │ │ │ │ │ │ Compound Corliss, condensing │ │16.5│ 14│12.5│ 12.1│12.5│ 100 │ │22.2│ 19│17.0│ 16.5│17.0│ 150 │ │ │ │ │ │ │ Single cylinder, rotary four │ │ │ │ │ │ │ 100 valve, non-condensing │ 75│26.2│22.3│21.3│ 21.6│22.8│ │ 400│35.0│27.2│26.4│ 26.0│26.8│ 180 │ │ │ │ │ │ │ Rotary four valve, tandem │ │ │ │ │ │ │ 100 compound non-condensing │ 125│32.0│22.0│ 20│18.25│18.5│ │ 600│40.0│28.3│23.2│ 22.5│22.7│ 150 │ │ │ │ │ │ │ Cross compound, non-condensing │ │ │ │ │ │ │ 100 rotary four valve │ 125│ 25│ 21│19.1│ 18.5│19.0│ │ 600│39.4│ 28│22.3│ 20.6│20.7│ 150 │ │ │ │ │ │ │ Single cylinder, poppett valve,│ │ │ │ │ │ │ 100 non-condensing │ 120│22.7│20.5│19.7│ 19.1│20.1│ │ 600│28.5│26.0│25.0│ 24.3│25.5│ 150 │ │ │ │ │ │ │ Single cylinder, poppett valve,│ │ │ │ │ │ │ 100 condensing │ 120│18.5│16.7│16.1│ 15.6│16.4│ │ 600│24.6│22.3│21.4│ 20.8│21.9│ 150 │ │ │ │ │ │ │ Compound condensing, poppett │ │ │ │ │ │ │ 100 valve │ 200│14.2│13.0│12.5│ 12.2│12.9│ │ 1200│18.4│16.9│16.3│ 15.9│16.8│ 150 │ │ │ │ │ │ │ Uniflow │ 125│14.6│13.7│13.4│ 13.4│13.3│ 150 │ 600│15.0│14.3│13.7│ 13.5│14.0│ │ │ │ │ │ │ │ Steam turbines, condensing, │ │ │ │ │ │ │ 125 Allis-Chalmers │ 300│ │ 24│ 17│ 160│16.5│ │ 2000│ │31.9│26.3│ 23.8│23.0│ 175 │ │ │ │ │ │ │ Steam turbines, condensing, │ │ │ │ │ │ │ 125 Westinghouse │ 300│ │13.7│12.8│ 12.2│12.6│ │ 2000│ │18.2│16.9│ 16.2│16.8│ 175 │ │ │ │ │ │ │ Steam turbines, high pressure, │ │ │ │ │ │ │ non-con., 12″ to 36″ wheel, │4 to 8│ │ │ │ │ │ 1000 to 3600 R.P.M. │stages│ │ │ │ 28 5│ │ │ │ │ │ │116.5│ │ │ │ │ │ │ │ │ Ditto. Condensing, 26–inch │ │ │ │ │ 17 3│ │ │ │ │ │ │112.0│ │ │ │ │ │ │ │ │ ───────────────────────────────┴──────┴────┴────┴────┴─────┴────┴──────

The steam consumption per indicated horse-power, better known as the water rate of the engine, for various types of engines at full and at part load is shown in Fig. 64. The steam consumption of other types at full load is shown in Table 27. The indicated horse-power (I.H.P.) of a steam engine is the product of the mean effective pressure (M.E.P.), the area of the steam pistons, the length of the stroke, and the number of strokes per unit of time. A common form of this expression is,

I.H.P = PLAN⁄33,000,

in which P = the M.E.P. in pounds per square inch;

L = the length of the stroke in inches;

A = the sum of the areas of the pistons in square inches;

N = the number of revolutions per minute.

The I.H.P. multiplied by the mechanical efficiency of the machine will give the brake or water horse-power, that is, the horse-power delivered by the machine. The product of the M.E.P., the sum of the areas of the steam pistons and the mechanical efficiency of the machine, should equal the product of the total head of water pumped against expressed in pounds per square inch and the sum of the areas of the water pistons or plungers. The M.E.P. is determined from indicator cards taken from the steam cylinders during operation. These cards show the steam pressure on the head and crank ends of each cylinder at all points during the stroke.

=81. Steam Turbines.=—Among the advantages in the use of steam turbines as compared with reciprocating steam engines for driving centrifugal pumps are their simplicity of operation, the small floor space needed, their freedom from vibration requiring a relatively light foundation, and their ability to operate successfully and economically either condensing or non-condensing under varying steam pressure. They can be operated with steam at atmospheric or low pressure, thus taking the exhaust from other engines. The greatest economy of operation for the turbine alone will be obtained by operating with high pressure, superheated steam and with a vacuum of 28 inches. In large units the economy of operation of steam turbines is equal to that of the best type of reciprocating engines. In order to develop the highest economy turbines are operated at speeds from about 3,600 to 10,000 r.p.m. or greater, the smaller turbines operating at the higher speeds. As these speeds are usually too great for the operation of centrifugal pumps for lifting sewage, reduction gears must be introduced between the turbine and the pump. Although the best form of spiral-cut reduction gears may obtain efficiencies of 95 to 98 per cent, or even higher, their use, particularly in small units, is an undesirable feature of the steam turbine for driving pumps.

The steam consumption of DeLaval turbines of different powers, and the steam consumption of a 450 horse-power DeLaval turbine at different loads are shown in Fig. 64. Some steam consumptions of other turbines are recorded in Table 27. It is to be noted that the steam consumption of the 450 horse-power turbine at part loads is not markedly greater than that at full loads. This is an advantage of steam turbines as compared with reciprocating engines. The steam consumption of any turbine is dependent on the conditions of operation and is lower the higher the vacuum into which the exhaust takes place.

There are two types of turbines in general use, the single stage or impulse machines, and the compound or reaction type. The DeLaval is a well-known make of the single stage or impulse type. The principle of its operation is indicated in Fig. 65, which is the trade mark of the DeLaval Steam Turbine Co. The energy of the steam is transmitted to the wheel due to the high velocity of the steam impinging against the vanes. In the compound or reaction type of machine the steam expands from one stage to the next imparting its energy to the wheel by virtue of its expansion in the passages of the turbine. For this reason the single-stage or impulse type is operated at higher speeds than the compound or reaction machines.

=82. Steam Boilers.=—Among the important points to be considered in the selection of a steam boiler for a sewage pumping station are: the necessary power; the quality of the feed water; the available floor space; the steam pressure to be carried; and the quality and character of the fuel. Tubular boilers of the type shown in Fig. 66, are lower in first cost than other types of boilers. They are not ordinarily built in units larger than 250 to 300 horse-power and where more power is desired a number of units must be used. They are objectionable because of the relatively large floor space required, and because of their relatively poor economy of operation. The efficiencies of water-tube boilers of different types are given in Table 28. Large power units of the water-tube type, as shown in Fig. 67, although more expensive in first cost, require less floor space. Almost any desired steam pressure can be obtained from either type but water-tube boilers are more commonly used for high pressures. The grate or stoker can be arranged to burn almost any kind of fuel under either water-tube or fire-tube boilers. The use of poor quality of water in water-tube boilers is undesirable as the tubes are more likely to become clogged than the larger passages of the fire-tube boilers. If necessary, a feed-water purification plant should be installed, as it is usually cheaper to take the impurities out of the water than to take the scale out of the boiler.

Not less than two boiler units should be used in any power station, regardless of the demands for power, and if the feed water is bad, three or even four units should be provided, as two units may be down at any time. An appreciable factor of safety is provided by the ability of a boiler to be operated at 30 to 50 per cent overload, if sufficient draft is available, but with resulting reduction in the economy of operation. The number of units provided should be such that the maximum load on the pumping station can be carried with at least one in every 6 units or less, out of service for repairs or other cause.

TABLE 28

EFFICIENCIES OF STEAM BOILERS

From Marks’ Mechanical Engineer’s Handbook ────────┬───────────┬──────────┬──────┬────────┬──────┬──────┬────────── Type │Horse-power│ Furnace │ │ │ │Evap. │ │ │ │ │ │ │ from │ │ │ │ │ │ │and at│ │ │ │ │ │B.T.U.│ 212° │ Combined │ │ │ Sq. │Per Cent│ per │ per │Efficiency │ │ │ Ft. │of Rated│ Lb. │ Lb. │of Boiler │ │ │Grate │Capacity│ Dry │ Dry │ and │ │ │ Area │D’v’l’d │ Coal │ Coal │ Furnace ────────┼───────────┼──────────┼──────┼────────┼──────┼──────┼────────── Babcock │ 300│Hand-fired│ │ │ │ │ & │ │ │ │ │ │ │ Wilcox│ │ │ 84│ 118.7│11,912│ 8.81│ 71.8 Babcock │ 640│Hand-fired│ │ │ │ │ & │ │ │ │ │ │ │ Wilcox│ │ │ 118│ 121.5│14,602│ 10.83│ 72.0 Stirling│ 1128│B. & W. │ │ │ │ │ │ │ chain │ │ │ │ │ │ │ grate │ 187│ 198.3│12,130│ 9.51│ 76.1 Rust │ 335│Hand-fired│ 68│ 210.5│13,202│ 9.42│ 68.9 Heine │ 400│Green │ │ │ │ │ │ │ chain │ │ │ │ │ │ │ grate │ 83.5│ 123.8│11,608│ 8.79│ 73.5 Maximum efficiency recorded │ │ │ │ │ 83 ───────────────────────────────┴──────┴────────┴──────┴──────┴──────────

The steam delivered by a boiler is the basis of the measurement of its capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U. per hour. It is approximately equal to the raising of 30 pounds of water per hour from a temperature of 100° Fahrenheit, to steam at a pressure of 70 pounds per square inch, or to 34 pounds of water per hour changed to steam from and at 212° Fahrenheit, at atmospheric pressure. The horse-power of a boiler is sometimes approximated by the area of its grate or heating surface. Such a method of measuring has a low degree of accuracy on account of the variations in the quality of the fuel, and the rate of combustion. For example, the rate of combustion under a locomotive boiler is high and there is less than ⅒th of a square foot of grate area and about 4.5 square feet of heating surface per boiler horse-power. The Scotch Marine type of boiler used on steam ships, has slightly more grate area and slightly less heating surface than the locomotive type of boiler, because the rate of combustion is lower. Stationary water-tube boilers may have 2 to 3 times as much grate area and heating surface per horse-power as is found in locomotive boilers. If a poor type of fuel is to be used the area of the grate should be increased about inversely as the heat content of the fuel. The approximate heat content of various types of fuels is shown in Table 29.

TABLE 29

APPROXIMATE HEAT VALUE OF FUELS

─────────────────────────────────────┬────────────────┬──────────────── Fuel │ │Pounds of Water │ │Evaporated from │ │ and at 212° F. │ │ All heat │B.T.U. per Pound│ utilized ─────────────────────────────────────┼────────────────┼──────────────── Anthracite │ 13,500│ 14.0 Semi-bituminous, Pennsylvania │ 15,000│ 15.5 Semi-bituminous, best, West Virginia │ 15,000│ 15.8 Bituminous, best, Pennsylvania │ 14,450│ 15.0 Bituminous, poor, Illinois │ 10,500│ 10.9 Lignite, best, Utah │ 11,000│ 11.4 Lignite, poor, Oregon │ 8,500│ 8.8 Wood, best oak │ 9,300│ 9.6 Wood, poor ash │ 8,500│ 8.8 ─────────────────────────────────────┴────────────────┴────────────────

=83. Air Ejectors.=—The Ansonia compressed-air sewage ejector is shown in Fig. 68. In its operation, sewage enters the reservoir through the inlet pipe at the right, the air displaced being expelled slowly through the air valve marked B. The rising sewage lifts the float which actuates the balanced piston valve in the pipe above the reservoir when the reservoir fills. The lifting of the valve admits compressed air to the reservoir. The air pressure closes valve A and the inlet valve at the right, and ejects the sewage through the discharge pipe at the left. As the float drops with the descending sewage it shuts off the air supply and opens the air exhaust through the small pipe at the top center. Sewage is prevented from flowing back into the reservoir by the check valve in the discharge pipe. Other ejectors operating on a similar principle are the Ellis, the Pacific, the Priestmann and the Shone.

=84. Electric Motors.=—The most common form of alternating current electric motor used for driving sewage pumps where continuous operation and steady loads are met is the squirrel-cage polyphase induction motor. These motors operate at a nearly constant speed which should be selected to develop the maximum efficiency of the pump and motor set. While Fig. 59 shows the best efficiency under varying heads to be obtained with variable speed, the advantages of cost, attention, and availability make the use of a constant speed motor common.[47] This type of motor is undesirable where stopping and starting are frequent because it has a relatively small starting torque and it requires a large starting current. Such motors can be constructed in small sizes for high starting torques by increasing the resistance of the rotor, but at the expense of the efficiency of operation.

Alternating current motors are more generally used than direct-current motors because of the greater economy of transmission of alternating current, but where direct current is available constant speed shunt wound motors should be adopted.

In the selection of a motor to drive a centrifugal pump it is important that the motor have not only the requisite power, but that its speed will develop the maximum efficiency from the pump and motor combined. If the pump and motor operate on the same shaft the speed of the two machines must be the same. If the two are belt connected, the size of the pulleys may be selected so as to give the required speed. If the motor is to be connected to a power pump an adequate automatic pressure relief valve should be provided on the discharge pipe from the pump, to prevent the overloading of the motor or bursting of the pump in case of a sudden stoppage in the pipe. The motor must be selected to suit the conditions of voltage, cycle, and phase on the line. Transformers are available to step the voltage up or down to practically any value. Rotary converters are used to change direct to alternating current or vice versa.

=85. Internal Combustion Engines.=—Internal combustion engines are used for driving pumps. Units are available in size from fractions of 1 horse-power to 2,000 horse-power or more, although the use of the larger sizes is exceptional. These engines are not commonly used for sewage pumping but when used they are ordinarily belt connected to a centrifugal pump, or to an electric generator which in turn drives electric motors which operate centrifugal pumps. This type of engine is more commonly adapted to small loads, although not entirely confined to this field, as they serve admirably as emergency units to supplement an electrically equipped pumping station. The fuel efficiency of internal combustion engines is higher than for steam engines as is indicated in Table 30, but the fuel is more expensive.

The four-cycle gas engine shown in Fig. 69 is the type most commonly used. Its horse-power is the product of: the mean effective pressure, the length of the stroke, the area of the piston, and the number of explosions per second divided by 550. The M.E.P. is dependent on the character of the fuel used and the compression of the gas before ignition. Producer gas will furnish mean effective pressures between 60 and 70 pounds per square inch, natural gas and gasoline, 85 to 90 pounds per square inch, and alcohol from 95 to 110 pounds per square inch.

TABLE 30

COMPARATIVE FUEL COSTS FOR PRIME MOVERS

───────────────────────────────────────┬───────────────┬─────────────── Type of Engine │ Quantity of │Cost of Fuel in │ Fuel per H.P. │ Cents per │ Hour │ Horse-power │ │ Hour ───────────────────────────────────────┼───────────────┼─────────────── Reciprocating steam engines, simple, │ 21 to 8 lb. │ 4.2 to 1.6 non-condensing, 25 to 200 H.P. │ coal │ Triple condensing, 2000 to 10,000 │2.3 to 1.9 lb. │ 0.46 to 0.37 H.P. │ coal │ ───────────────────────────────────────┼───────────────┼─────────────── Steam turbines, high pressure, │ │ non-condensing, │ │ 200 to 500 K.W. │6.5 to 4.2 lb. │ 1.3 to 0.86 │ coal │ 500 to 3000 K.W. │2.6 to 1.9 lb. │ 0.52 to 0.37 │ coal │ Condensing 5000 to 20,000 K.W. │1.8 to 1.43 lb.│ 0.36 to 0.28 │ coal │ ───────────────────────────────────────┼───────────────┼─────────────── Gas engines │ │ Natural gas, 50 to 200 H.P. │ 19 to 11 cu. │ │ ft. │ Producer gas, 50 to 200 H.P. │ 2 to 1.5 cu. │ │ ft. │ Illuminating gas, 10 to 75 H.P. │ 26 to 19 cu. │ 2.1 to 1.5 │ ft. │ Gasoline, 10 to 75 H.P. │ 1.5 to 0.8 │ 5.6 to 3.0 │ pints │ ───────────────────────────────────────┼───────────────┼─────────────── Oil engines, 100 to 500 H.P. │1.1 to 0.75 lb.│ │ oil │ ───────────────────────────────────────┴───────────────┴─────────────── NOTE.—Coal assumed at $4.00 per ton, illuminating gas at 80 cents per thousand cubic feet, and gasoline at 30 cents per gallon.

The Diesel Engine is the most efficient of internal combustion engines. The original aim of the inventor, Dr. Rudolph Diesel, was to avoid the explosive effect of the ordinary internal combustion engine by injecting a fuel into air so highly compressed that its heat would ignite the fuel, causing slow combustion of the fuel thus utilizing its energy to a greater extent. The fuel and air were to be so proportioned as to require no cooling. Although the ideal condition has not been attained, the heat efficiency of Diesel engines is high. They will consume from 0.3 to 0.5 of a pound of oil (containing 18,000 B.T.U. per pound) per brake horse-power hour, giving an effective heat efficiency of 25 to 30 per cent. Although not now in extensive use in the United States it is probable that this engine will be more generally adopted for conditions suitable for internal combustion engines.

=86. Selection of Pumping Machinery.=—Centrifugal pumps are particularly adapted to the lifting of sewage because of their large passages, and their lack of valves. The low lifts, nearly constant head, and the possibility of equalizing the load by means of reservoirs are particularly suited to efficient operation of centrifugal pumps. They require less floor space than reciprocating pumps of the same capacity, and because of their freedom from vibration they do not demand so heavy a foundation. The discharge from the pump is continuous thus relieving the piping from vibration. In case of emergency the discharge valve can be shut off without shutting down the pump, an important point in “fool proof” operation.

Volute pumps are better adapted to pumping sewage as their passages are more free and they are better suited to the low lifts met. Gritty and solid matter will cause wear on the diffusion vanes of turbine pumps in spite of the most careful design. Although turbine pumps can possibly be built with higher efficiency than volute pumps, their efficiency at part load falls rapidly and the fluctuations of sewage flow are sufficient to affect the economy of operation. Turbine pumps are more expensive and heavier than volute pumps on account of the increased size necessitated by the diffusion vanes.

Multi-stage pumps are used for high lifts and are seldom if ever required in sewage pumping. As ordinarily manufactured, each stage is good for an additional 40 to 100 pounds pressure, but wide variations in the limiting pressures between stages are to be found.

Reciprocating plunger pumps are sometimes used for sewage pumping where the character of the sewage is such that the valves will not be clogged nor parts of the pump corroded. These pumps are seldom used in small installations or for low lifts. They are not adapted to automatic or long distance control as are electrically driven centrifugal pumps. The use of reciprocating pumps for sewage pumping is practically restricted to very large pumping stations with capacities in the neighborhood of 50,000,000 gallons per day or more. Steam-driven pumps are the most common of the reciprocating type, but power pumps are sometimes used in special cases for small installations and may be driven by either a steam or gas engine or an electric motor.

Compressed air ejectors, as described in Art. 83 are used for lifting sewage and other drainage from the basement of buildings below the sewer level.

Centrifugal pumps electrically driven are, as a rule, the most satisfactory for sewage pumping. Electric drive lends itself to control by automatic devices, which are particularly convenient in small pumping stations. The control can be arranged so that the pump is operated only at full load and high efficiency, and when not operating no power is being consumed, as is not the case with a steam pump where steam pressure must be maintained at all times. The electric driven pump is thrown into operation by a float controlled switch which is closed when the reservoir fills, and opens when the pump has emptied the reservoir. The choice between steam and electric power for large pumping stations is a matter of relative reliability and economy.

The selection of the proper type of pump, whether reciprocating or otherwise, requires some experience in the consideration of the factors involved. Fig. 70 is of some assistance. In discussing this figure, Chester states:

“Fig. 70 attempts to represent graphically, the writer’s ideas under general conditions, of the machines that should be selected for certain capacities for both principal engine and alternate and the station duty they may be expected to produce, but you must realize that this intends the principal engine doing at least 90 per cent of the work and that the head, the cost of coal, the load factor, the cost of real estate ... the boiler pressure, and the space available, and finally ... the funds available, are factors which may shift both the horizontal and curved lines. In the field of low service pumps of 10,000,000 capacity or over, the centrifugal pump reigns supreme, and for constant low heads of 20,000,000 capacity or over the turbine driven centrifugal usurps the field.”

A reciprocating pump of any type would have to be specially built for pumping sewage not carefully screened or otherwise treated, as the valves, ordinarily used in such pumps for lifting water, would clog. The vertical triple-expansion pumping engine with special valves and for large installations, and the centrifugal pump for large or small installations are the only suitable types for pumping sewage. With steam turbine or electric drive the centrifugal has the field to itself.

=87. Costs of Pumping Machinery.=—The cost of pumping machinery can not be stated accurately as the many factors involved vary with the fluctuations in the prices of raw materials, transportation, labor, etc. The actual purchase price of machinery can be found accurately only from the seller. The costs given in this chapter are useful principally for comparative purposes and for exercise in the making of estimates. The costs of complete pumping stations are shown in Table 31.[48] These figures represent costs in 1911.

TABLE 31

COSTS OF COMPLETE PUMPING STATIONS

These costs include the best type of triple-expansion engines, high-pressure boilers, brick or inexpensive stone building with slate roof, chimney and intake. Cost of land is not included. ─────────────────┬─────────────────┬─────────────────┬───────────────── Discharge │ Horse-power per │ │ Pressure, Lbs. │ Million Gals. │Cost, Dollars per│Cost, Dollars per per Sq. In. │ Pumped │ Horse-power │ Million Gallons ─────────────────┼─────────────────┼─────────────────┼───────────────── 30│ 12│ 562│ 6,750 40│ 16│ 438│ 7,000 50│ 20│ 362│ 7,250 60│ 24│ 312│ 7,500 70│ 28│ 277│ 7,750 80│ 32│ 250│ 8,000 90│ 36│ 229│ 8,250 100│ 40│ 213│ 8,500 110│ 44│ 200│ 8,750 120│ 48│ 187│ 9,000 130│ 52│ 192│ 10,000 │ │ │ ─────────────────┴─────────────────┴─────────────────┴─────────────────

=88. Cost Comparisons of Different Designs.=—In the design of a pumping station and its equipment the relative costs of different designs should be compared, and the least expensive design selected, due consideration being given to serviceability, reliability, and other factors without definite financial value. In comparing the costs of different types of machinery, all items in connection with the pumping station should be considered. For example, the cost of an electrically driven centrifugal pump and equipment may be less than the total cost of a steam driven reciprocating pump and equipment because of the saving in the cost of boilers, boiler house, etc., but a comparison of the capitalized cost of the two might show in favor of the reciprocating steam pump because of the lower cost of operation.

The total cost of a plant, or any portion thereof, may be considered as made up of three parts: (1) The first cost, (2) operation and maintenance and, (3) renewal. The total cost S can be expressed as

S = C + O⁄r + R,

in which C = the first cost;

O = the annual expenditure for operation and maintenance;

R = the amount set aside to cover renewal;

r = the rate of interest.

S is called the capitalized cost of a plant. The annual payment necessary to perpetuate a plant is

A = Sr = Cr + O + Rr.

The value of R is useful when expressed in terms of the life of the plant or machine and the current rate of interest. It is sometimes called the depreciation factor or capitalized depreciation. If it is borne in mind that R is the amount to be set aside at compound interest for the life of the plant, at the end of which time the accrued interest should be sufficient to renew the plant, it is evident that

R(1 + R)^n − R = C

or R = C⁄((1+r)^n − 1)

in which n is the period of usefulness, or life of the plant, expressed in years, no allowance being made for scrap value.

A comparison of the annual expense of three different plants is shown in Table 32. It is evident from this comparison that the machinery with the least first cost is not always the least expensive when all items are considered.

A sinking fund is a sum of money to which additions are made annually for the purpose of renewing a plant at the expiration of its period of usefulness. The annual payment into the sinking fund is equivalent to the term Rr in the expression for annual cost, or in terms of C, r, and n, the annual payment is

Cr⁄((1 + r)^n − 1).

It is the same as the capitalized depreciation multiplied by the rate of interest. The expression r⁄((1 + r)^n − 1) is sometimes called the rate of depreciation.

The present worth of a machine is the difference between its first cost and the present value of the sinking fund. If m represents the present age of a plant in years, then the present worth is

P = C(1 – ((1 + r)^n − 1)⁄((1 + r)^m − 1)).

TABLE 32

COMPARISON OF COSTS OF THREE DIFFERENT PUMPING STATIONS. NOMINAL CAPACITY THIRTY MILLION GALLONS PER DAY RAISED THIRTY FEET

────────────────┬────────────────────────────────── Equipment │ Plant A ────────────────┼────────────────────────────────── │One Acre of Land. Brick Building, │ Steel Trussed Roof, Slate │ Covered. Cross Compound │ Condensing Horizontal Pumping │ Engine ────────────────┼───────┬──────────┬───────┬─────── │Annual │ Years of │Sinking│ Total │Payment│Usefulness│ Fund │ │ on │ │Payment│ │ First │ │ │ │ Cost │ │ │ ────────────────┼───────┼──────────┼───────┼─────── Land │ 100│ │ 0│ 100 Permanent │ 1188│ 50│ 1080│ 2,260 Structures[49]│ │ │ │ Pumps and │ 440│ 15│ 435│ 875 Machinery │ │ │ │ Boilers │ 280│ 10│ 446│ 726 Labor │ │ │ │ 14,000 Fuel │ │ │ │ 5,500 Repairs, etc. │ │ │ │ 480 ────────────────┼───────┼──────────┼───────┼─────── Total │ │ │ │ 23,941 ────────────────┴───────┴──────────┴───────┴───────

────────────────┬────────────────────────────────── Equipment │ Plant B ────────────────┼────────────────────────────────── │One Acre of Land. Brick Building. │ Steel Trussed Roof, Slate │ Covered. Compound Condensing Low │ Duty Horizontal Pumping Engine │ ────────────────┼───────┬──────────┬───────┬─────── │Annual │ Years of │Sinking│ Total │Payment│Usefulness│ Fund │ │ on │ │Payment│ │ First │ │ │ │ Cost │ │ │ ────────────────┼───────┼──────────┼───────┼─────── Land │ 100│ │ 0│ 100 Permanent │ 1180│ 50│ 1080│ 2,260 Structures[49]│ │ │ │ Pumps and │ 390│ 15│ 395│ 785 Machinery │ │ │ │ Boilers │ 252│ 10│ 400│ 652 Labor │ │ │ │ 14,000 Fuel │ │ │ │ 7,200 Repairs, etc. │ │ │ │ 400 ────────────────┼───────┼──────────┼───────┼─────── Total │ │ │ │ 25,497 ────────────────┴───────┴──────────┴───────┴───────

────────────────┬────────────────────────────────── Equipment │ Plant C ────────────────┼────────────────────────────────── │One Acre of Land. Frame Building, │ Shingle Roof. Compound Duplex │ Non-Condensing Pumping Engine. │ │ ────────────────┼───────┬──────────┬───────┬─────── │Annual │ Years of │Sinking│ Total │Payment│Usefulness│ Fund │ │ on │ │Payment│ │ First │ │ │ │ Cost │ │ │ ────────────────┼───────┼──────────┼───────┼─────── Land │ 100│ │ 0│ 100 Permanent │ 810│ 50│ 775│ 1,585 Structures[49]│ │ │ │ Pumps and │ 360│ 15│ 352│ 712 Machinery │ │ │ │ Boilers │ 308│ 10│ 490│ 798 Labor │ │ │ │ 14,000 Fuel │ │ │ │ 8,200 Repairs, etc. │ │ │ │ 550 ────────────────┼───────┼──────────┼───────┼─────── Total │ │ │ │ 25,945 ────────────────┴───────┴──────────┴───────┴───────

Where straight-line depreciation is spoken of it is assumed that the worth of a machine depreciates an equal part of its first cost each year. For example, if the life of a plant is assumed to be 20 years, straight-line depreciation will assume that the plant loses 1/20 of its original value annually. The present worth of a plant under this assumption would be the product of its first cost and the ratio between its remaining life and its total life. This method of estimating depreciation and worth is frequently used, particularly for short-lived plants and for simplicity in bookkeeping, but it is less logical than the method given above.

=89. Number and Capacity of Pumping Units.=—In order to select the number and capacity of pumping units for the best economy, a comparison of the costs of different combinations of units should be made and the most economical combination determined by trial. The principles outlined in the preceding articles should be observed in making these comparisons. In a steam pumping station, when the number of units operating is less than the average daily maximum for the period, steam must nevertheless be kept on a sufficient number of boilers to operate the maximum number of pumps. This, and corresponding standby losses must not be overlooked, as they may show that a smaller number of larger units is ultimately more economical.

TABLE 33

SUMMARY OF FLUCTUATIONS OF SEWAGE FLOW AT A PROPOSED PUMPING STATION

─────────────────┬─────────────────┬─────────────────┬───────────────── Number of Days │Flow in Thousand │ │ Loads Occurred in│ Gallons per │ │ One Year │ Minute │ Lift in Feet │ Horse-power ─────────────────┼─────────────────┼─────────────────┼───────────────── 1│ 293│ 6.0│ 450 8│ 163│ 8.6│ 354 15│ 119│ 10.0│ 300 18│ 106│ 10.6│ 284 23│ 88│ 11.2│ 249 31│ 69│ 12.2│ 211 32│ 65│ 12.4│ 204 45│ 51│ 13.4│ 173 41│ 50│ 13.5│ 169 30│ 45│ 13.8│ 158 28│ 44│ 13.9│ 154 23│ 40│ 14.2│ 143 21│ 38│ 14.4│ 137 18│ 35│ 14.6│ 129 12│ 29│ 15.0│ 111 8│ 24│ 15.6│ 95 5│ 20│ 16.0│ 79 3│ 16│ 16.5│ 65 2│ 14│ 16.8│ 58 1│ 6.5│ 18.0│ 29 ─────────────────┴─────────────────┴─────────────────┴───────────────── Total horse-power days for one year, 102,000. Average load in horse-power, 280.

TABLE 34

POSSIBLE COMBINATIONS OF FIVE PUMPING UNITS TO CARE FOR THE LOADS SHOWN IN TABLE 33[50]

──────────────────────────────────┬─────────────── 40 Horse-power │ Load Type 1[51] │ ────────┬──────┬───────────┬──────┼───────┬─────── Per Cent│Pounds│ Load in │Pounds│Number │ Total of Rated│Steam │Horse-power│Steam,│of Days│ Load Capacity│ per │ │Units │Load is│Carried │ H.P. │ │10,000│Carried│ on │ Hour │ │Pounds│in Year│ these │ │ │ │ │Days in │ │ │ │ │ H.P. ────────┼──────┼───────────┼──────┼───────┼─────── 151│ 45│ 60.4│ 6.5│ 1│ 681 120│ 44│ 48│ 40.5│ 8│ 542 102│ 45│ 40.8│ 66.1│ 15│ 458 96│ 45│ 38.4│ 74.8│ 18│ 434 98│ 45│ 39.2│ 97.5│ 23│ 381 │ │ │ │ 31│ 322 │ │ │ │ 32│ 312 │ │ │ │ 45│ 264 │ │ │ │ 41│ 258 101│ 45│ 40.4│ 131│ 30│ 242 98│ 45│ 39.2│ 119│ 28│ 235 │ │ │ │ 23│ 218 │ │ │ │ 21│ 210 │ │ │ │ 18│ 198 │ │ │ │ 12│ 170 104│ 45│ 41.6│ 20.9│ 8│ 145 │ │ │ │ 5│ 121 │ │ │ │ 3│ 100 99│ 45│ 39.6│ 8.5│ 2│ 89 113│ 44│ 45.2│ 4.8│ 1│ 45 │ │ │ ————│ │ Sub-total │ 596.6│ │ Grand total in pounds, 65,700,000 ──────────────────────────────────────────────────

──────────────────────────────────┬─────────────── 50 Horse-power │ Load Type 1[51] │ ────────┬──────┬───────────┬──────┼───────┬─────── Per Cent│Pounds│ Load in │Pounds│Number │ Total of Rated│Steam │Horse-power│Steam,│of Days│ Load Capacity│ per │ │Units │Load is│Carried │ H.P. │ │10,000│Carried│ on │ Hour │ │Pounds│in Year│ these │ │ │ │ │Days in │ │ │ │ │ H.P. ────────┼──────┼───────────┼──────┼───────┼─────── 151│ 45│ 75.5│ 8.2│ 1│ 681 120│ 44│ 60.0│ 50.7│ 8│ 542 102│ 45│ 51.0│ 82.7│ 15│ 458 90│ 45│ 48.0│ 93.5│ 18│ 434 98│ 45│ 49.0│ 122.0│ 23│ 381 104│ 45│ 52.0│ 174.5│ 31│ 322 101│ 45│ 50.5│ 174.8│ 32│ 312 │ │ │ │ 45│ 264 103│ 45│ 51.5│ 228│ 41│ 258 │ │ │ │ 30│ 242 │ │ │ │ 28│ 235 │ │ │ │ 23│ 218 │ │ │ │ 21│ 210 │ │ │ │ 18│ 198 │ │ │ │ 12│ 170 │ │ │ │ 8│ 145 109│ 44│ 54.5│ 28.8│ 5│ 121 │ │ │ │ 3│ 100 99│ 45│ 49.5│ 10.7│ 2│ 89 │ │ │ │ 1│ 45 │ │ │ ————│ │ │ 973.9│ │

──────────────────────────────────────────────────

──────────────────────────────────┬─────────────── 60 Horse-power │ Load Type 1[51] │ ────────┬──────┬───────────┬──────┼───────┬─────── Per Cent│Pounds│ Load in │Pounds│Number │ Total of Rated│Steam │Horse-power│Steam,│of Days│ Load Capacity│ per │ │Units │Load is│Carried │ H.P. │ │10,000│Carried│ on │ Hour │ │Pounds│in Year│ these │ │ │ │ │Days in │ │ │ │ │ H.P. ────────┼──────┼───────────┼──────┼───────┼─────── 151│ 45│ 90.6│ 9.8│ 1│ 681 120│ 44│ 72.0│ 60.8│ 8│ 542 102│ 45│ 61.2│ 99.2│ 15│ 458 96│ 45│ 57.6│ 112│ 18│ 434 │ │ │ │ 23│ 381 104│ 45│ 62.4│ 209.0│ 31│ 322 101│ 45│ 60.6│ 210│ 32│ 312 102│ 45│ 61.2│ 325│ 45│ 264 │ │ │ │ 41│ 258 │ │ │ │ 30│ 242 │ │ │ │ 28│ 235 │ │ │ │ 23│ 218 │ │ │ │ 21│ 210 │ │ │ │ 18│ 198 106│ 45│ 63.6│ 137│ 12│ 170 │ │ │ │ 8│ 145 109│ 44│ 65.4│ 34.5│ 5│ 121 │ │ │ │ 3│ 100 │ │ │ │ 2│ 89 │ │ │ │ 1│ 45 │ │ │ ————│ │ │1197.3│ │

──────────────────────────────────────────────────

──────────────────────────────────┬─────────────── 100 Horse-power │ Load Type 4[51] │ ────────┬──────┬───────────┬──────┼───────┬─────── Per Cent│Pounds│ Load in │Pounds│Number │ Total of Rated│Steam │Horse-power│Steam,│of Days│ Load Capacity│ per │ │Units │Load is│Carried │ H.P. │ │10,000│Carried│ on │ Hour │ │Pounds│in Year│ these │ │ │ │ │Days in │ │ │ │ │ H.P. ────────┼──────┼───────────┼──────┼───────┼─────── 151│ 28│ 151│ 10.2│ 1│ 681 120│ 25│ 120│ 57.5│ 8│ 542 102│ 25│ 102│ 62.5│ 15│ 458 96│ 25│ 96│ 103.8│ 18│ 434 98│ 25│ 98│ 135.1│ 23│ 381 │ │ │ │ 31│ 322 │ │ │ │ 32│ 312 │ │ │ │ 45│ 264 │ │ │ │ 41│ 258 │ │ │ │ 30│ 242 │ │ │ │ 28│ 235 │ │ │ │ 23│ 218 │ │ │ │ 21│ 210 │ │ │ │ 18│ 198 106│ 25│ 106│ 76.5│ 12│ 170 104│ 25│ 104│ 29.1│ 8│ 145 │ │ │ │ 5│ 121 100│ 25│ 100│ 32.4│ 3│ 100 │ │ │ │ 2│ 89 │ │ │ │ 1│ 45 │ │ │ ————│ │ │ 507.1│ │

──────────────────────────────────────────────────

──────────────────────────────────┬─────────────── 200 Horse-power │ Load Type 5[51] │ ────────┬──────┬───────────┬──────┼───────┬─────── Per Cent│Pounds│ Load in │Pounds│Number │ Total of Rated│Steam │Horse-power│Steam,│of Days│ Load Capacity│ per │ │Units │Load is│Carried │ H.P. │ │10,000│Carried│ on │ Hour │ │Pounds│in Year│ these │ │ │ │ │Days in │ │ │ │ │ H.P. ────────┼──────┼───────────┼──────┼───────┼─────── 151│ 23│ 302│ 16.7│ 1│ 681 120│ 20│ 240│ 92.0│ 8│ 542 102│ 20│ 204│ 147│ 15│ 458 96│ 20│ 192│ 166│ 18│ 434 98│ 20│ 196│ 216│ 23│ 381 104│ 20│ 208│ 309.5│ 31│ 322 101│ 20│ 202│ 310│ 32│ 312 102│ 20│ 204│ 481│ 45│ 264 103│ 20│ 206│ 405│ 41│ 258 101│ 20│ 202│ 291│ 30│ 242 98│ 20│ 196│ 264│ 28│ 235 109│ 20│ 218│ 241│ 23│ 218 105│ 20│ 210│ 212│ 21│ 210 99│ 20│ 198│ 171│ 18│ 198 │ │ │ │ 12│ 170 │ │ │ │ 8│ 145 │ │ │ │ 5│ 121 │ │ │ │ 3│ 100 │ │ │ │ 2│ 89 │ │ │ │ 1│ 45 │ │ │ ————│ │ │3322.2│ │

──────────────────────────────────────────────────

TABLE 35

FINANCIAL COMPARISON OF PUMPING EQUIPMENTS

The loads to be cared for are shown in Table 34. An emergency unit is supplied to bring the overload capacity of the plant, less the largest unit, equal to the maximum load on the plant. No unit will be overloaded more than fifty per cent of its rated capacity.

───────────┬───────────┬───────────┬───────────┬───────────┬─────────── Number of │ │ │ │ │ Units │ │ │ │ │ Exclusive │ │ │ │ │ of │ │ │ │ │ Emergency │ │ │ │ │ Unit │ 5 │ 4 │ 3 │ 2 │ 1 ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── Capacity │ 40 h.p.,│ │ │ │ and Type of│ Type 1│ │ │ │ Units │ 50 h.p.,│ 50 h.p.,│ │ │ │ Type 1│ Type 1│ │ │ │ 60 h.p.,│ 100 h.p.,│ 50 h.p.,│ │ │ Type 1│ Type 4│ Type 1│ │ │ 100 h.p.,│ 125 h.p.,│ 150 h.p.,│ 200 h.p.,│ │ Type 4│ Type 4│ Type 5│ Type 5│ │ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p., │ Type 5│ Type 5│ Type 6│ Type 6│ Type 7 ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── Emergency │ │ │ │ │ Unit, │ │ │ │ │ Capacity │ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p., and Type │ Type 5│ Type 5│ Type 6│ Type 6│ Type 7 ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── Annual │ │ │ │ │ payments,│ │ │ │ │ Dollars │ │ │ │ │ First │ │ │ │ │ cost of│ │ │ │ │ pumps │ 1,560│ 1,660│ 1,480│ 1,440│ 1,500 Renewal │ │ │ │ │ of │ │ │ │ │ pumps │ 1,340│ 1,430│ 1,270│ 1,240│ 1,290 First │ │ │ │ │ cost, │ │ │ │ │ boilers│ 1,024│ 1,089│ 1,125│ 1,115│ 1,410 Renewal, │ │ │ │ │ boilers│ 800│ 935│ 966│ 958│ 1,210 Fuel │ 13,140│ 11,860│ 10,490│ 9,420│ 9,400 Repairs, │ │ │ │ │ oil, │ │ │ │ │ etc. │ 2,000│ 1,800│ 1,500│ 1,300│ 1,200 Labor │ 35,000│ 31,500│ 29,500│ 27,000│ 27,000 Emergency│ │ │ │ │ unit. │ │ │ │ │ First │ │ │ │ │ cost │ 640│ 560│ 800│ 800│ 1,500 Emergency│ │ │ │ │ unit. │ │ │ │ │ Renewal│ 550│ 480│ 690│ 690│ 1,290 ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── Total │ 56,134│ 51,314│ 47,821│ 43,963│ 45,800 ───────────┴───────────┴───────────┴───────────┴───────────┴───────────

Type 1. Simple duplex, non-condensing, horizontal.

Type 4. Compound condensing low duty horizontal.

Type 5. Low duty, triple, condensing, horizontal.

Type 6. Cross compound, condensing, horizontal.

Type 7. High duty, triple, condensing, vertical.

For example, the sewage flow expected at a proposed pumping station is shown in Table 33. The steps involved in the selection of the number and capacity of pumping units to care for these quantities are as follows: (1) Determine the rated capacity of the equipment to be provided. In this case the capacity will be taken as 450 horse-power, which is the maximum load to be placed on the pumps. (2) Select any number of units of such different types and capacities as are available for comparison, and arrange them in different combinations so that each unit will operate as nearly as possible at its rated capacity. The work involved in such a study for 5 units is shown in Table 34. The weight of steam consumed per indicated horse-power hour corresponding to the per cent of the rated capacity at which the unit is operating is read from Fig. 64 or other data. (3) Repeat this step for other numbers and types of units. (4) Prepare a table showing the annual costs of combinations of different numbers and types of units as shown for this example in Table 35. The figures in Table 35 show that the least expensive of the combinations of the units studied is one 200 horse-power unit, and one 250 horse-power unit, with a 250 horse-power unit in reserve. It is to be noted that a reserve unit has been provided in each combination, the capacity of which is equal to that of the largest unit of the combination.

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