| TABLE 33 | |||
|---|---|---|---|
| Summary of Fluctuations of Sewage Flow at a Proposed Pumping Station | |||
| Number of Days Loads Occurred in One Year | Flow in Thousand Gallons per 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 Type 1[[51]] | 50 Horse-power Type 1[[51]] | 60 Horse-power Type 1[[51]] | 100 Horse-power Type 4[[51]] | 200 Horse-power Type 5[[51]] | Load | ||||||||||||||||
| Per Cent of Rated Capacity | Pounds Steam per H.P. Hour | Load in Horse-power | Pounds Steam, Units 10,000 Pounds | Per Cent of Rated Capacity | Pounds Steam per H.P. Hour | Load in Horse-power | Pounds Steam, Units 10,000 Pounds | Per Cent of Rated Capacity | Pounds Steam per H.P. Hour | Load in Horse-power | Pounds Steam, Units 10,000 Pounds | Per Cent of Rated Capacity | Pounds Steam per H.P. Hour | Load in Horse-power | Pounds Steam, Units 10,000 Pounds | Per Cent of Rated Capacity | Pounds Steam per H.P. Hour | Load in Horse-power | Pounds Steam, Units 10,000 Pounds | Number of Days Load is Carried in Year | Total Load Carried on these Days in H.P. |
| 151 | 45 | 60.4 | 6.5 | 151 | 45 | 75.5 | 8.2 | 151 | 45 | 90.6 | 9.8 | 151 | 28 | 151 | 10.2 | 151 | 23 | 302 | 16.7 | 1 | 681 |
| 120 | 44 | 48 | 40.5 | 120 | 44 | 60.0 | 50.7 | 120 | 44 | 72.0 | 60.8 | 120 | 25 | 120 | 57.5 | 120 | 20 | 240 | 92.0 | 8 | 542 |
| 102 | 45 | 40.8 | 66.1 | 102 | 45 | 51.0 | 82.7 | 102 | 45 | 61.2 | 99.2 | 102 | 25 | 102 | 62.5 | 102 | 20 | 204 | 147 | 15 | 458 |
| 96 | 45 | 38.4 | 74.8 | 90 | 45 | 48.0 | 93.5 | 96 | 45 | 57.6 | 112 | 96 | 25 | 96 | 103.8 | 96 | 20 | 192 | 166 | 18 | 434 |
| 98 | 45 | 39.2 | 97.5 | 98 | 45 | 49.0 | 122.0 | 98 | 25 | 98 | 135.1 | 98 | 20 | 196 | 216 | 23 | 381 | ||||
| 104 | 45 | 52.0 | 174.5 | 104 | 45 | 62.4 | 209.0 | 104 | 20 | 208 | 309.5 | 31 | 322 | ||||||||
| 101 | 45 | 50.5 | 174.8 | 101 | 45 | 60.6 | 210 | 101 | 20 | 202 | 310 | 32 | 312 | ||||||||
| 102 | 45 | 61.2 | 325 | 102 | 20 | 204 | 481 | 45 | 264 | ||||||||||||
| 103 | 45 | 51.5 | 228 | 103 | 20 | 206 | 405 | 41 | 258 | ||||||||||||
| 101 | 45 | 40.4 | 131 | 101 | 20 | 202 | 291 | 30 | 242 | ||||||||||||
| 98 | 45 | 39.2 | 119 | 98 | 20 | 196 | 264 | 28 | 235 | ||||||||||||
| 109 | 20 | 218 | 241 | 23 | 218 | ||||||||||||||||
| 105 | 20 | 210 | 212 | 21 | 210 | ||||||||||||||||
| 99 | 20 | 198 | 171 | 18 | 198 | ||||||||||||||||
| 106 | 45 | 63.6 | 137 | 106 | 25 | 106 | 76.5 | 12 | 170 | ||||||||||||
| 104 | 45 | 41.6 | 20.9 | 104 | 25 | 104 | 29.1 | 8 | 145 | ||||||||||||
| 109 | 44 | 54.5 | 28.8 | 109 | 44 | 65.4 | 34.5 | 5 | 121 | ||||||||||||
| 100 | 25 | 100 | 32.4 | 3 | 100 | ||||||||||||||||
| 99 | 45 | 39.6 | 8.5 | 99 | 45 | 49.5 | 10.7 | 2 | 89 | ||||||||||||
| 113 | 44 | 45.2 | 4.8 | 1 | 45 | ||||||||||||||||
| Sub-total | 596.6 | 973.9 | 1197.3 | 507.1 | 3322.2 | ||||||||||||||||
| Grand total in pounds, 65,700,000 | |||||||||||||||||||||
| 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 and Type of Units | 40 h.p., Type 1 50 h.p., Type 1 60 h.p., Type 1 100 h.p., Type 4 200 h.p., Type 5 | 50 h.p., Type 1 100 h.p., Type 4 125 h.p., Type 4 175 h.p., Type 5 | 50 h.p., Type 1 150 h.p., Type 5 250 h.p., Type 6 | 200 h.p., Type 5 250 h.p., Type 6 | 450 h.p., Type 7 | |
| Emergency Unit, Capacity and Type | 200 h.p., Type 5 | 175 h.p., Type 5 | 250 h.p., Type 6 | 250 h.p., Type 6 | 450 h.p., 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.
CHAPTER VIII
MATERIALS FOR SEWERS
90. Materials.—The materials most commonly used for the manufacture of sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood are also used, but only under special conditions. For pipes built in the trench, concrete, concrete blocks, brick, and vitrified clay blocks are used. Concrete is being used to-day more than bricks or blocks because it is cheaper. A decade or more ago all large sewers were built of bricks. Vitrified clay and concrete are used for manufactured pipe 42 inches and less in diameter. Concrete is used almost exclusively for larger sizes of pipe, particularly for pipe constructed in place, although a brick invert lining is advisable when high velocities of flow are expected.
The character of the external load, the velocity of flow and the quality of sewage are important factors in determining the material to be used in the construction of sewers. Reinforced concrete should be used for large sewers near the surface subjected to heavy moving loads. A high velocity of flow with erosive suspended matter demand a brick wearing surface on the invert. Many engineers consider concrete less suitable than vitrified clay or brick for conveying septic sewage or acid industrial wastes, as concrete deteriorates more rapidly under such conditions. Concrete should be used on soft yielding foundations, whereas a hard compact earth, which can be cut to the form of the sewer, is suitable to the use of brick or concrete.
Cast-iron pipe with lead joints is used for sewers flowing under pressure, or where movements of the soil are to be expected. If the sewage is not flowing under pressure, cement joints are sometimes used in the cast-iron pipe. Movements of the soil are to be expected on side hills, under railroad tracks, etc. Steel pipe is used on long outfalls or under other conditions where external loads are light and the cost is less than for other materials. Because of the thin plates used and the liability to corrosion steel is not frequently used. It should never be deeply buried nor externally loaded because of its weakness in resisting such forces. Like wood pipe, its lightness is favorable to use on bridges, but the greater heat conductivity of steel than wood necessitates protection against freezing in exposed positions. Wood is preferable only where the economy of its use is pronounced and the pipe is running full at all times. It is desirable that the wood pipe should be always submerged as the life of alternately wet and dry wood is short.
Corrugated galvanized iron and unglazed tile have been used for sewers, but usually only in emergencies or as a makeshift. Corrugated iron is not suitable on account of its roughness and liability to corrosion, and unglazed tile because of its lack of strength.