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.
Fig. 49.—Calumet Sewage Pumping Station, Chicago, Illinois.
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.
Fig. 50.—Section through de Laval Single-Stage, Double Suction Centrifugal Pump.
Fig. 51.—Types of Centrifugal Pumps.
Fig. 52.—Section of a Multi-Stage Centrifugal Pump.
Courtesy DeLaval Steam Turbine Co.
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.
Fig. 53.—Triplex Power Pump.
Courtesy, The Deming Co.
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.
Fig. 54.—Water End of Inside Center-Packed Plunger Pump.
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.
Fig. 55—Water End of Outside Center-Packed Plunger Pump.
Courtesy Allis-Chalmers Co.
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.
Fig. 56.—Marine Type Thrust Bearing.
Courtesy, DeLaval Steam Turbine Co.
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.
Fig. 57.—Piston Type of Thrust Balancing Device.
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.
Fig. 58.—Characteristics of Centrifugal Pumps with Different Styles of Impellers at Constant Speed.
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.
Fig. 59.—Efficiency and Characteristic Curves of a Centrifugal Pump at Different Speeds.
Fig. 60.—Efficiencies of Centrifugal Pumps.
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.
Fig. 61.—Centrifugal Pump in Manhole at Duluth, Minn.
Eng. Contracting, Vol. 43, 1915, p. 310.
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.
Fig. 62.—Interior Arrangement of the Calumet Sewage Pumping Station, Chicago.
Eng. News-Record, Vol. 85, 1920, p. 872.
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
Fig. 63.—Section of Duplex Piston Steam Pump.
Courtesy, The John H. McGowan Co.
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.
Fig. 64.—Diagram Showing Rates of Steam Consumption for Different Size Units under Different Loads.
| TABLE 27 | |||||||
|---|---|---|---|---|---|---|---|
| Water Rates of Prime Movers at Full and Part Loads | |||||||
| Type of Engine | Power, K.W. | Per Cent of Full Load | Boiler Press. Lbs. | ||||
| 25 | 50 | 75 | 100 | 125 | |||
| Single cylinder, high speed, non-condensing | 25 | 33 | 27 | 26.3 | 27.0 | 27.5 | 100 to 150 |
| 250 | 42 | 37.5 | 35 | 34.0 | 34.0 | ||
| Automatic, flat four valve, high speed | 150 | 32 | 30 | 26.5 | 29.0 | 100 to 125 | |
| 250 | 33 | 31 | 28 | 30.0 | |||
| Tandem compound condensing, high speed | 125 | 23 | 19 | 17 | 18 | 100 to 150 | |
| 25 | 20 | 19.5 | 21 | ||||
| Cross compound, condensing, high speed | 30 | 26 | 24 | 23 | 23.5 | 125 | |
| Cross compound, non-condensing, high speed | 39 | 31 | 27 | 26 | 27.5 | 125 | |
| Single cylinder Corliss, condensing | 120 | 23.7 | 20.4 | 19 | 18.5 | 19.0 | 100 |
| 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 valve, non-condensing | 75 | 26.2 | 22.3 | 21.3 | 21.6 | 22.8 | 100 |
| 400 | 35.0 | 27.2 | 26.4 | 26.0 | 26.8 | 180 | |
| Rotary four valve, tandem compound non-condensing | 125 | 32.0 | 22.0 | 20 | 18.25 | 18.5 | 100 |
| 600 | 40.0 | 28.3 | 23.2 | 22.5 | 22.7 | 150 | |
| Cross compound, non-condensing rotary four valve | 125 | 25 | 21 | 19.1 | 18.5 | 19.0 | 100 |
| 600 | 39.4 | 28 | 22.3 | 20.6 | 20.7 | 150 | |
| Single cylinder, poppett valve, non-condensing | 120 | 22.7 | 20.5 | 19.7 | 19.1 | 20.1 | 100 |
| 600 | 28.5 | 26.0 | 25.0 | 24.3 | 25.5 | 150 | |
| Single cylinder, poppett valve, condensing | 120 | 18.5 | 16.7 | 16.1 | 15.6 | 16.4 | 100 |
| 600 | 24.6 | 22.3 | 21.4 | 20.8 | 21.9 | 150 | |
| Compound condensing, poppett valve | 200 | 14.2 | 13.0 | 12.5 | 12.2 | 12.9 | 100 |
| 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, Allis-Chalmers | 300 | 24 | 17 | 160 | 16.5 | 125 | |
| 2000 | 31.9 | 26.3 | 23.8 | 23.0 | 175 | ||
| Steam turbines, condensing, Westinghouse | 300 | 13.7 | 12.8 | 12.2 | 12.6 | 125 | |
| 2000 | 18.2 | 16.9 | 16.2 | 16.8 | 175 | ||
| Steam turbines, high pressure, non-con., 12″ to 36″ wheel, 1000 to 3600 R.P.M. | 4 to 8 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,