20 feet, with suitable bearings at uniform intervals. These bearings are lubricated by separate oil pipes. In some designs the impellers consist of screw shaped vanes and diffusion vanes mounted along the shaft and discharge pipes. The pump head supports the discharge pipe and attached pump casing and carries the thrust bearing from which the shaft and impellers are suspended, and also supports the motor, engine or drive pulley. To develop the desired head at the ground surface, it is sometimes economical to modify the pump head to include a vertical shaft centrifugal pump taking suction from the deep well pump and boosting the water to any required pressure. In this case the deep well pump only lifts the supply to the surface. With suitable mounting the pump can be driven by electric motor or internal combustion engine. The capacity varies from 200 g.p.m. from a 6-inch to approximately 3500 g.p.m. from a 24-inch well. With this type of pump it is practicable to develop these quantities from any depth ordinarily encountered, it being only necessary to provide a sufficient number of stages to overcome the head pumped against. The necessary stages may be all in the well, but against high heads, the boosting stages are better located at the ground surface. The best efficiency of this type of pump is approximately 60 to 65 per cent for the pump proper and about 52 to 55 per cent overall efficiency with motor drive. This type of pump will give satisfactory service and require comparatively small expense for maintenance. For satisfactory operation it is essential that the alignment in the well be perfect; otherwise wear on bearings will be destructive. Pumping by compressed air. An eduction pipe for the discharge of water is suspended in the well casing with one-half or more of its length below the free water surface. Compressed air is admitted to the bottom of the eduction pipe through a small air pipe. The head of water in the well outside the eduction pipe just overbalances the longer, less dense column of mixed air and water inside the eduction pipe, causing the mixture to overflow at the top. The height from the water level in the well during pumping to the point of discharge is termed the "lift," He and the depth from this water level to the point of air admission is termed the running submergence, H. The ratio H2/(H ̧ + He), expressed in percentage, is termed the percentage of submergency. The air pressure at the bottom of the air pipe is equal to, and solely dependent upon, the water pressure due to submergence. In a properly designed air lift pump the eduction pipe should be proportioned for the proper expansion of the air to prevent excessive velocity at the discharge and should allow smooth passage for the air and water. The following table gives the ratio of lift to submergency: For lifts up to 50 feet-70 to 66 per cent submergence Submergence is the distance below the pumping level the air is injected into the water. The total lift is the distance above the pumping level the air travels with the water before it is separated from it. The starting pressure is controlled by the starting submergence, which is the column of water between the static level and the point at which the air is injected into the water. The operating pressure is controlled by the operating submergence and the friction in the air lines between the compressor and the point at which the air is injected into the water. A convenient, but not efficient, method of delivering the water to a reservoir located at a distance from a group of wells is by means of a booster separator located in the discharge at the well head in which by centrifugal action the air used to elevate the water is separated and a pressure maintained upon the surface of the water in the booster (before the air is allowed to escape to the atmosphere) sufficient to force the water through the horizontal discharge into a common flow line. The advantages of an air lift are: There are no moving parts in the wells, the air compressor is located in the engine room where it is under the care of an engineer, and sand carried with the water is not detrimental to the equipment. As to economy, the maintenance costs are low and the efficiency is sustained over a long period. The water may be pumped at a widely varying rate with relatively small loss in efficiency. The characteristics of the water are frequently improved due to the aeration. The energy applied (isothermal air horsepower) to an air-lift pump is that due to the isothermal expansion of the applied air, from the pressure at the foot piece to atmospheric pressure. The quantity of air is taken as the quantity of free air at the water temperature. The efficiency is taken as the ratio of useful work done to applied energy or water h.p. isothermal air h.p. Energy losses in the compressor and air piping may be considered separately, and should not be charged to the pump. Good efficiencies with best submergence and capacities of 150 g.p.m. or more, range from 45 for lifts of 500 feet to 60 per cent for lifts of 150 feet or less. The average efficiency of 22 wells at Memphis since operation was begun and including all air leakage in transmission pipes has been approximately 58 per cent, with lifts of 85 feet and capacities of 500 g.p.m. Similar results are obtained at Madison, Wis. The following formula will be found convenient for a rough estimate of the amount of free air required per gallon of water delivered by an air lift: This variable constant C may range from 327 for low lifts and high submergence to 188 for high lifts and low submergence and it also may vary with the character of the piping, the fluid to be pumped and other variants, so that experience must indicate how it should be selected. It is, therefore, advisable to consult some one having experience in this class of work for the most satisfactory results. The terms regularly used in referring to air lift pumping are explained more fully below: Static head. Normal water level when not pumping, measured from surface or top of well casing. Drop. Point to which the water level drops below the static head while being pumped. Pumping head. Level of water when pumping as compared to ground surface or top of well casing. Static head + drop head. = Elevation. Point above the ground surface or top of well casing to which water is raised. Total lift. Distance water is elevated from level when pumping to point of discharge, at an elevation, and includes: Elevation + static head + drop total lift. = = Lift. Distance the water is elevated from level when pumping to point of discharge at surface, and includes: Static head + drop lift (pumping head). Submergence. Distance below the pumping head at which the air picks up the water. 100 per cent. The vertical distance the air travels with the water from point introduced to point discharged, and includes: Total lift or lift + submergence 100 per cent. = CHAPTER XVI SERVICES Water service pipe practice has not become standardized to nearly the same extent as wiring or plumbing practice. Safety and sanitation perhaps are not so dependent upon the character of service pipe construction, but health may be affected, and considerations of economy and adequacy of service combine to favor as close adherence to standards as local conditions will allow. Installation of services, from the main to the curb or property line, by the municipality or water company is becoming increasingly common. Uniformity of construction and economy are among the advantages following the elimination of work by private plumbers in public streets. Much may be said, also, in favor of the owner of the water system installing the pipe through the foundation wall of the building to be connected. Some cities, particularly in New England, follow this practice. Opinions differ concerning the desirability of installing services at intervals in advance of permanent paving. Circumstances should govern. There can be little question of the desirability of laying connections for all existing houses even though they may not be used immediately, nor for platted lots where building is active. Where land is not subdivided, or where there is small prospect of early building operations, it will be better usually to tear up paving when necessary than to lay pipes that may lie unused for years and perhaps always. Galvanized wrought iron and steel, unlined or lined with lead, tin or cement; lead, unlined or lined with tin; cast iron; and copper and brass are used for service piping. Unlined galvanized iron and steel pipe services are commoner than any other kind. They are comparatively cheap and are not easily injured, but some waters and soils cause them to deteriorate rapidly. Active waters attack the metal and are discolored at the tap. Galvanized iron or steel pipes often become clogged to such an extent as to impair service materially. Such pipe is lined with lead, tin or cement to prevent rusting and tuberculation. |