The acceleration and retardation of the plunger at the ends of the stroke causes corresponding retardations and accelerations in the supply and delivery columns, and if these are long con siderable momentum effects may be experienced. If the speed is sufficiently high the columns may be unable to maintain contact with the plunger and "cavitation" is set up, which gives rise to considerable shock in the pump. To avoid this an air vessel should always be fitted on the suction side of the pump, and often on the delivery side. This equalizes the flow along the pipe lines.
For low speed pumps, disc valves of rubber or metal, which are returned to their seat either by the action of a spring or by their own elasticity, are usually used. For high speed pumps mechani cally operated valves are often used.
Pumps of the rotary type are well adapted for working over a wide range of speeds with comparatively low head and with widely varying discharges. They have the advantage over the reciprocating pump that the discharge is prac tically continuous, while the efficiency does not fall off nearly so quickly with discharge variations as the centrifugal pump.
There are two main types of rotary pump. In one a drum mounted eccentrically in its casing carries one or more sliding diaphragms which make contact with the interior of the casing.
In the other type two rotating drums are used mounted on parallel axes and coupled together by two equal gear wheels mounted on the shafts outside the casing.
In the Humphrey Gas Pump water is raised by the direct action of the pressure accompanying the explosion and expansion of a gas-air mixture. At the beginning of the power stroke a compressed charge of air and gas in the combustion chamber C (fig. 4) is ignited by an electric spark and expands forcing the column of water D between the com bustion chamber and the delivery tank DT towards the tank and discharging part of it in the process. Due to the momentum of this column, expansion continues until the pressure at the lower part of the water column falls to atmospheric, when the suction valves V, communicating with the suction tank ST, open and water is drawn through them into the pipe. At the same time the exhaust valve is opened. When the momentum of the water column is exhausted it oscillates back, driving the products of combustion out of the combustion chamber until it reaches the exhaust valve, which it closes by impact; on the next rebound of the water column the gas inlet valve opens and a fresh charge is drawn in ; at the end of the outward oscillation the gas inlet valve is closed and the returning oscillation compresses the charge which is fired as the column comes to rest.
This consists of an open ended vertical lift pipe, of which the lower end is submerged in the liquid to be raised, and the upper end delivers into a discharge tank at the height which is required. Air is supplied from an air compressor through an air pipe to the lower end of the lift pipe, and rising in the form of small bubbles through the liquid in this pipe forms a mixture having a specific gravity lower than that of the liquid itself. The pressure of the liquid surrounding the pipe then raises the lighter mixture above the supply level and out of the top of the lift pipe. The possible lift depends upon the amount of air supplied and upon the depth of submersion of the bottom of the lift pipe. For best results the depth of submersion should be approximately 1.5 times the height of the lift. For high efficiency the air should be supplied to the lift pipe through a foot-piece having a series of small openings so arranged as to distribute the air in a series of small bubbles through the liquid. If well designed such a pump is capable of converting some 6o% of the energy in the compressed air at the foot-piece into useful work in lifting water. Air lift pumps have been constructed for lifting against heads of upwards of 400 ft., and with lift pipe diameters up to about i 5 inches.
In the jet pump a high pressure water supply is conducted to a small nozzle, whence it issues into a suction cham ber as a jet having a high velocity and a relatively low pressure. The suction chamber surrounding the jet is connected to the suc tion reservoir. If the pressure at the jet is sufficiently low, water is lifted into this chamber and is forced by the impact of the high velocity jet into a discharge pipe, whose suction gradually in creases in the direction of flow. There the kinetic energy is par tially converted into pressure energy, enabling the pump to dis charge against a head considerably higher than that of the suc tion reservoir. Owing to the loss due to eddy formation accom panying the admixture of the high and low velocity water, the efficiency of such a pump is low and does not usually exceed about 20%. When the fluid used for the high pressure jet is steam, the pump becomes the boiler feed injector so largely used in locomotive work.