Displacement Pump

In reciprocating displacement pumps, water is drawn into the pump chamber or cylinder on the suction stroke of the piston or plunger inside the pump chamber and then the water is pushed out on the discharge stroke. This is a simplex or single-acting reciprocating pump. An air chamber (Figure 2.17) should be provided on the discharge side of the pump to prevent excessive water hammer caused by the quick-closing flap or ball valve; by the quick closing or opening of a gate valve, float valve, or pressure-reducing valve; and the sudden shutdown of a pump. The air chamber or other surge suppressor will protect piping and equipment on the line and will tend to even out the intermittent flow of water. See "Water Hammer," this chapter. Reciprocating pumps are also of the duplex type wherein water is pumped on both the forward and backward stroke, and of the triplex type, in which three pistons pump water. The motive power may be manual; a steam, gas, gasoline, or oil engine; an electric motor; or a windmill. The typical hand pump and deep-well plunger or piston pumps over wells are displacement pumps.

A rotary pump is also a displacement pump, since the water is drawn in and forced out by the revolution of a cam, screw, gear, or vane. It is not used to any great extent to pump water.

Displacement pumps have certain advantages over centrifugal pumps. The quantity of water delivered does not vary with the head against which the pump is operating but depends on the power of the driving engine or motor. A pressure-relief valve is necessary on the discharge side of the pump to prevent excessive pressure in the line and possible bursting of a pressure tank or water line. They are easily primed and operate smoothly under suction lifts as high as 22 feet. Practical suction lifts at different elevations are given in Table 2.12.

Displacement pumps are flexible and economical. The quantity of water pumped can be increased by increasing the speed of the pump, and the head can vary within wide limits without decreasing the efficiency of the pump. A displacement pump can deliver relatively small quantities of water as high as

Pump discharge I Row
Air Chamber Dimensions

Discharge Pipe

Inside Diameter of Air Chamber

Total Height

a

b

2"

8"

3'0"

4"

9"

2j"

8"

3'6"

4"

12"

3"

10"

4'0"

5"

15"

4"

10"

5'0"

6"

21"

5"

12"

6'0"

6"

27"

6"

16"

TO"

6"

33"

FIGURE 2.17 Air chamber dimensions for reciprocating pumps. (Source: Water Supply and Water Purification, T.M. 5-295, War Department, Washington, DC, 1942.)

FIGURE 2.17 Air chamber dimensions for reciprocating pumps. (Source: Water Supply and Water Purification, T.M. 5-295, War Department, Washington, DC, 1942.)

800 to 1,000 feet. Its maximum capacity is 300 gpm, although horizontal piston pumps are available in sizes of 500 to 3,000 gpm. The overall efficiency of a plunger pump varies from 30 percent for the smaller sizes to 60 to 90 percent for the larger sizes with electric motor drive. It is particularly suited to pumping small quantities of water against high heads and can, if necessary, pump air with water. This type of pump is no longer widely used.

TABLE 2.12 Atmospheric Pressure and Practical Suction Lift

Elevation Above Sea level Atmospheric Pressure

Design Suction Life (ft)

ft miles lb/in.2 ft of water Displacement Centrifugal Turbine

TABLE 2.12 Atmospheric Pressure and Practical Suction Lift

Elevation Above Sea level Atmospheric Pressure

Design Suction Life (ft)

ft miles lb/in.2 ft of water Displacement Centrifugal Turbine

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