Piston Pumps

Piston pumps are used for moderately high pressures. They are used in some special services, such as emergency and shutdown services, where pipelines must be completely emptied. Mixtures of liquid and vapor are encountered in these services, and the reciprocating pump is the most reliable.

Figure 11 shows the major components of a piston pump: the cylinder, the piston that moves back and forth, and the suction and discharge valves in the ends of the cylinder. The left side of Figure 11 shows the intake stroke. During the intake stroke, the piston moves away from the end of the cylinder, increasing the volume inside the cylinder. The decreasing pressure inside the cylinder causes the suction valve to open and liquid to flow into the cylinder. The higher pressure on the discharge side keeps the discharge valve closed and this is no flow out of the pump.

The right side of Figure 11 shows the discharge stroke. Now the piston moves toward the end of the cylinder, forcing the liquid out of the cylinder. The higher pressure inside the cylinder pushes open the discharge valve but keeps the suction valve shut.

Some piston pumps are double acting in order to increase capacity and reduce pulsations. Figure 12 shows a double acting piston pump. There are cylinder chambers on both sides of the piston. While the left end of the cylinder is going through the suction stroke, the right end of the cylinder is discharging. The opposite happens when the piston moves back in the other direction. There is flow out of the pump in both directions of piston travel.

Other important components of a piston pump are:
• Piston rod – transfers the energy from the drive mechanism to the piston.
• Packing – surrounds the rod at the point where it enters the cylinder to prevent liquid leakage.
• Drive mechanism – includes a motor and a mechanism for changing the rotating motion to a reciprocating motion. See Figure 13.

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