Positive displacement pumps move a fixed volume of liquid per unit of time. Although less common than centrifugal pumps, they are used for certain special applications within Saudi Aramco. They are used for services with relatively low flow rates, services with high differential pressures, or for liquids that have high viscosities.
The two major types of positive displacement pumps used by Saudi Aramco are reciprocating pumps and rotary pumps.
In a reciprocating pump, a piston moves back and forth within a cylinder. Liquid enters the cylinder during the suction or intake stroke. Liquid is forced out of the cylinder during the discharge stroke. Refer to Figure 11.