Plunger pumps are used for very high pressure applications, 1000 psig or greater. Examples are glycol circulation and amine circulation in gas plants. See Figure 14. A plunger of constant diameter replaces the piston and piston rod. This provides the mechanical strength needed for high pressure.
Plunger pumps are single acting, that is, they pump on the forward stroke only. A pump that has a single cylinder is called simplex. A pump with two cylinders operating in parallel is a duplex, and three cylinders in parallel form a triplex.
Multiple cylinders in parallel have two advantages:
• Higher capacities are possible without large diameter cylinders, an advantage at high pressures.
• The magnitude of flow rate pulsations is reduced because discharge strokes are staggered.