A plunger pump (Figure 1.23) is similar to a piston pump, except the reciprocating member is a plunger rather than a piston. The plunger is single-acting (i.e., only has liquid discharging during the forward stroke), and the plunger is sealed with packing in the cylinder walls.
Generally speaking, plunger pumps are used for higher pressure applications than piston pumps. They are capable of the highest pressures obtainable with a PD pump, with some very special applications achieving pressures greater than 50,000 psi. Plunger pumps generally run at higher speeds than piston pumps. They are therefore usually a lower capital cost alternative to a piston pump if both are being considered. However, the plunger pump may have higher maintenance expense and lower abrasion resistance than a piston alternative.
For all types of reciprocating pumps, rather large pressure pulsations are produced. These pulsations become more smoothed out the higher the number of reciprocating members. Even so, quite often it is necessary to fit pulsation dampener devices downstream of the pump discharge. A pulsation dampener is a vessel that is separated in the middle with a bladder or membrane and that has air or nitrogen or a neutral gas on the upper half of the vessel. The membrane flexes, and the air compresses and dampens out the pulsations produced as the reciprocating pump strokes.