Reciprocating positive displacement pumps are generally categorized in four ways: direct-acting
or indirect-acting; simplex or duplex; single-acting or double-acting; and power pumps.
Direct-Acting and Indirect-Acting Pumps
Some reciprocating pumps are powered by prime movers that also have reciprocating
motion, such as a reciprocating pump powered by a reciprocating steam piston. The piston
rod of the steam piston may be directly connected to the liquid piston of the pump or it may
be indirectly connected with a beam or linkage. Direct-acting pumps have a plunger on the
liquid (pump) end that is directly driven by the pump rod (also the piston rod or extension
thereof) and carries the piston of the power end. Indirect-acting pumps are driven by means
of a beam or linkage connected to and actuated by the power piston rod of a separate
Simplex and Duplex Pumps
A simplex pump, sometimes referred to as a single pump, is a pump having a single liquid
(pump) cylinder. A duplex pump is the equivalent of two simplex pumps placed side by
side on the same foundation.
The driving of the pistons of a duplex pump is arranged in such a manner that when one
piston is on its upstroke the other piston is on its downstroke, and vice versa. This
arrangement doubles the capacity of the duplex pump compared to a simplex pump of
Single-Acting and Double-Acting Pumps
A single-acting pump is one that takes a suction, filling the pump cylinder on the stroke in
only one direction, called the suction stroke, and then forces the liquid out of the cylinder
on the return stroke, called the discharge stroke. A double-acting pump is one that, as it
fills one end of the liquid cylinder, is discharging liquid from the other end of the cylinder.
On the return stroke, the end of the cylinder just emptied is filled, and the end just filled
is emptied. One possible arrangement for single-acting and double-acting pumps is shown
in Figure 13.
Power pumps convert rotary motion to low speed reciprocating motion by reduction
gearing, a crankshaft, connecting rods and crossheads. Plungers or pistons are driven by
the crosshead drives. Rod and piston construction, similar to duplex double-acting steam
pumps, is used by the liquid ends of the low pressure, higher capacity units. The higher
pressure units are normally single-acting plungers, and usually employ three (triplex)
plungers. Three or more plungers substantially reduce flow pulsations relative to simplex
and even duplex pumps.
They can be driven by either electric motors or turbines. They are relatively expensive pumps
and can rarely be justified on the basis of efficiency over centrifugal pumps. However, they are
frequently justified over steam reciprocating pumps where continuous duty service is needed due
to the high steam requirements of direct-acting steam pumps.
In general, the effective flow rate of reciprocating pumps decreases as the viscosity of the fluid
being pumped increases because the speed of the pump must be reduced. In contrast to
centrifugal pumps, the differential pressure generated by reciprocating pumps is independent of
fluid density. It is dependent entirely on the amount of force exerted on the piston. For more
information on viscosity, density, and positive displacement pump theory, refer to the handbook
on Thermodynamics, Heat Transfer, and Fluid Flow.