In a reciprocating pump, a volume of liquid is drawn into the cylinder through the suction valve on the intake stroke, and is discharged under positive pressure through the outlet valves on the discharge stroke. The discharge from a reciprocating pump is pulsating and changes only when the speed of the pump is changed. This is because the intake is always a constant volume.
Often an air chamber is connected on the discharge side of the pump to provide a more even flow by equalizing the pressure surges. Reciprocating pumps are often used for sludge and slurry. One construction style of a reciprocating pump is the direct-acting steam pump.
This consists of a steam cylinder end in line with a liquid cylinder end, with a straight rod connection between the steam piston and the pump piston or plunger. The pistons are double acting, which means that each side pumps on every stroke.
Another construction style is the power pump, which converts rotary motion to low speed reciprocating motion using a speed reducing gear. The power pump can be either single or double-acting. A single-acting design discharges liquid only on one side of the piston or plunger; only one suction stroke and one discharge stroke per revolution of the crankshaft can occur. The double-acting design applies suction and discharges on both sides of the piston, resulting in two suctions and discharges per crankshaft revolution. Power pumps are generally very efficient and can develop high pressures. These pumps do, however, tend to be expensive.
Typical reciprocating pumps are:
- [Plunger pumps] – a reciprocating plunger pushes the fluid through one or two open valves, closed by suction on the way back.
- [Diaphragm pumps] – similar to plunger pumps, where the plunger pressurizes hydraulic oil which is used to flex a diaphragm in the pumping cylinder. Diaphragm valves are used to pump hazardous and toxic fluids.
- [Piston pumps] displacement pumps – usually simple devices for pumping small amounts of liquid or gel manually. The common hand soap dispenser is such a pump.
- [Radial piston pumps] - a form of hydraulic pump where pistons extend in a radial direction.