Positive Displacement Pumps displace a known quantity of liquid with each revolution of the pumping elements. This is done by trapping liquid between the pumping elements and a stationary casing. Pumping element design include gears, lobes, rotary pistons, vanes and screws.
Positive Displacement Pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation.
The positive displacement pumps can be divided in two main classes
In general, Positive Displacement Pumps, unlike centrifugal pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure.
A Positive Displacement Pump must not be operated against a closed valve on the discharge side of the pump because it has no shut-off head like centrifugal pumps. A Positive Displacement Pump operating against a closed discharge valve will continue to produce flow until the pressure in the discharge line has increased until the line bursts or the pump is severely damaged - or both.
A relief or safety valve on the discharge side of the Positive Displacement Pump is therefore absolute necessary. The relief valve can be internal or external. The pump manufacturer has normally the option to supply internal relief or safety valves. The internal valve should in general only be used as a safety precaution, an external relief valve installed in the discharge line with a return line back to the suction line or supply tank is recommended.
Typical reciprocating pumps include
- plunger pumps
- diaphragm pumps
Plunger pumps comprise of a cylinder with a reciprocating plunger in it. In the head of the cylinder the suction and discharge valves are mounted. In the suction stroke the plunger retracts and the suction valves opens causing suction of fluid into the cylinder. In the forward stroke the plunger push the liquid out the discharge valve.
With only one cylinder the fluid flow varies between maximum flow when the plunger moves through the middle positions, and zero flow when the plunger is in the end positions. A lot of energy is wasted when the fluid is accelerated in the piping system. Vibration and "water hammers" may be a serious problem. In general the problems are compensated by using two or more cylinders not working in phase with each other.
There are two main types of diaphragm pump design:- hydraulically flexed, where a plunger pressurizes hydraulic oil which is used to flex a diaphragm in the pumping cylinder, or mechanically flexed where the diaphragm is connected to a solenoid or a conrod.
Typical rotary positive displacement pumps include
- gear pumps
- lobe pumps
- vane pumps
- progressive cavity pumps
- peripheral pumps
- screw pumps
In gear pumps the liquid is trapped by the opening between the gear teeth of two gears and the casing of the pump on the suction side. On the pressure side the fluid is squeezed out when the teeth of the two gears are rotated against each other. The motor provides the drive for one gear.
The lobe pumps operates similar to the gear pump, but with two lobes driven by external timing gears. The lobes do not make contact.
Progressive cavity pumps consist of a metal rotor rotating within an elastomer-lined or elastic stator. When the rotor turns, progressive chambers from suction end to discharge end are formed between the rotor and stator, moving the fluid.
Some designs of positive displacement pumps combine the design elements of reciprocating pumps with the design elements of rotary positive displacement pumps. The resulting rotating reciprocating pump has no mechanical valves, like a rotary positive displacement pump, but has accurate
output and typical service intervals more commonly associated with a reciprocating positive displacement pump. An example of this design is
manufactured by FMI
See our Positive Displacement Pumps by clicking on the links below
Reciprocating Positive Displacement Pumps