Positive Displacement Pumps

A Positive Displacement Pump has 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 a constant given each cycle of operation.

The positive displacement pumps can be divided in two main classes

• reciprocating
• rotary

The positive displacement principle applies whether the pump is a

• rotary lobe pump
• progressing cavity pump
• rotary gear pump
• piston pump
• diaphragm pump
• screw pump
• gear pump
• vane pump
• regenerative (peripheral) pump
• peristaltic

A Positive Displacement Pump, unlike a Centrifugal or Roto-dynamic Pump, will produce the same flow at a given speed (RPM) no matter the discharge pressure.

• A Positive Displacement Pumps is a "constant flow machine"

A Positive Displacement Pump must never operate against closed valves on the discharge side of the pump - it has no shut-off head like Centrifugal Pumps. A Positive Displacement Pump operating against closed discharge valves continues to produce flow until the pressure in the discharge line is 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 absolute necessary. The relief valve can be internal or external the pump. An 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 highly recommended.

Reciprocating Pumps

Typical reciprocating pumps are

• plunger pumps
• diaphragm pumps

Plunger pumps consists 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.

In a diaphragm pump the plunger pressurizes hydraulic oil which is used to flex a diaphragm in the pumping cylinder. Diaphragm pumps are used to pump hazardous and toxic fluids.  

Speed Correction in Reciprocating Pumps vs. Viscosities 

Example - Speed Correction of Reciprocation Pump vs. Fuel Oil Viscosity

The viscosity of fuel oil nr. 2 at 20 ^oC is about 150 SSU. The viscoity of fuel oil type 5 at 100 ^oC is about 4000 SSU. According the table above the speed of the reciprocating pump should be reduced with approximately 30%.  

Speed Correction in Reciprocating Pumps vs. Water Temperatures

Rotary Pumps

Typical rotary pumps are

• gear pumps
• lobe pumps
• vane pumps
• progressive cavity pumps
• peripheral pumps
• screw pumps

In a gear pump the liquid is trapped by the opening between the gear teeth of two identical gears and the chasing 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.

A lobe pump operates similar to a gear pump, but with two lobes driven by external timing gears. The lobes do not make contact.

A progressive cavity pump 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. 

Speed Correction in Rotary Pumps vs. Viscosities