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Positive Displacement Pumps

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Introduction to Positive Displacement Pumps

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

  • reciprocating
  • rotary
reciprocating positive displacement pump
rotary positive displacement pumps
rotary positive displacement pump

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.

Reciprocating Pumps

Typical reciprocating pumps include

  • plunger pumps
  • diaphragm pumps
reciprocating positive displacement 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.

Rotary Pumps

Typical rotary positive displacement pumps include

  • gear pumps
  • lobe pumps
  • vane pumps
  • progressive cavity pumps
  • peripheral pumps
  • screw pumps
rotary displacement gear pump rotary positive displacement gear pump

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

Codip Tubular Diaphragm Pumps Flows up to 300 l/min Maximum Discharge Head 10 Bar
FMI Valveless Metering Pumps Flows up to 4600 ml/min Maximum Discharge Head 6.9 Bar
Nikkiso MX Series Metering Pumps Flows up to 114 l/min Maximum Discharge Head 500 Bar
Wanner Hydra-Cell High Pressure Diaphragm Pumps Flows up to 130 l/min Maximum Discharge Pressure 172 Bar

Rotary Positive Displacement Pumps

HNPM Internal Gear Pumps Flows up to 1152 ml/min Maximum Discharge Head 150 Bar
Micropump External Gear Pumps Flows up to 40 l/min Maximum Discharge Head 8.7 Bar
Liquiflo External Gear Pumps Flows up to 200 l/min Maximum Discharge Head 24 Bar
Viking Gear Pumps Flows up to 345 m3/hr Maximum Discharge Head 172 Bar
Viking Rotary Vane Pumps Flows up to 36 m3/hr Maximum Discharge Head 14 Bar
3M Rotary Vane Pumps Flows up to 3 m3/hr Maximum Discharge Head 25 Bar

 

Some useful guides on Positive Displacement Pumping

When to select a Positive Displacement Pump
Positive Displacement Pump installation & operation advice sheet
Overview of types of Rotary Positive Displacement Pump
Comparing four types of Positive Displacement Pump
Viking Pump comparison between Rotary Positive Displacement Pumps and Centrifugal Pumps
Viking Pump guide on when to use a PD pump vs a Centrifugal Pump
A guide to liquids commonly pumped by Rotary Positive Displacement Pumps
Pumping abrasive liquids with Rotary Positive Displacement Pumps
Pumping fuel oils with Rotary Positive Displacement Pumps
Pumping high temperature liquids with Rotary Positive Displacement Pumps
Pumping shear sensitive liquids with Rotary Positive Displacement Pumps
Liquid containment for Rotary Positive Displacement Pumps
Gear Pumps for metering applications
NPSHR and its effect on Rotary Positive Displacement Pumps
Why use relief valves with Liquiflo Gear Pumps
 

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