Positive Displacement Pump

What are the types of Positive Displacement Pumps?

The main different types of Positive Displacement Pumps are;

• Progressive Cavity Pump (PC Pump)
• Gear Pump – Internal & External
• Vane Pump (Impeller Pump)
• Rotary Lobe Pump
• Screw Pump
• Diaphragm Pump
• Peristaltic Pump (Hose pump)

Progressive Cavity Pump has a rotor rotating within a housing called a stator. The rotor is always metallic and the stator is made up of a rubber type of material. It looks somewhat like a screw thread – the fluid is between the cavities and the rotary motion of the rotor forces the fluid through from one end to the other. It has a low to moderate capacity, low to high pressure, good solids handling capability, one seal, low shear, constant flow and a low pulsation.

Gear pumps are available in 2 configurations;

1)Internal gear which has a moderate capacity, low to moderate pressure, max 14 Bar/ Spl 17 Bar, no solids handling capability, one seal and has a constant flow.

2)External gear has a moderate capacity, moderate to high pressure, Standard 20 Bar /Spl 250 Bar. It has one seal, no solids handling capability and a constant flow.

Vane Pump (Impeller Pump) has moderate capacity, low pressure, moderate solids handling capability, one seal and a constant flow.

Rotary Lobe Pump has moderate to high capacity, low to moderate pressure, good solids handling capability, two/four seals, a constant flow and moderate pulsation.

Screw Pump – the screw pump has multi versions known as multi screw pumps featuring moderate to high capacity, high pressures, only lubricative liquids, no solids handling capability, one seal and a constant flow.

Diaphragm Pump – Air Operated Diaphragm Pump has low to moderate capacity, low to moderate pressure, very low efficiencies, no seal and high pulsation.

Peristaltic Pump (Hose pump) has moderate capacity, low pressure, good solids handling capability, low shear, no seal and high pulsation.

Can Positive Displacement Pumps run dry?

Certain Positive Displacement Pumps can run dry  i.e Air Operated Diaphragm pumps have no parts requiring lubrication or no close clearances between parts. Peristaltic pumps can run dry as the hose is lubricated in a bath of its own fluid. Other types of PD pumps should not be run dry.

Is NPSH required for Positive Displacement Pumps

Every pump has a NPSH (Net pressure suction head) required to ensure reliable and trouble-free operation without damage caused by cavitation therefore therefore the  system should be designed to ensure there is a sufficient margin between NPSHA (Net pressure suction head available) and NPSHR (Net pressure suction head required).

What is the Positive Displacement pump discharge pressure control?

Unlike a centrifugal pump which produces pressure, a positive displacement pump does not produce pressure – it is the system itself that develops pressure from the pressure drop which then creates a back pressure which largely depends on the flow rate through the system i.e higher flow rates will result in higher losses and as a result a higher back pressure. The back pressure is also dependent on the pressure in the vessel at the point of discharge i.e a hydrogen blanket present or steam. The pressure is controlled largely by the pumping rate, therefore, pressure is controlled by varying the speed of the pump. In cases where the variable speed drive is not deployed the system pressure will be controlled to a degree by the setting of the pressure relief valve.

What are Positive Displacement Metering Pumps?

Positive Displacement Metering pumps are usually used where a high degree of accuracy is required e.g in dosing applications where pH control is required e.g Waste Water Treatment Plants or where filling lines require accuracy of volumes of fluid dispersed into containers.