Positive Displacement Pump
A Positive Displacement pump (PD pump) is a mechanical device which displaces a known quantity of liquid for every revolution or cycle that the pump completes. The flow rate through a positive displacement pump is directly proportional to its speed and number of cycles over a given time.
How does a positive displacement pump work?
A positive displacement pump works by using a screw, a blade, a vane, a lobe, a gear or diaphragm. It creates a chamber or cavity between the pumping elements and the cavity in which the fluid is temporarily stored is moved by the reciprocating or rotary motion along the pipe to its destination.
API 676 Rotary Positive Displacement Pumps applies to progressive cavity pumps.
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.
What are the main applications of Positive Displacement pumps?
Positive Displacement pumps are generally used for fluids with a relatively high viscosity. They can be used where high accuracy is required e.g metering or dosing. They can also be used where high pressures are required i.e high pressure washing. Waste Water Treatment is another application e.g Netzsch Tornado Rotary Lobe Pump
What are the differences between a Positive displacement pump vs a Centrifugal pump?
The main advantages of a Positive Displacement Pump is that it can handle highly viscous fluids whereas a Centrifugal Pump would be inefficient and require high driver powers. PD pumps also have a good volumetric efficiency & driver power is kept to a minimum. The flow rate is easily adjustable via a speed control because the flow rate is directly proportional to its speed. Driver sizing is not as critical as with a Centrifugal pump because the pump will deliver the known quantity of fluids regardless of system back pressure (losses). A Positive Displacement Pump can produce a very low shear action in the case of sensitive fluids.
The main disadvantages of a Positive Displacement Pump vs a Centrifugal pump is that dry running can be catastrophic due to either close clearances of parts or in the case of progressive cavity pumps the interference fit between the rotor and stator. All PD pumps require the installation of a pressure relief valve to prevent failure of the pump or piping in case of accident or closure of the delivery valve or blockage in the piping. Main pd pumps will produce pulsations which can lead to undesirable effects i.e vibration, product damage & water hammer. PD pumps have a limited flow range ~1000m³/hr vs 180,000m³/hr Centrifugal Pumps. The material of construction of PD pumps are more limited in range of available materials than that of Centrifugal Pumps and finally PD pumps have limited solids handling ability in terms of size and/or content.
Does a Positive Displacement Pump require priming?
A Positive Displacement Pump will usually self-prime due to the very small clearances which exist within the pump. This will help it pull a vacuum and thus expel the air through the pump until the liquid reaches the pump. Care should be taken on the suction line i.e installation of a “goose neck” which will ensure there is some liquid in the pump during the priming cycle which will prevent dry running & consequently failure.
An Air Operated Diaphragm pump (AODD pump) is able to self-prime without any liquid being present but this capability to lift is limited if the line is empty of fluid.
Max. suction lift [mwc] – dry = 2.5 and wet = 5.5
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.
How we can help you?
Flexachem are the leading distributors for Netzsch Positive Displacements Pumps in Ireland – Progressive Cavity Pump, Rotary Lobe Pump (Industrial applications) & Screw Pump. We also supply Inoxpa Sanitary Rotary Lobe Pump and Flotronic Air Operated Diaphragm Pumps for the Food & Beverage & Pharma sectors.
We provide localised technical support & on-site service engineering to support the operational needs of our clients. We also hold a heavily stocked inventory to help take the pressure off you in the event of unexpected emergencies. Why not contact one of our Pump Specialists if you have a particular application in mind.
Contact our Pump Specialists:
Phil Soltan (External) – mob: 086 185 3782
Adrian McSweeney (Internal)
Paul-Fox Morris (Internal)
Fiona Foley (Internal) – Pump & Service Support
Tel: (021) 4617260