What Is a Positive Displacement Pump?
A positive displacement (PD) pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms, or vanes.
Positive displacement pumps add energy to a fluid by applying force to the liquid with a mechanical device such as a piston or plunger. A positive displacement pump decreases the volume containing the liquid until the resulting liquid pressure equals the pressure in the discharge system.
That is, the liquid is compressed mechanically, causing a direct rise in potential energy. Most positive displacement pumps are reciprocating pumps in which the linear motion of a piston or plunger in a cylinder causes the displacement.
In rotary pumps, another common positive displacement pump, a circular motion causes the displacement. There are several manufacturers of positive displacement pumps which are often found in high-pressure services.
How Does Positive Displacement Pump Work?
A positive displacement pump will draw fluid into the pump chamber at an inlet valve and then discharge it through an outlet valve. Typically, this may involve a rotary, reciprocating, or diaphragm system that moves the fluid through the pump. It is a repeating cycle that generates consistent flow rates. Different pumps will use different methods to achieve this action.
A positive displacement pump makes a fluid move by trapping a fixed amount of the fluid and forcing (displacing) that trapped volume into a discharge pipe or discharge system.
Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume remains constant through each cycle of pump operation.
Positive Displacement pumps do not use impellers but rely on rotating or reciprocating parts to directly push the liquid in an enclosed cavity until enough pressure is built up to move the liquid into the discharge system.
The pump does not rely on raising the velocity of the fluid as the centrifugal pump does by moving the liquid through the impeller. Consequently, the fluid velocity inside a positive displacement pump is much lower than that of a centrifugal pump. This is often a desirable feature for certain applications, such as when needing to pump a media containing fragile solids.
Positive displacement pump characteristics
Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, can theoretically produce the same flow at a given speed (RPM) no matter what the discharge pressure. Therefore, positive displacement pumps can be regarded as constant flow devices. However, a slight increase in internal leakage as the pressure increases can prevent a truly constant flow rate.
In application, a positive displacement pump must not be allowed to operate against a closed valve on the discharge side of the pump, because it has no shutoff head like a centrifugal pump.
With the pump operating against a closed discharge valve, it will continue to produce flow and the pressure in the discharge line will increase until the pipeline can either fracture or the pump can become severely damaged, or both.
To prevent this, a relief or safety valve on the discharge side of the positive displacement pump is therefore often necessary. This relief valve can be positioned either internally or externally to the pump. The pump manufacturer normally has the option to supply internal relief or safety valves.
An internal valve is usually only used as a safety precaution, but an external relief valve in the discharge line, with a return line back to the suction line or supply tank, will provide increased safety.
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)
Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
- 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.
5 Reasons to Choose a Positive Displacement Pump
The decision to choose a positive displacement pump over a centrifugal is not always a clear one. They each have very different behaviors and if not familiar with them, it may be difficult to understand how a positive displacement pump could fit into your process.
Positive displacement pumps are a more efficient choice than centrifugal pumps in some situations. If any of the conditions or applications below are in your process, a positive displacement pump should probably be used.
1. High Viscosity
Centrifugal have issues pumping viscous liquids, becoming very inefficient at even modest levels. Positive displacement pumps, on the other hand, have no problem moving thick liquids.
2. Low Flow
Oftentimes, a centrifugal pump is run off its BEP when a lower flow is desired, much to the pump’s detriment. Running a centrifugal off its BEP can cause excessive energy consumption, damage to the pump, and overall poor performance.
A positive displacement pump, however, is well suited for these conditions, providing a constant flow of fluid at a given pump speed. If you’re trying to get the pressure or flow you need by operating a centrifugal pump off its best efficiency point (BEP), a positive displacement pump may be a better choice.
Positive displacement pumps are an ideal choice for metering applications. They deliver constant flow, allowing them to easily meet process requirements. Some common types of positive displacement pumps used for metering are:
4. High Pressure Requirements
Positive displacement pumps are excellent for applications that require high pressure, with some models producing over 1,000 psi (2,300 ft). Due to the positive displacement pump’s design, however, if kept in operation against a closed discharge valve, it will continue to build pressure until the line bursts, the pump is damaged, or both.
5. Shear Sensitive Liquids
Centrifugal pumps generally operate at higher speeds when compared to positive displacement pumps. the higher speeds can shear liquids, making centrifugal pumps a poor choice for liquids like tomato paste and latex paint. Positive displacement pumps operating at lower speeds can be gentler on products and are usually preferred in these types of applications.
Applications of Positive Displacement Pumps
These pumps are commonly used to pump high viscosity fluids where precise dosing otherwise high force output can be necessary. Not like centrifugal pumps, the outputs of these pumps are not affected by force thus they also have chosen in any condition where the supply is unequal. The best positive displacement pump examples are piston, plunger, diaphragm, gear, lobe, screw, and vane.
- Piston and Plunger pumps are used to pump low viscosity fluids, paint spraying, oil production, and high force washing.
- Diaphragm pump can be used for metering, spraying, treatment of water, oils, and paints.
- Gear pumps are used for pumping the high viscosity fluids within the petrochemical, food industries, paints, oils, etc.
- Lobe pump is used in food and chemical industries pharmaceutical, biotechnology, sanitary, etc.
- Screw pump is used in fuel transferring, production of oil, irrigation, etc
- Vane pump is used in low viscosity liquids, fuel loading, & transmission, etc.
A positive displacement (PD) pump is used to move a liquid frequently with a set volume, with the help of valves otherwise seals by moving it automatically throughout the system. The action of pumping is repeated & can be driven by screws, pistons, lobes, gears, vanes, diaphragms. These pumps are mainly used where highly viscous fluids are required.
Advantages Of Positive Displacement Pumps
There are many benefits of positive displacement pumps for varying applications. Below we outline the main reasons many operations choose to use them.
- PD pumps are sometimes called constant-volume pumps because they maintain a constant speed and flow. Even if the system pressure varies, the flow remains constant.
- PD pumps can handle a variety of fluid types: high, low and variable viscosity; shear sensitive fluids; solids; and liquids with a high percentage of air or gas entrainment.
- Their capacity is not affected by the operation pressure.
- They are excellent for applications with flows below 100-gpm and above 100-psi.
- They can be 10 to 40 points more efficient than centrifugal pumps when handling viscous fluids.
- PD pumps are able to self-prime.
- They can be designed as a sealless pump.
Disadvantages of Positive Displacement Pumps
- These are less efficient pumps than dynamic pumps.
- Low efficiency.
- These can’t deliver pulsating free flow.
- These are not best for high-pressure applications.
- They generate high noise.
- Low discharge capability.