A positive displacement (PD) pump transports fluid through a system by periodically enclosing a set volume of fluid and moving it mechanically. Pistons, screws, gears, rollers, diaphragms, and vanes can all drive the pumping motion.
In this article, you will learn the working principles of a positive displacement pump, its functionality vs. a centrifugal pump, and examples.
Working Principles of Positive Displacement Pump
Liquid flows into positive displacement pumps as the cavity on the suction side expands, and fluid flows out of the discharge side as the cavity collapses. Each cycle of operation maintains a consistent volume. Reciprocal and rotary positive displacement pumps are the two primary positive displacement pumps.
On the discharge side of the pump, positive displacement pumps should not run against closed valves. They do not have a shut-off head like centrifugal pumps. If that’s the case, running against closed discharge valves keeps the flow going until the pressure in the discharge line is raised to the point where the line bursts or the pump is severely damaged.
Positive Displacement Pump vs. Centrifugal Pump
What is a Centrifugal Pump?
A centrifugal pump is a mechanical device that moves a fluid by transferring rotational energy from one or more impellers or driven rotors.
Both a PD pump and a centrifugal pump move fluids. However, there are some significant distinctions between them.
Mechanics of PD Pump vs. Centrifugal Pump
The mode of operation provides a clear difference between the two. Centrifugal pumps impart velocity to the liquid, resulting in pressure at the outflow. PD pumps collect and transport small fluid volumes from the suction to the discharge port. Thus, centrifugal pumps generate pressure via flow, whereas PD pumps generate flow via pressure.
Performance of PD Pump vs. Centrifugal Pump
Since flow results in pressure, the flow of centrifugal pumps varies as the pressure changes. Because PD pumps work in the opposite direction, changes in pressure do not affect their flow. The speed of both pump types can change to adjust the flow.
A rotating PD pump’s flow rate is proportional to its speed (typically more than 10:1) and is essentially unaffected by differential pressure. This is not the case with centrifugal pumps which are much more sensitive to inlet conditions.
Positive Displacement Pump vs. Centrifugal Pump Consistency
The efficiency of centrifugal pumps diminishes as fluid viscosity rises. Frictional losses within the pump cause this. As a result, these pumps rarely utilize on viscosities of more than 850 cSt and are a favorite for large scale industrial air applications. A centrifugal pump’s performance begins to deteriorate once viscosity reaches 100 cSt.
On the other hand, PD pumps rise in efficiency with fluid density.
Positive Displacement Pump vs. Centrifugal Pump Efficiency
Centrifugal pumps are at their most efficient when they are running at full speed. The efficiency reduces as the pressure level rises or falls. This pump is adequate within a range of 80-110 percent of its BEP. The efficiency of positive displacement pumps increases as the pressure increases.
Inlet Conditions of Both Pump
A centrifugal pump cannot deliver a GVF (Gas Volume Fraction) greater than 15%. As a result, the liquid must pump into the suction piping and the pump. It is impossible for a dry pump to prime on its own.
On the other hand, a negative pressure forms at the inlet port of positive displacement pumps (suction). The pump must be filled with liquid at least once. NPSHr (Net Positive Suction Head Required) values typically range in only a few feet, thus enabling the handling of gas and low density applications.
Positive Displacement Pump Examples
Examples of the best PD pump include piston and plunger, diaphragm, gear, lobe, screw, and vane.
Piston and Plunger Pump
A reciprocating positive displacement pump with pistons and plungers is known as a piston and plunger pump. It pumps low viscosity fluids, paint spraying, oil production, and high force washing.
Next, a diaphragm pump works by combining the reciprocating motion of a diaphragm composed of Teflon, thermoplastic, or rubber with the corresponding valves on both sides of the diaphragm (globe valves, flap valves, butterfly valves, check valves, or any other form of the valve). It is used for metering, spraying, treatment of water, oils, and paints.
This type of positive displacement pump transfers fluid by enclosing a set volume with interlocking cogs or gears and mechanically transferring it with a cyclic pumping motion. Gear pumps see their primary application in pumping high viscosity fluids within the petrochemical, food, and heavy industries. They are well suited for the pumping of paints and oils.
Rotary lobe pumps are positive-displacement pumps that transfer liquids by rotating two or more lobes around parallel shafts in the pump’s body. Rotary lobe pumps see use in pharmaceutical, biotechnology, sanitary, food, and chemical industries.
A screw pump, also known as a water screw, is a positive-displacement (PD) pump that moves fluid solids or liquids along the axis of one or more screws. It sees use in fuel transfer, petrochemical, and irrigation applications.
A rotary vane pump is a positive-displacement pump with vanes positioned on a rotating rotor inside a chamber. The vane pump transmits low viscosity liquids well.