Positive Displacement Pump Working Principle:
Positive displacement pump trap liquid at the suction of the pump and pushes that quantity of liquid out to the discharge of the pump. The pressure produced by a positive displacement pump depends upon the amount of force exerted on the piston. Positive displacement pump sucks liquid and then pushed liquid forward due to the thrust applied on it by a moving member. Discharge of the liquid pumped only depends on the speed of the pump.
Centrifugal pumps have many operational and maintenance advantages over positive displacement pumps. Positive displacement pumps cannot replace with centrifugal pumps. These services involve the requirement for very high pressure, the constant preset delivery which is not affected by system changes, and pumping of various viscous liquids.
Positive displacement pumps divided into two main categories based on their design and operation.
- Reciprocating Pumps
- Rotary Pumps
1. Reciprocating Pumps:
Reciprocating Pumps consist of the cylinder and piston assembly. The piston moves forward and backward in the cylinder. This to and fro motion derived from connecting rod. Connecting rod drives through crank (crank revolves at the uniform speed). The pump has automatic control valves to regulate the flow of liquid into the cylinder and out again.
The volume of liquid moved through the pump in one cycle (one suction stroke and one discharge stroke) is equal to the change in the liquid volume of the cylinder as the piston moves from its outermost left position to its outermost right position. These pumps are obsolete now a day for industrial use due to high capital cost and running/ maintenance cost.
2. Rotary Pumps:
Rotary pumps have one or more closely fitting elements. Fitting elements continuously revolve in a fixed casing and deliver the fluid continuously. These pumps are not suitable for high-pressure operation due to more internal losses as compared to the reciprocating pump. Vane pump, screw pump, lobe pump, and gear pump are the examples of rotary pumps.
Positive Displacement Pump Characteristic Curve:
Positive displacement pumps deliver a definite volume of liquid for each suction and discharge stroke of pump operation. Therefore, the only factor that effects flow rate in an ideal positive displacement pump is the speed at which it operates. The flow resistance through the system in which the pump is operating will not affect the pump flow rate.
The dashed line in Figure shows actual positive displacement pump performance. The dashed line shows the fact that as the discharge pressure of the pump increases, some amount of liquid will leak from the discharge of the pump back to the pump suction. This leakage in liquid reducing the effective flow rate of the pump. The rate at which liquid leaks from the positive displacement pump discharge to back its suction is called slippage.
Positive Displacement Pump Protections:
These pumps protected from overpressurization through a relief valve. Relief valve locates on the upstream side of the pump discharge valve. Positive displacement pump cannot work with blocked or shut off discharge without immediately damaging the weakest member. That is why the discharge must be fitted with a relief valve which will open and prevent over pressurization and subsequent damage. Sometimes the pump is supplied with an internal relief valve but more often it is a responsibility of the user to fit the system with a relief valve which should be as close to the pump as possible without any other obstacle in the piping like a valve, pipe fitting or a heat exchanger.
The relief valve prevents system and pumps damage if the pump discharge valve is shut during pump operation. Relief valve also protects the pump during clogged or choked strainer condition.