The positive displacement pump provides an approximate constant flow at fixed speed, despite changes in the counter pressure. Two main types of positive displacement pumps exist:
- Rotary pumps
- Reciprocating pumps
This is further broken up into sub-groups:
- Helical rotor or progressive cavity pumps
- Peristaltic or hose pumps
- Rotary Lobe or gear pumps
- Diaphragm pumps
- Piston pumps
- Bucket pumps (windmills)
The difference in performance of a centrifugal pump (non positive displacement), compared to a rotary pump and a reciprocating pump is illustrated in Figure 1. Depending on which of these pumps you are dealing with, a small change in the pump’s counter pressure results in differences in the flow.
The flow from a centrifugal pump will change considerably. The flow of a rotary pump will change a little, while the flow of a reciprocating pump will hardly change at all.
So, why is there a difference between the pump curves for reciprocating pumps and rotary pumps?
The actual seal face surface is larger for rotary pumps than for reciprocating pumps. Even though the two pumps are designed with the same tolerances, the gap loss (fluid slippage) of the rotary pump is larger.
These pumps are typically designed to the finest tolerances to obtain the highest possible efficiency and suction capability. However, in some cases, it is necessary to increase the tolerances, for example when the pumps have to handle highly viscous liquids, liquids containing particles or liquids of high temperature.
Positive displacement pumps all pulsate, meaning that their volume flow within a cycle is not constant. The variation in flow and speed leads to pressure fluctuations due to resistance in the pipe system and in valves.
A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe.
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 decreases. The volume is constant through each cycle of operation.
Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, theoretically can produce the same flow at a given speed (RPM) no matter what the discharge pressure. Thus, positive displacement pumps are constant flow machines. However, a slight increase in internal leakage as the pressure increases prevents a truly constant or linear flow rate.
A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both.
A pressure relief or safety valve on the discharge side of the positive displacement pump is therefore necessary. The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves. The internal valve is usually only used as a safety precaution. An external relief valve in the
discharge line, with a return line back to the suction line or supply tank provides increased safety.
Grundfos utilize two types of positive displacement pumps in our equipment
- Helical rotor in our SQ range of bore hole pumps
- Diaphragm in our DDA,DMX, DMH, etc. chemical dosing range of pumps