Pumps can be broadly classified into two basic categories: Centrifugal pumps and Positive Displacement pumps. Centrifugal pumps have rotating equipment parts imparting momentum to the fluid particles which is later converted to pressure. For centrifugal pumps the discharge flowrate and differential head developed in the pumps are strongly dependant on each other. Whereas for positive displacement pumps, the discharge flowrate is nearly independent of the downstream pressure.
Depending on the fluid service, the flowrate handled and differential pressure requirements, either of the pump types can be used effectively.
Following figure gives an idea of the basic structure of a centrifugal pump.
Positive displacement pumps have different types as indicated in the following figures.
External gear pump
When it comes to selecting between centrifugal pumps and positive displacement pumps a number of factors have to be taken into consideration.
1. Fluid service handled: Centrifugal pumps are preferred for a wide range of fluids from clean and clear non-abrasive fluids to abrasive fluids with high solid content. But centrifugal pumps run into limitations when it comes to handling highly viscous fluids and they have very low tolerance for handling entrained gases. Positive displacement pumps (reciprocating and rotary type) are preferred for handling highly viscous fluids and fluids with entrained gases. I general, it can be stated that positive displacement pumps are equipped to handle a wider range of fluids.
2. Flowrates handled: Centrifugal pumps are preferred for handling of higher flowrates. But centrifugal pumps have limitations on handling of lower flowrates. Positive displacement pumps are preferred when it comes to low flowrates.
3. Differential pressure head: A single stage centrifugal pump cannot deliver very high discharge pressure unless it is operated at very high speeds, which can turn out to be expensive. The operation of a positive displacement pump is nearly independent of the discharge pressure encountered downstream. Positive displacement can achieve very high differential pressures and hence they are preferred for such applications.
4. Efficiency: A Centrifugal pump can operate at the best efficiency for a narrow range of flowrate and differential head values. The positive displacement pumps are not limited in such ways.
5. Space constraints: Centrifugal pumps are compact and require less space compared to reciprocating pumps handling similar flowrates. In case of positive displacement pumps, rotary type of pumps should be preferred to reciprocating pumps as rotary pumps take up less space.
6. Pulsating flow: Centrifugal pumps deliver a smooth flow. Reciprocating pumps deliver a pulsating flow profile thus requiring a pulsation dampener at the pump discharge. Rotary pumps are preferred to reciprocating pumps as they can deliver a smooth flow as well.
7. Costs: Centrifugal pumps are most widely used and regarded as economical. They have low initial cost, low maintenance cost but high power cost. Out of positive displacement pumps, reciprocating pumps are the most expensive alternatives. Reciprocating pumps high initial and maintenance cost, but they consume lower power. Rotary type of positive displacement pumps are always an attractive alternative even to the centrifugal pumps. Rotary pumps have low initial, maintenance and power costs.
8. Summary: Centrifugal pumps have been found suitable for the widest range of application and are most widely used. A variety of users prefer to use centrifugal pumps because of their familiarity with these pumps. The comparison done here indicates that often times positive displacement pumps, especially the rotary type pumps can prove to be an attractive alternative to centrifugal pumps. Use of positive displacement pumps should be specially considered for cases with low flows, entrained gases, highly viscous fluid, high differential head.