Positive Displacement Pumps
These pumps work on principle of trapping the fluid and pushing it forward/out. Fixed volume of fluid is trapped in suction side and pushed/squeezed to the discharge side. These pumps can be:
1. Reciprocating type.
2. Gear type.
3. Vane type.
4. Screw type.
5. Diaphragm type.
Since these pumps work on moving fixed volume of fluid over time, the amount of fluid transferred will depend on the speed of the pump (stroke/sec or rev/sec). These pumps have very fine clearances between the moving and stationary parts in order to minimise the internal leakages. Positive displacement pumps develop high pressure and should be always operated with discharge valve in open condition. Special attention is given to the Safety valve that forms integral part of the pump body.
In order to maintain good efficiency of these pump the clearances have to be maintained very well. So it is of utmost importance to keep suction filters clean and maintain integrity of filters and any dirt/ foreign objects will cause abrasion thereby increasing clearance. This will cause internal leaks and reduce the fixed volume that can be trapped.
For reciprocating pumps suction, delivery valves and piston rings must be kept in good condition. These pump creates negative suction pressure/partial vacuum on suction side, so it is of utmost importance to keep eye on the tank levels. The pump should not be allowed to run dry as it will cause metal to metal contact/ metal to rubber contact leading to either pump seizure or weardown of the clearances.