Why the name comes positive displacement pump?

Question

Pumps like vane pumps, gear pumps are cumulatively named as positive displacement pump why it is named so?

Answer 1

The piston positively displaces the oil or water .

Answer 2

There are several things to think about. The head as defined is the total head produced by the pump but that is only a consideration when it comes to selecting the driver size and the design pressure for the case. Head is expressed in meters of the liquid being pumped and can be converted to pressure using the specific gravity of the liquid. In addition to being sure the pump case will contain the pressure (with some safety factor) you need to be sure the piston, rod and crankshaft will stand up to the pressure acting on the piston at the peak of the discharge stroke. Most positive displacement pumps deliver a fixed volume unless you devise some way for the piston to have a variable stroke or you by pass some of the liquid back to the suction as a means of controlling the flow. The other way to control flow is to have a variable speed driver. One final thing I might mention. With a PD pump the pump will produce what ever discharge pressure is required regardless of flow rate. It will continue to produce pressure until the case fails, the drive train fails or the motor is over loaded. For this reason they always need a pressure safety valve on the discharge. This is sometimes built into the pump and in other cases is added on the discharge piping right at that discharge of the pump. There are other types of PD pumps including gear pumps, diaphragm and peristaltic pumps. You should look these up on the web before you go to far with the piston design. There are also some so called piston PD pumps that are really plunger pumps. You should check these out as well.

Answer 3

positive displacement means displacement or discharge or work will be always on the positive side of displacement(motion)