3 replies to this topic

[Subject_Name_Here]

Hi everyone,

 

Just had a few general questions about positive displacement pumps, or more specifically, external gear pumps.

 

1. Am I correct in assuming that a gear pump will produce a constant flow of water regardless of the suction head as long as the inlet's filled?

2. Also, is it ok for a gear pump to run dry? What about air-operated diaphragm pumps? I know with centrifugals this is not the case, but I'm not exactly sure how this is harmful to the pump.

3. Are positive displacement pumps more or less prone to cavitation?

 

Any input would be much appreciated. Thank you.

[ankur2061]

1. At a constant speed a PD pump will provide a constant volumetric flow theoretically. However, 'slip' increases in PD pumps over a period of time due to wear and tear and the increase in 'slip' will cause the volumetric flow to decrease over long periods of operation.

 

2. Gear pumps do not get affected in the way that centrifugal pumps are affected due to running dry. However running a gear pump dry can lead to following problems:

 

a. Air leakage into the system. If the liquid being pumped is incompatible with air causing oxidative degradation and / or creating an explosive process liquid - air mixture, the whole pumping system is endangered from operation and / or safety point of view.

 

b. Liquids provide lubrication and heat dissipation to the rotating parts in contact with the process. Lack of lubrication by no liquid flow means heat build-up and excessive and rapid wear and tear. This can lead to premature failure.

 

c. Dry running can lead to excessive noise and vibration which again can lead to mechanical failure. This is one form of cavitation only but the severity is somewhat less compared to a centrifugal pump.

 

3. PD pumps are more capable of handling dissolved gases compared to centrifugal pumps which is self-explanatory.

 

Hope this helps.

 

Regards,

Ankur.

[Subject_Name_Here]

Thank you!

[TS1979]

Pump damage depends on pump design. Pneumatic pump is less likely to be damaged by running dry. Other kind pumps are more likely to be damaged when you run the pump dry. Remember that energy input to the pump has to go somewhere. If you don't have liquid to transfer, the energy will convert to heat and damage the pump.

 

Generally speaking, positive displacement pumps are more prone to cavitation because in the NPSH calculation formula, PD pump needs to consider liquid accelerating head loss.