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[justcrazy85]

Dear all,

I am a young chemical engineer at the beggining of my carrier. I need to design a volumetirc pumping system, but unfortunately all my academic studies about pumps were focused on centrifugal ones, so my background on this topis is poor.
I have 5 questions:

1) are positive displacement pumps a kind of volumetric pumps? which is the main difference between positive displacement pumps, metering pumps, and piston pumps?

2) In case of centrifugal pumps, discharge pressure changes by liquid according to its specific gravity, but the discharge head (in meters) is always the same for any liquid, once you have defined the pumping system. Is it true even in case of volumetric pumps (metering, Positive displacemente) ?

3) which is cavitation effect on volumetric pumps? I guess that pump will work anyway, but discharge pressure shall be lower in case of cavitation. Moreover you could have pump failures due to vibrations as with centrifugal. Is it true?

4) I heard that in case of incipient cavitation a centrifugal pump will absorbe more power. Can you explain me the reason? is it true even in case of volumetric pumps?

5) My volumetric pump has to elaborate a flammable liquid, so it is very important to avoid cavitation foir safety reasons. I was suggested to install a device to measure the power absorbed by pump,so that if cavitation is occurring then pump will absorbe more energy and the system will automatically shut down the pump. Do you think this system is efficient?


Thanks for your help and sorry for my long message

Regards and Wishes

Antonio

[GS81Process]

1. Positive displacement (PD) pumps operate at a constant volumetric flow rate. Metering and piston pumps are types of PD pumps.

2. Discharge pressure of a centrifugal pump is typically controlled by discharge throttling. This does not work for PD pumps. PD pumps can pressurize a fixed volume of liquid up to an indefinitely high pressure and so they require a PSV. The operating curve of a PD pump is like a straight vertical line, in comparison with a centrifugal pump curve. PD pumps are typically controlled by recirculating a portion of the fluid to the suction, or by adjusting the pump speed via VFD.

3. Cavitation on PD pumps is as serious as it for centrifugal pumps. It will damage the pump. Acceleration head of the fluid has to been considered in a PD pump's NPSHr. The NPSHa for a PD pump must exceed the NPSHr (plus appropriate margin)- same as for a centrifugal pump.

4. Not sure about this.

5. I've never heard of using pump power to predict cavitation and I don't think it would work. Typically cavitation is avoided by ensuring via piping design that the pump NPSHa will always be greater than NPSHr (plus margin) for the worst case (typically highest temperature) fluid operation.