Does cavitation occur in positive displacement pumps?

[u/OutrageousBack9507]

Our prof asked us this question and i really don’t know what’s the correct answer. Can you help me?

[u/somber_soul]

Cavitation occurs anytime suction pressure goes below vapor pressure to form bubbles followed by subsequent repressurization in the pump.

So is it possible? Totally.

[u/yakimawashington]

Yup. Destroyed a pump head of a positive displacement pump due to cavitation when I was a new engineer at my current job lol.

Luckily, I work in R&D and this particular pump head was to a bench-scale pump, and only cost a few hundred dollars to replace.

[u/EmergencyAnything715 → u/yakimawashington]

Most of the pd pumps we use at a facility are for small chemical injection. Couple thousand to replace.

[u/ric_marcotik]

Yes it does

[u/Kelvininin]

It absolutely can.

[u/Exxists]

Adding to the “yes it can” answers. The additional factor that can make it worse in positive displacement pumps is that many of them move the fluid with an oscillatory flowrate. That makes the acceleration term of Bernoulli’s equation a significant contributor to pressure loss in the suction that needs to be accounted for in NPSHr.

[u/elcollin]

It totally can. But you can also pump boiling fluids with a PD pump without destroying it provided fluid velocities are low enough and materials of construction are appropriate. Gas boosters get used for this in my experience - gotta deal with the heat generated.

[u/Nstreethoodlums]

Not to mention the fact that a lot of PD pumps are specifically designed to move fluids with negligible vapor pressures AND the suction is often uniquely designed to stay “flooded” such that the fluid never experiences pressures much below the feed condition

[u/Zealot_Zack]

Yes, it is a huge deal -> -also in a way which is magnified the oscillatory stop-starts of PD pumps. You can easily google "cavitation in a gear pump" and see video with cavitation during intake - like here https://www.youtube.com/watch?v=C3-BKvswQkE

Couple more concepts to run over that might help:

For centrifugal pumps, NPSHr is defined by ASME by a 1% loss in discharge TDH, not by the onset of cavitation. Cavitation is present sometimes with minor consequences.

For positive displace pumps, the centrifugal mechanical energy balance with Bernoulli's principle is shifted, not just for cavitation. During an intake stroke, the flow rate is only for a short burst so it is exaggerated compared to the continuous basis and you can't calculate the kinetic energy average with an time-average velocity (because squared term throws off the average). For example, if the average is 1 m/s but intake is 10% of time then energy balance is off: 10² * (1/10) != 1²

Additionally, the entire column of fluid at the pump feed needs to be accelerated from a stop, and this change in momentum requires energy on the suction side. This is often approximated with an "acceleration head" term, but that is more a pseudo-factor than a true fundamentals based approach.

[u/Ritterbruder2]

Cavitation can occur anytime the cross-sectional flow area narrows and then widens again.

The narrowing of flow increases the fluid velocity, which decreases the static pressure. Bubbles will form if the static pressure drops below fluid vapor pressure.

When the flow area widens again, the pressure is recovered to close to what it was before the narrowing occurred. This increase in pressure causes the bubbles to collapse back into its liquid form, which is what results in cavitation.

[u/GeorgeTheWild]

Lewa has a very good video on their diaphragm pd pump. At about the 4 min mark, they show closing the suction valve can cavitate the pump and wreck the diaphragm

Lewa pd pump video

[u/MuddyflyWatersman]

yes . in fact with pulsing flow pumps the flow rate is an average. Every strokes there's a high acceleration force when the flow starts and then stops with every stroke.... there's a special way you calculate NPSH for diaphragm type pumps due to this. .. see a pulsafeeder technical sheet on it.... it's far worse than a continuous flow pump..... it depends on the pipe size because all the liquid in the piping has to be accelerated.......

[u/Peclet1]

Yes. Vapor lock also affects PD dosing pumps as well, though this condition is typically caused by air in the suction lines not being bled.

[u/AICHEngineer]

For sure, PD pumps still sweep a volume. If suction pressure is below NPSHr for the pump, youll get invitation along the swept edge. That will lead to rapidly reduce pump performance, since youll increase your slip (all pd pumps have some slip back around the rotors to the suction side)