Negative pressure at axial piston pump inlet

[hydroman247]

After much Googling, I can't find a solid answer anywhere.

What is the accepted pressure on the suction side of an axial piston pump? There are plenty of charts out there with velocity ranges suggested but no real useful information regarding actual pressure.

Looking through pump catalogues doesn't give too much detail either, not from the ones I have read anyway. I am measuring 0.9 Bar absolute at the inlet to a pump. I want to know how far I am from the limit and what other peoples experiences are in this area.

Thank you.

 

[hydtools]

Parker data shows min. inlet pressure .8bar absolute for their P2/P3 pumps.

http://www.launchrun.com/hpd/pdfs/P2P3.pdf

Ted

 

[MikeHalloran]

Most of them will self-prime eventually, up to some fraction of an atmosphere of lift of whatever fluid you're trying to pump, but they won't be happy doing it forever.

I have used a 7HP centrifugal pump to supercharge the inlet of a 150HP piston pump to meet its NPSHR. I'm not the first person to do that.





Mike Halloran
Pembroke Pines, FL, USA

 

[HPost]

At 0.9 BAR absolute, you are close to the minimum inlet pressure for most axial piston pumps. As an example, the minimum inlet pressure for a Rexroth A11 pump is 0.8 BAR absolute.

With a charge pump, the minimum for this pump is 0.7 BAR absolute.

As you move below the minimum inlet pressure, the negative pressure on the slippers starts to pull them away from the pistons. That creates a wear pattern, the piston and slipper separate more, which creates a "slapping" motion as the outlet pressure forces them together again. The wear increases until the slippers are pulled off the pistons. Then "BANG", the pump is dead.

Don't go below 0.8 BAR absolute if you want to preserve the life of the pump.

Cheers

HPost CEng MIMechE

 

[hydroman247]

Thanks guys. Interesting that a closed loop pump doesn't require as much pressure on the inlet side relative to an open loop pump. In that A11 example anyway.

 

[kcj]

Is the lower allowable absolute pressure for the closed loop pump specified at the inlet to the piston sections, or more likely inlet to charge pump? Charge pumps are usually gear or gerotor steady flow and steady acceleration of fluid. Piston open loop sections has the 'gulp factor' of accelerating flow all along the inlet hose and inlet through the casting, as the pump goes on and foff stroke.

 

[HPost]

Yes, the 0.6 BAR quoted is for the charge pump inlet.

It would be impossible to have 0.6 BAR abs on the main pump inlet with a charge pump installed.

Much below 0.6 BAR there is a risk of boiling the oil in the pump inlet. Especially at high velocity and with hot oil.

In reference to kcj's comments on 'gulp factor'...The inlet pipework and the pump casing should be designed to keep the fluid velocity below the maximum even with the pump at max displacement. Also, adding hoses to a pump inlet is asking for trouble. Should always rigid tube.

 

[kcj]

Trying to say that the lower absolute pressure rating for hst compared to open loop pump is not now a rating for the variable pump portion (variable piston, on and off stroke) but at the inlet of a constant flow fixed gerotor or gear pump. Without the acceleration and deceleration of fluid, the style of pump can take a bit lower inlet pressure.
Even with large inlet lines, which we routinely do in mobile equipment due to cold temps, the fluid still changes velocity in a variable flow pump. Larger lines minimize the change.

 

[HPost]

I wasn't disagreeing, just adding some context...