Is intermittent cavitation ok?

[SBC1969]

Is a little bit of cavitation OK in a hydraulic system?
I am sizing an orifice valve and found that the flow will begin to choke (like a gas) at a cavitation number ([Pu-Pv]/[Pu-Pd]) of about 1.5 or more. After a bit of research I think I can conclude that cavitation is going on.
The valve will be used in a landing gear actuator so it will not see constant flow. Is intermittent cavitation safe? Is there any published information about how long it will take for any damage to occur?

 

[EngineerTex]

This is a bit surprising. Do you mean that the pressure drops so low that the fluid boils? My knowledge of cavitation is the phenomenon where a pump (or propeller) is moving faster than the fluid is able to fill in the area behind the moving vane, which causes a vacuum. It is the interface of high (differential) pressures between the fluid and the void which causes the pitting and component failure associated with cavitation.

To answer part of your question, to my knowledge, the problem with cavitation is not that it damages the fluid.

Is it a fixed plate orifice or is it an adjustable orifice such as you would find in a cartridge valve? I may just be ignorant of the phenomenon, but I have experience with a fair amount of cylinders, pumps, manifolds, cushioning schemes and all sorts of relief valves and counterbalance valves and in all of that, I've not heard of cavitation across any of the valves I use. That's not to say that it hasn't happened, but I can say that in pressure drops of 5,000 psi across valves which hold a load upwards of 600,000 lbs, the flow over a relief valve or counterbalance valve is pretty similar to an orifice valve. The components wear, it's true, and you should keep a maintenance schedule, but I'm not familiar with any problems further than the normal wear which occurs in any valve anywhere else in the world.

Can you explain further how the cavitation occurs?

Engineering is not the science behind building. It is the science behind not building.

 

[Hydromechdude]

From what I've read (not experienced), 'liquid flowing through any orifice will cavitate whenever its velocity causes the pressure in the throat of the orifice to drop below the vapour pressure of the flowing liquid.' This is independent of a high supply pressure and high back pressure on the orifice.

Formula in the Lee Company Technical Hydraulic Handbook is:

P2 = (0.062*deltaP / (L^2 *(Min. passage)^4))) + Vapour pressure (psia)

P2 = Downstream pressure (psia)
deltaP = P1 - P2
P1 = Upstream pressure (psia)
L = Lohm rate of fluid restrictor

 

[SBC1969]

"Can you explain further how the cavitation occurs?"

This is a good basic explanation.

http://www.ewp.rpi.edu/hartford/~ernesto/F2006/EP/Aids/Papers/Elder/Koivula.pdf

 

[EngineerTex]

Well, I certainly do have egg on my face now. In fact, I feel pretty embarrassed about not knowing about the problem. Very interesting paper.

And having put my foot in my mouth once, I feel licensed to throw out a suggestion.

Can you consider using two orifice plates in series, seprated by a length of tubing? The pressure drop across each orifice would be reduced by half and perhaps doing this would address the issue.

As for experience with the issue, I do know that we have a number of sharp-edge orifices in all of our hydraulic cylinders, used for cushioning at the end of the stroke. In all of the (many) problems we have had and addressed, this one has never come up. That info may be completely irrelevant to you, but I've never had the experience of seeing this problem.

Engineering is not the science behind building. It is the science behind not building.

 

[hydtools]

I have seen localized cavitation erosion in equipment I designed. If the erosion could not be moved to a non-critical area, the material was hardened to better tolerate the erosion.
If you are not seeing erosion, I would not expect the infrequent cavitation to be a problem.
If you are concerned that cavitation exists at all, consider the suggestion to use series orifices to reduce the pressure drop across each. You may be able to increase the diameter of each orfice to reduce fluid velocity also.

Ted

 

[SBC1969]

I do not think that lee formula is correct

 

[Gkranz]

To give you an answer to your questions:

Yes intermittent cavitations are permitted, so long as they are not severe and not happen for long periods of time, several minutes or longer: run the Reynolds number to see where you are at, the lower the better.

Damage will occur immediately when cavation is present, make your orifice out of 4140 steel alloy and its affects will be reduced.

Most likely will need new landing gear cylinder seals long before you wear out the orifice?


Best Regards,
Westerndynamics.com