cavitation defination 5

[bsfc9]

while troubleshooting an engine's oiling system problem on our engine dyno. the phrase "pump cavitation" is thrown around rather loosely. can anyone simply define cavitation as it would apply to spur gear type oil pump. this is a "dry sump" oiling system in that the pump is external of the engine and is fed from a tank of hot(240 degree F), deaerated oil. thanks

 

[BRIS]

I apologise for straying further from the original question: I am not a pump designer I am a civil/hydraulic engineer involved in the design of pumping stations and hydraulic structures - dam spillways and the like and it is many years since I studied cavitation. But I always understood that the presence of gas (air in my case) in the fluid reduced the risk of damage from cavitation. For this reason I have designed many hydraulic structures with aeration devices - spillways, hollow jet valves, outlet gates etc. My understanding is that the gas expanding in the negative (sub atmospheric) pressure zone forms a cushion reducing the damaging affects of vapour bubble collapse (if there is air in the cavity in can't collapse)and also in free surface flow situations such as high velocity flow on a spillway the presence of dissolved air prevents the pressure dropping to vapour pressure. Now this may not be the same in a pump where negative pressures are a factor of the pump and pumping system but I believe that for the majority of hydraulic systems dissolved gas content has a beneficial effect on the potential erosion effects of cavitation .

However, the presence of dissolved gas may have a detrimental influence on the onset of cavitation (I user the word cavitation but in my view this is a misnomer - it is dissolved gas coming out of solution). And this can be of concern when considering the onset of choking cavitation where the fluid becomes a mix of gas and vapour. – A feature of cavitation that has so far not been discussed!!.

Brian

 

[PUMPDESIGNER]

I have some good literature on large scale cavitation too, was not going to post it but I will now when I get the chance.

Gas bubbles in the line can help prevent cavitation if the gas bubbles are injected in such a fashion as to raise NPSHa. This has been used to cure cavitation problems on pumps.

PUMPDESIGNER

 

[25362]

Experts advise adding heat to reduce cavitation severity ! This action would increase pressure and temperature and decrease the vapor/liquid density ratio to push cavitation back to an acceptable level. The striking example is water, to which an increase from 40 to 60^oF will decrease the vapor/liquid density ratio by 50%.

 

[BRIS]

Adding heat will cause cavitation to start sooner. i.e the point of incipient cavitation will start be at a higher pressure. Choking cavitation where the fluid vaporises will also start at a higher pressure - at 20 degree C vapour pressure is 2500 N/m2 and at 90 degree c it is 79,000 N/m2. At 20 degree c the pressure has to drop to .025 bars absolute at 90 degree c it only has to drop to 0.79 bar absolute for the water to vaporise.

Adding heat reduces the severity of the cavitation but in most cases cavitation will still cause rapid erosion of metal surfaces that are in cotact with the imploding vapour bubbles.

Since adding heat causes the onset of cavitation at a higher pressure and increases the risk of choking cavitation it is not really a solution.

Brian