Hi guys,
I am facing a certain condition in which I am unable to acertain the cause of loss of suction & reduction of flow and discharge pressure of a pump abruptly which leads to cavitation.

The pump is not showing any symptoms which conclude for any wear & tear in pump.
Other wise the pump is running normally if restarted by procedure.
The person operating the pump says,when during cavitation if the pump is filled with flushing medium for hrs together then it again regain its suction & normality.
The pump is also running in its design temperature range.The suction strainers & pipes are also clean.

Except for NPSHA,NPSHR calulations & theories what are the other possibility of this cavitation and how can they be solved???????

Kindly, help me ASAP.

1) Do you notice increase in vibration, or "pumping rocks" noise when supposedly cavitating?
2) How is the impeller secured to the shaft? If collet mount instead of keyed, that may be your issue.

What kind of pump do you have? It works well with flushing medium. What is the normal pumping fluid?

1gibson---
there is no increase in vibration until pump cavitates & then vibration increases due to it
Also the impeller is keyed to shaft.

micalbrch

This is a two stage pump with first impeller having double suction and pump having top-top configuration.
The normal fluid being pumped is preflashed crude oil at temp.of 165C. and specific gravity of 0.7888.

Very light crude for that high temperature. Vapor pressure is?

If it ain't broke, don't fix it. If it's not safe ... make it that way.

BIGInch----
That is the maximum design temperature.The pumping range has varied from 145C to 160C at some of the times.
The vapour pressure range is 3.28-3.84 kg/cm2a.

That's the vapor pressure at 165C?
Anyway, you are sure that NPSHa was calculated correctly?

If it ain't broke, don't fix it. If it's not safe ... make it that way.

It sounds to me like the fluid entering the pump is degassing somewhere in the volute and that you might derive some benefit from venting the volute to a flash accumulator (or similar scheme) or by slipping some of the fluid from discharge back to suction.

Failing that (perhaps unorthodox) suggestion, review the suction system and look for any occurrences of sloping towards the pump or pocketing. If the pump is indeed top-top, then fluid entering the pump suction is going downward somewhere. In a double suction pump, in combination with this, you might end up with suction side fluid traveling at low enough velocity to facilitate gas break-out, leading to your problem.

I wonder if this is one of those rare occasions where a decrease in line size on the suction side might actually provide some benefit.

You might also look at something like a Fluid Kinetics SSC (or equal) suction stabilizer with automatic degassing controls.

An earlier thread with a similar pumped fluid suggested simply pulling the suction strainer if the fluid pumped proves to be clean enough. There are a few inexpensive things to try before you need tostartgetting truly worried.

Can you provide a sketch of the inlet piping? Is the inlet compliant with ANSI/HI 9.8? Is there an elbow immediately before the pump suction that may cause turbulence?

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

Put your ear to the pump piping and listen. If it does not sound full you have offgassing

If you can't put your ear to the piping, get a wooden dowell, a hammer handle, saw of a piece of broom handle, something wood and put it to the pipe on one end and the other end to your ear.

rmw

A lot of people confuse cavitation with air bubbles INTRODUCED into the fluid becasue it sounds the same.

Cavitation by definition means the fluid will release bubbles because the fluid is under NEGATIVE pressure on the suction side of the pump. The bubbles collapse when the fluid is pressurized by the pump impellor. The result is the gravelly sound that everyone is familiar with. If it is cavitation you should be able to throttle the pump discharge and the gravelly sound should go away.


If air bubbles are somehow introduced into the fluid the bubbles can also be collapsed by pressurization by the pump impellor. The result is the same gravelly sound as cavitation but it is not cavitation because the damage is not the same. To check, if you throttle the pump discharge the gravelly sound should not change no matter how far you throttle it in.


Basically the two conditons have similar symptons but different solutions are applied.

Quality Time

I disagree with you completely. The introduction of air into pump suctions has been used to reduce the noise from cavitation. it provides a cushioning effect.
Cavitation occurs when the vapour bubbles collapses and a very high velocity jet of fluid is fired across the buckling sphere and impacts on the surface of the pump on the other side of the sphere. Whereas an air bubble would be compressed by incease pressure but will not collapse. It takes longer for air to go back into solution than that.

Suggested reading is by Grist http://www.google.com.au/url?sa=t&rct=j&q=...

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

Quality Time
Seems we have come round again to the same discussion re entrained air that has been extensively discussed previously
http://www.eng-tips.com/viewthread.cfm?qid=285396.

Compressing entrained air / gas will never cause damage that in any way looks like cavitation damage nor will it sound like cavitation.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)