8 replies to this topic

[hazmee]

dear everyone..

i've done a simulation using hysys for urea plant. my problem is, cavitation had occurred on the valve when the pressure differences is really high (from 145 bar to 2 kpa)

my question is, how can we avoid this from happen.

thanks.

[Andree]

1. bigger full bore valve (if cavitation is a result of velocity increase)
2. lower temperature of liquid (but this does not improve the situation much)

[Qalander (Chem)]

1. bigger full bore valve (if cavitation is a result of velocity increase)
2. lower temperature of liquid (but this does not improve the situation much)

We witnessed/suffered similar valve cavitation frequent damages on

High differential pressure Steam Let down service valve(From 600 psig to 150 psig)

at the previous employer's site.

Hope this proves helpful

[ankur2061]

hazmee,

For severe service valves (high delta P), angle valves prove useful as they are able to handle high differential pressures much better than conventional valves (globe or plug).

Hope this helps.

Regards,
Ankur.

[JoeWong]

When you state PRESSURE especially low pressure like 2 kPa, please make sure state gauge or absolute. 2 kPa abs or 2 kPa gauge (103.325 kPa abs) make large differences in certain aspect. Make it a practice. You will not regret.

When discuss about cavitation, one of the key information is fluid vapour pressure of fluid. Without knowing the vapor pressure, you will miss this information to define the potential of cavitation. The second key information is the pressure recovery parameter of the valve.


Now... to avoid cavitation, the operating pressure in any point (from valve inlet to valve outlet) shall be always below vapor pressure. Keep in mind, there is pressure recovery in a valve. Some valves have large pressure recovery and some valves have low recovery. The lowest is to have a valve with no recovery. Read control valve handbook ie. Fisher CV handbook, if you are not clear with pressure recovery. Pressure recovery will define the lowest pressure that your valve will experience. To avoid cavitation, you have to ensure this lowest pressure is still higher than fluid vapor pressure.

Two ways help to avoid / minimise cavitation:
i) lower the fluid vapor pressure i.e. deep cool fluid... or heat the fluid at high temperature to vaporise and gas/vapor follow by cooling...or other means to drop the vapor pressure

ii) increase minimum pressure in the flow path by introduce device in the valve i.e. anti-cavitation trim to reduce the pressure recovery to minimum.

[Qalander (Chem)]

Thanks Dears ankur/ Joe,

I really anticipated your valuable input(s) will benefit the OP& Others indeed!

Regards
Qalander

[hazmee]

thanks for the views and comments. i hope this might help in the next presentation.

[Art Montemayor]

This is not an uncommon situation - especially in thermal power plants.

Joe Wong has come close to the specific answer in his post. As would be suspected, Fisher Controls has, for many years, been called upon to furnish control valves for the power industry - where a great deal of control applicaitons involve exactly this type of problem. Draining or controlling high pressure condensate to a low pressure often encounters cavitation.

There is a specific Fisher Controls Catalog dedicated especially for the power industry and it focuses directly on this application with specific valve types and descriptions as well as design criteria. You might be able to obtain it by contacting Fisher Controls or a representative. Or, I suspect, you could also obtain similar catalogs or information from European competitors of Fisher. Japanese valve suppliers may also be a source of information. Everyone in the world is plagued by this phenomena and they must be able to respond to it. I have applied the Fisher valve trim to this application with good results. The method used is a series of internal expansions that gradually reduce the pressure level with special internal trim.

My copy of the Fisher Controls catalog was loaned to a colleague who hasn't returned it yet and this is a good opportunity to remind him that its time to return it.

I hope this information helps.

[Qalander (Chem)]

Thanks indeed Art. ; for a comprehensive,yet focused reply!
Best regards
Qalander