Mitigation and Selection of Control Valve for Cavitation Issues

[Pavan Kumar]

Hi All,

I am specifying a control valve for water service. Because of the available pressure differences, calculated by performing pressure drop calculations, severe cavitation is predicted based on the values of the cavitation index calculated. Below is the summary of the data:


Case Flow Rate P1 P2 PV Sigma
(Imp gpm) (psig) (psig) (psia)
Max Flow 147.34 30.51 4.606 0.508 1.726
Norm Flow 86.65 45.06 -1.764 0.337 1.27
Min Flow 25.96 52.89 -4.97 0.220 1.164

where P1 is inlet pressure of the control valve
P2 is the outlet pressure
PV is the vapor pressure at inlet temperature
Sigma is cavitation index, calculated as Sigma = (P1-PV)/(P1-P2)

As per ISA quidelines Cavitation index should greater than 1.5 for single stage trim to work.

I am looking for suggestions to mitigate for cavitation and control valve body style selection which are suitable to withstand cavitation. I mean which body style would be lower pressure recovery factor among globe, ball, segmented ball valve, eccentric rotary plug valve etc.


Thanks and Regards,
Pavan Kumar

 

[mk3223]

It seems that it's a valve sizing issue due to the high pressure drop as well as the negative outlet pressure. IMO, you may need to try the different size of valve or valve trim to eliminate the cavitation issue.

 

[LittleInch]

Well only a globe valve is really a control valve type out of all the ones you mention.

You might need a plug and cage, but I would just put these on a data sheet and send to a few vendors and let them tell you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nick_Gray_88]

You need a multi-stage cage-type control valve to control the pressure drop and avoid cavitation from occurring.

We could go through how many stages etc. but the design methods are closely guarded secrets, and we won’t win any friends sharing them online...

Like LittleInch says, take it to a company who do it for a living for a proper solution.

 

[Pavan Kumar]

Hi Nick_Gray_88,

If there is cavitation we can mitigate it by increasing the back pressure by adding an orifice or throttling the downstream valve to the control valve if it is not too much pressure drop required to add this back pressure. That was my plan. In my case the vendor - Fisher came back and saying that they wrongly estimated the cavitation when in reality there is no cavitation. I will take their word but when I calculated the cavitation index I found the values were less than 1.5 for normal and min flow cases. Sources on the internet like the link copied below says that the cavitation index should be greater than 2 to eliminate cavitation. What is your experience and what are your thoughts?.

https://www.flotechinc.com/understanding-predicting-cavitation-control-valves/

Thanks and Regards,
Pavan Kumar

 

[Nick_Gray_88]

I would say greater than 2 - 2.5 and you’ll see no cavitation.
The issue really becomes what trim and what hole pattern is on the trim in the valve?
If you have a multi-stage trim then the pressure-drop per stage is what should be used to find your cavitation index.