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Pilot Operated Safety Valve Calculation

safety valve pilot operated

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#1 rs20170808@gmail.com

rs20170808@gmail.com

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Posted 08 October 2017 - 12:08 AM

Hi

 

I have a pump discharge PSV which is set at 180 psig. The PSV is pilot operated. It is connected to a header which has max operating pressure of 120 psig. So, the backpressure is 120 + built up backpressure. I calculated built up backpressure + backpressure = 125 psig. This is 69% of set pressure. When calculating size of relief valve, we have a backpressure correction factor (Kw). I could not find in API what Kw value to be used? My questions are:

 

1. Is my backpressure assumption correct

2. How to assume Kw..is it safe to assume 1 for Kw

 

thanks, 

 

Rs



#2 ChemEng01

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Posted 08 October 2017 - 12:26 AM

Typically for pilot PSV the backpressure correction factor will be less than 1 at Total backpressures greater than ~ 40-50% so it is not ok to assume 1. Best thing to do is get the vendor catalogue for the PSV and look for the back pressure correction curve. Or email the vendor and ask for it.

 

With regards to your Total back pressure this is superimposed + built-up back pressure which is what you have done.

 

Also,

 

Make sure your flow is still critical at 65% back pressure. It may be sub-critical. If so the subcritical sizing equations should be used. Alternatively API does give another backpressure correction curve which can be used with the critical flow sizing equations.


Edited by ChemEng01, 08 October 2017 - 12:30 AM.


#3 rs20170808@gmail.com

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Posted 08 October 2017 - 01:02 AM

 thanks for ur reply. 

 

For liquids I could see the following formula from API (in SI units below, they also have formula in USC units):

 

 

Attached File  11.png   2.84KB   2 downloads

 

Is there any other formula, we need to use when Pbackressure > Pcritical 

 

thanks,

 

rs



#4 fallah

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Posted 08 October 2017 - 02:49 AM

 

I have a pump discharge PSV which is set at 180 psig. The PSV is pilot operated. It is connected to a header which has max operating pressure of 120 psig. So, the backpressure is 120 + built up backpressure. I calculated built up backpressure + backpressure = 125 psig. This is 69% of set pressure. When calculating size of relief valve, we have a backpressure correction factor (Kw). I could not find in API what Kw value to be used? My questions are:

 

1. Is my backpressure assumption correct

2. How to assume Kw..is it safe to assume 1 for Kw

 

 

Rs,

 

For pilot operated PSV in liquid services, such as your case, no special correction due to back pressure isn't required.



#5 rs20170808@gmail.com

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Posted 08 October 2017 - 03:10 AM

fallah, thanks for your reply

 

so basically i can use the equation from API for liquids as below with Kw = 1. 

 

Attached File  11.png   2.84KB   9 downloads

 



#6 fallah

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Posted 08 October 2017 - 03:13 AM

fallah, thanks for your reply

 

so basically i can use the equation from API for liquids as below with Kw = 1. 

 

attachicon.gif11.png

 

Yes, you can...
 






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