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Psv Sizing For Fire Case

psv sizing

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#1 hinarris

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Posted 23 January 2015 - 07:33 AM

I am doing a sizing calculation for a PSV required on a filter separator vessel. The PSV has to be sized for fire case.

 

I have sized the PSV as per API 521 for vapor flow and obtained a relieving flow of 260kg/h and an orifice area of 0.017in2.

 

This area is much smaller than the smallest API designated orifice 0.11in2 (orifice D). Can I choose orifice D for this purpose or is it possible to have a smaller valve custom designed by valve suppliers? Is the API code dictating to use orifce D as the smallest orifice to be used?

 

I believe if the calculated area is less than 60% of selected orifice, the valve will chatter.

 

Please let me know your comments.

 

Regards,

Kashyap.



#2 fallah

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Posted 23 January 2015 - 07:59 AM

Kashyap,

 

API 526 valves are a subset of code valves and there are many code valves other than API 526 valves. It's adequate for a code valve had been undergone capacity certification according to ASME tests protocol.

 

Then for the situation you described you haven't to use the required PSV from API 526 category. You can select from another certified manufacturer's code valves in which the smallest PSV has a designated orifice area close to the required area obtained from the sizing you has performed. 


Edited by fallah, 23 January 2015 - 08:09 AM.


#3 amolbhvsr

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Posted 23 January 2015 - 08:55 AM

You may get all certified relief valve details in Red Book (National Board certification) with manufacturer details.

#4 samroo

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Posted 08 February 2015 - 12:55 PM

Dear all,

 

A vessel on hydrocarbon service having design pressure of 50 Psig which is more than source pressure (15 Psig). The block flow does not applicable on this vessel.

Vessel is in remote area, Does fire case applicable?

Or the vessel design can be finalized without PSV as per ASME Section VIII?



#5 fallah

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Posted 08 February 2015 - 01:50 PM

samroo,

 

It's left to the user to determine if the fire case is applicable to mentioned vessel. Being in fire zone, possibility of HC leakage and existing of ignition source in vicinity...are among the parameters based on which the user can make proper decision for fire case credibility...

 

As per ASME Section VIII, regardless of if any over pressure scenario is applicable, each pressure vessel shall be equipped with a pressure safety device to protect the vessel against over pressure...



#6 samroo

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Posted 08 February 2015 - 03:11 PM

Thank you very much for the detail answer



#7 latexman

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Posted 08 February 2015 - 10:44 PM

Kashyap,

 

Another option is to purchase a standard size PSV with a restricted lift.  A restricted-lift PSV has a mechanical stop (collar) that fits around the stem, limiting the lift and effective orifice area.  Some PSV manufacturers do that, but not all.  In U.S., I believe Consolidated (Dresser) does that on some of their more popular PSVs.



#8 aqassam

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Posted 13 February 2015 - 04:25 PM

There's a good article describing valve chatter by Smith & Burgess:

 

http://smithburgess....nlet-Piping.pdf

 

This describes the origin of the "required capacity at least 60% of valve actual capacity"as a good design rule for PSVs. 

 

However, you'll notice that significant valve chatter may not be a concern until you're at 25-30% of valve actual capacity. 

 

Also, many people think of fire as an emergency contingency where you are less concerned with chattering (since there is a fire in the facility). You should always check what guidelines your company works with. 

 

Can I ask what source you are using for the 60% guideline for PSV design? 






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