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Determine Flow Capacity for Safety Relief ValveHelpful Member!(2) 

tc7 (Mechanical)
24 Aug 09 13:59
We have a large receiver (75 cubic ft) that is charged to 3000 psig by a compressor that has a free air delivery specification of 30CFM. Once the receiver reaches 3000psig, it cuts out until the pressure drops to 2750psig.

The receiver will feed a new work station that requires pressure to be reduced to 600 psig.   Downstrean of the reducing valve, we will add a saftey relief valve. The set pressure of this safety relief valve will be according to ASME B31.1 or Section VIII criteria. That is the easy part.  

The hard part is that I do not know what flow capacity this safety relief valve must have.  If all we had to do is worry about the compressor flow, it would be miniscule (about 0.15CFM @3000psig)  but with a receiver in the line, which is fully charged, how do you determine the required flow capacity of the safety relief valve so that the downstream equipment is protected?  It seems to me that the receiver will act as an infinite energy source capable of high flow for a long long time. How do you do this? Please advise.

Thanks.



 
Helpful Member!  Chance17 (Chemical)
24 Aug 09 14:19
Reducing valve failure is the required flow for the safety valve.
Helpful Member!  skearse (Chemical)
24 Aug 09 14:19
Determine the max flow through your reducing valve based on the high side pressure (check with the vendor and get the full flow/fail wide open Cv values).
tc7 (Mechanical)
25 Aug 09 14:57
Ok thanks for that and was able to correctly inquire of the regulating valve company what their max "wide open" or failure flow will be.

When dealing with a huge pressure drop on air supply lines (from 3000psig to atmosphere) that are passing saturated air, will the safety relief valve flow performance be hampered by ice forming? My quess is "yes" and if so, what is a typical solution for this problem? Thanks.  
 
meanderson3d (Chemical)
16 Sep 09 16:16
One thing you have to be careful of is your reducing valve being able to have more than one trim size.  If you currently have a trim size that is smaller than maximum, someone can go in there later and install a larger trim if they decide they need more flow.  Then your relief valve will be undersized.

Three possible solutions to this:

(1) Size the relief valve based on flow through the maximum available trim in the reducing valve

(2) Place a sign on the reducing valve stating, "Do not exceed X size trim in this valve."

(3) Have a good management of change procedure that includes checking all affected relief valves when changing trim size in pressure reducing valves.

To answer your question about ice forming: If ice is going to form, it will probably be at the reducing valve, which will actually help you in a relief situation.  The relief valve will be set at a lower pressure, so I don't think ice would be a problem there.

Andy
New Orleans, LA
Petronyx Consulting Engineers, LLC
http://www.petronyx.com

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