RV sizing for regulator failure case

[ValveEngineer123]

I am sizing an RV for a regulator failure scenario. All the literature I can find basically states that to determine the required capacity of the RV, one must calculate the flow rate through the wide open regulator and size the RV to relieve that flow rate. It seems as if the physical interpretation of this would be that the entire flow through the failed regulator would now flow through the RV.

1) Can any credit be given due to the fact that the original flow path is still open and will flow at least the original flow rate (pre-regulator failure) through it? i.e. Can an RV with a lower capacity than the wide open regulator be used as long as the sum of the flow out equals the flow through the failed regulator?

2) If I sized the RV in this manner, would there be any pressure accumulation in the line beyond the RV setpoint?


The fluid is service water at ambient temperature and 80psig, with the regulator set to 10psig and a flow rate of 28 USgpm (flow rate prior to regulator failure).

Any advice is much appreciated

 

[GrimesFrank]

ValveEngineer,
What you're sizing is an RV for an isolable/restricted flow pressure boundary (piping, PV, HX, etc.). The scenario input is that the flow into said boundary is full open flow downstream of a failed regulator. You would need to size the RV for full flow whether there was a regulator or not. What you are taking credit for is that the regulator creates a pressure drop (even at full open).

What you might be able to take credit for is flow out of the pressure boundary that cannot be isolated.

Today is gone. Today was fun.
Tomorrow is another one.
Every day, from here to there,
funny things are everywhere. ~'Dr.' Theodor Geisel

 

[ValveEngineer123]

Thanks Grimey,
That confirms what I would have instinctively thought, I just haven't seen a situation such as this discussed in any codes or reference documents.

 

[MortenA]

Another thing to remeber with regards to evaluating relief rate is that the downstream pressure is higher than usual (design pressure+10% usually) this means a lower dP over the regulator.

Best regards

Morten

 

[JimCasey]

You are also in a little gray area of the code here since (last time I checked) ASME VIII governs pressures from 15 psi up. If the regulator is set at 10, you really don't want to set the SRV any lower than 11.11 psi to account for 10% blowdown, but you can't set a codestamped valve at less than 15.
You can take credit for continuous system usage other than the SRV if that usage absolutely WILL be there at all times regardless of ANY consideration. If it's possible for something silly like for a forklift to fall out of a 747 and squash the downstream piping---you have a weak case.

 

[lizking]

I think there are 2 overpressure cases to consider: one is failed open regulator where you can take credit for minimum normal downstream use rates. The other is blocked outlet (if this is would cause overpressure) where you would take normal flow through regulator (you can't assume favorable responses from instrumentation in a relieving scenario). Also consider modulating relief valves in these circumstances to avoid chattering; vendor should be consulted.