Rated vs. Actual Capacities for determining pressure drops 2

[chemter]

Hello all,
I have a question concerning the calculation of relief valve discharge pressure drops. I've tried searching past threads, but I haven't seen any that discuss this particular question. If I've missed it, my apologies.

My question pertains to a conventional spring loaded RV in vapor service protecting a vessel with an allowed accumulation of 10%. The RV SP = MAWP. Therefore, at the relieving pressure, the rated and actual capacities will be equal. However, if the case would be for fire, then the allowed accumulation would be 21%. When determining the discharge pressure drop from a relief valve, should you use the relief valve's rated capacity (adjusted for actual fluid) at 10% overpressure or the actual capacity at 21% overpressure? I am aware of the API allowance of exceeding the 10% rule for these cases, but shouldn't the discharge pressure drop be based on the rated capacity since at this flow you run the risk of chattering and/or closing? Is one way or the other incorrect? I hope I have given enough info for this problem. If I have been unclear, please let me know. Thanks.

 

[pleckner]

After you pick the relief valve based on your controlling scenairo, you will find that the relief valve size is mostly larger than you need. The valve will come stamped with its rated flow. You must use this rated flow for both your inlet pipe and tail pipe line loss calculations. The stamped value flow rate is based on a 10% accumulation over set pressure. This is the only value that is guaranteed by the vendors. They do not guarantee the value based on 21% overpressure, only 10%.

 

[Latexman]

Your confusion is exactly why a standard is needed to determine the outlet (and inlet) pressure drop to compare to Code that governs PSV sizing. While it would be more conservative to meet the inlet (3%) and outlet pressure drop requirements at 21% accumulation, the standard is 10% accumulation.

Good luck,
Latexman

 

[chemter]

Thank you pleckner and latexman. I just wanted some reassurance that I was performing the calculations correctly.