Rated Capacity for Relief Device Sizing 1

[met11]

Hello everyone,

A few of us at our company are confused as to what rated capacity to use in sizing inlet/outlet piping. Our situation is best cleared up with 2 examples, let me know if you know the answer please as we are torn:

1. Say a relief valve is protecting a piece of equipment with an MAWP of 100 psig, and the relief valve is set at 100 psig and the only governing case is external fire. For sizing the valve, you are allowed a 21% overpressure (which in this case is equal to a 21% accumulation since the PSV is set at the MAWP). Would the "Rated" capacity stamped on the valve be calculated at 10% or 21% overpressure? It's my understanding that they would only do bench tests at 10% overpressure, and certify them for 10% overpressure. Or do they do them at 21% overpressure? Also, when calculating the pressure drop in the inlet/outlet piping, do they use a 10% or 21% overpressure for determining flow and pressure drop?

2. Now say a relief valve is set at 90 psig, protecting a piece of equipment with an MAWP of 100 psig (for some process/safety reason). The allowable accumulation pressure is 21% over the MAWP (121 psig) for the governing fire case. This corresponds to an overpressure of about 34.4%. Given an orifice area, would the rated capacity for the valve be calculated at 10% overpressure (99 psig) or 34.4% overpressure (121 psig)? What pressure/flow combination would be used for sizing the piping?

We've looked over API 520 and 521, as well as the Crosby handbook and cannot come to an agreement. Has anybody encountered this before and can answer with certainty as opposed to hypothesizing like we all are?

 

[don1980]

ASME Sec VIII (Appendix M) states that the pressure drop calculations are to be performed using the rated capacity (synonyms for "rated": nameplate, stamped, marked, certified).

ASME defines rated capacity at 10% overpressure (note: not 10% accumulation). There's an exceptional case in which ASME allows the rated capacity to be defined at 20% overpressure. Unfortunately, this can seldom be used because of the ASME restrictions. As you guessed, this is for cases in which the PSV is installed for fire protection. The catch is that ASME limits this to vessels "having no permanent supply connection and used for storage at ambient temperatures of nonrefrigerated liquidied compressed gasses".

So, to your questions:

Q1: Rated capacity is the value at 10% overpressure. Inlet pressure drop calculations have to use that same set of conditions (rated cap. at 10% overpressure). Outlet pressure drop is allowed to be as high as the actual overpressure, but no more (ref: API 520 pt1, 8th ed, 5.3.3.1.3). Since fire is your only scenario, the overpressure is 21%, and thus the allowable tailpipe losses are 21%.

Q2: The rated capacity is at 10% overpressure (99 psig). As explained in Q1, the inlet losses are calculated at 10% overpressure using rated flow, while the tailpipe losses can be up to 34.4 %.