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[mirandomka]

I am doing a rupture disk calculation and the fire relieving P (21%+ Set Pressure) is above critical pressure. Usually I use the rigorous supercritical fire expansion to find the peak required orifice area and the coincidental relief load and relieving temperature, if it is a PSV. But in this case I have a rupture disk instead, so it is not re-closing; and pressure drops once the disk is ruptured. Therefore, I don't feel it is adequate to use the rigorous supercritical fire method which assume the pressure is constant at relieving pressure. Beside my rupture disk is not discharging to the atmosphere, thus, the Kd=0.62 method/orifice area method cannot be used as per API520 pt1.

 

If I use the vapour fire method from API, again it may not be adequate as the vapour fire equation is developed from vapour critical nozzle equation which may not be adequate and may oversize.

 

I also checked my rupture disk worst case upstream burst pressure based on the worst case backpressure. The result shows it's below my relieving pressure and below critical condition (vapour generation calc is possible, however, maybe overkill or not adequate to say that is the required load at relieving condition).

 

Although I am quite sure the external fire is not the governing sizing scenario (this is a rupture disk on HX tube side for tube rupture case. Tube rupture case required relief volume rate ~1000 m^3/hr), meaning that the rupture disk is going to be oversized for fire case scenario anyway and I expect it will never reach to the relieving pressure in reality, it is strange to say fire case is not credible for pressure vessel...

 

So summary from above my questions are:

1. how to determine the relieving temperature and required relief load in this case?

2. Do I have to do the calculation to show the fire case is not governing? Can anyone tell me if  based on your experience?