Fire Case for Air/Water Mixture in Tank

[RJB32482]

Here's the case I have for an air/water separator tank.

Which case do I design the worst case scenario for in this tank (air, water, or two phase)? It is not that full with water during normal operation. MAWP is 120 psig.

Thanks.

 

[djack77494]

If you're designing your relief case for fire, then assume the vessel will be isolated. Next calculate the heat gain per the applicable codes. The pertinent equation (which I unfortunately don't have handy) assumes that the heat gain will occur over the wetted area. The codes will also help you define "wetted area". This assumption is reasonable since heat transfer to an inner surface where water is boiling is MUCH higher than heat transfer into the vapor space. Use the heat to boil water and have the PSV relieve the resulting vapor. This process will be occuring at 120psig + up to 16% overpressure. To be conservative, take the water level to be at the highest level that you'd normally reach. (I'd just assume the vessel is full of water at its boiling point at relieving pressure and go from there.)
Doug