Chemical Engineering Interview Question:

What are some guidelines for sizing a PSV for a fire scenario on a vessel in a refinery service?

Submitted by: Administrator

Sizing a PSV on your vessel is a matter of calculating how much heat is inputted from the fire. API-520 uses Q = FA0.82 where Q is BTU/hr, F is the insulation factor (commonly taken as 1.0 but can be less than 1.0 if your insulation will remain effective during the fire and not be dislodged by fire hoses) and finally, A is the external area in ft2. The vapor load is then the total heat input from the fire divided by the liquid's latent heat (BTU/lb).

As a fluid approaches its critical pressure, the latent heat as it boils decreases so the relieving flow rate increases. At the critical point, the latent heat goes to 0. Some companies simply use a minimum 50 BTU/lb latent heat others look at de-pressuring equipment, etc. One point is the protection, or potential lack of it, provided by a PSV during a fire. The boiling liquid in the vessel from the fire helps keep the metal 'cool' so it retains its strength. Once the liquid is gone or the flame impinges on the wall not in contact with liquid, the metal can quickly reach a temperature where it has insufficient strength to withstand the internal pressure and you have a BLEVE. Not something, you want to be around. As an added point to the information above, if 50 Btu/lb is not your company's minimum standard for latent heat, here is an alternative to calculate the latent heat:
Submitted by: Administrator

If you want to try and determine latent heat the most accurate thing would be to take a process simulation tool such as PROII, HYSYS, winsim or the like. Set up a stream with the relevant composition and pressure and make a bubble point calculation (=the program calculates the temperature where the liquid starts to boil at the PSV set point P+allowable overpressure). Then add a heater to the stream with a specified duty e.g. the resulting vapor stream and the duty that you specified can now be used to calculate the latent heat. Important: You have to specify a relatively low heat input because you want the latent heat for the first fraction that boils off
Submitted by: Administrator