Fire case, unwetted surface
Fire case, unwetted surface
(OP)
API 521 Section A.5.2.1 gives an equation for discharge area. For the F' term, "The recommended minimum value of F' is 0.01; when the minimum value is unknown, F'=0.45." What does that mean? I can calculate F' from the given equation, however I get a non-real result because T1 is greater than Tw.
T1=(P1/Pn)*Tn. P1 is relieving pressure, Pn and Tn are normal operating pressure and temperature, respectively. T1 gets so large because system operating pressure is so far below relieving pressure.
So, does this mean "the minimum value is unknown" and I should use F'=0.45?
Thanks!
T1=(P1/Pn)*Tn. P1 is relieving pressure, Pn and Tn are normal operating pressure and temperature, respectively. T1 gets so large because system operating pressure is so far below relieving pressure.
So, does this mean "the minimum value is unknown" and I should use F'=0.45?
Thanks!





RE: Fire case, unwetted surface
A relief vavle in gas filled vessels is of limited use. It will maintain the vessel pressure leading to ruputure regardless of the starting conditions. That is the reason depressuring systems are used to protect gas filled vessels subject to fire.
--Mike--
RE: Fire case, unwetted surface
RE: Fire case, unwetted surface
--Mike--
RE: Fire case, unwetted surface
If I just base the overpressure on set pressure to get a lower P1 value, T1 falls below Tw and all seems peachy.
Does the following make sense? With a low set pressure the heat input expanding the vapor causes the PSV to lift at a temperature based on the set pressure, not an inflated relieving pressure based on MAWP. So, in essence, I am beginning to depressure the vessel before the internal temperature reaches Tw (1100 F). If so, wouldn't we be reaching the goal of protecting from overpressure meeting the intent of the code? Of course understanding the vessel itself would be toast from the fire anyway. Hmmm....
RE: Fire case, unwetted surface
The issue of vessel failure will still remain unless you depresure the vessel or put the fire before the vessel wall gets too hot.
--Mike--