Fire case, unwetted surface 1

[MechBRR]

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!

 

[MikeClay]

When T1 is greater than Tw, the temperature in the vessel is higher than the allowable wall metal temperture. That is a non-real result. In otherwords, at the allowable temperature, the pressure will not reach the reliev valve set pressure. The vessel will continue to heat until the wall weakens enough for the vessel to fail.

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--

 

[MechBRR]

Thanks Mike for the response. I understood that the T1 was greater than Tw making the non-real result, but I wasn't connecting the dots that the internal pressure rise due to the heat input would not reach relieving pressure before the vessel would probably rupture. I'll do some more reading of API 521 to get some ideas for depressuring.

 

[MikeClay]

The comment on "non real" was not meant as a slap, but as a pun. The vessel contents can never be higher than the wall temperature. Thus the situation is non real, or maybe "imaginary".

--Mike--

 

[MechBRR]

Some second thoughts here. The reason the calculated T1 is so high is because relieving pressure,P1, is large compared to normal operating pressure,Pn. P1 gets so big because allowable overpressure is based on MAWP, not set pressure. This is a case where my set pressure is way below MAWP, which I was wondering about in thread1203-232829.

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....

 

[MikeClay]

You will satisfy the intent of the code for fire sizing the valve. But you will not get any "depressuring", you will just be relieving the excess pressure. The vessel pressure will remain somewhere between the set pressure and 21% over the set pressure.

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--