External Fire Plus Chemical Reaction

[Chance17]

I have a current design problem.
An external Fire can initiate a chemical reaction inside a pressure vessel.
The reaction is initiated at 450-F. The temperature at safety valve set pressure is about 500-F.
The heat of reaction is well known.
The heat input due to Fire is about equal to the chemical heat of reaction.
The mode is two phase flow.

My view of ASME Code - 21% accumulation is used for Fire only.
Because of chemical reaction, the allowable accumulation drops down to 10%.

I guess my view is the same for two phase flow.
The 21% accumulation applies only to vapor venting. For two phase flow cases, the accumuation drop down to 10%.

Well my client sees the code allows 21% accumulation for both cases.

Any comments would be apprecated.

 

[Latexman]

My company interprets ASME Code as fire plus reaction is 10% accumulation.

Good luck,
Latexman

 

[moltenmetal]

We're presuming there's more at stake here than just a larger valve, otherwise the answer is obvious- put in the larger valve sized for 10% accumulation for the combined case.

Is there another case which can cause the excessive temperature needed for the autothermal chemical reaction?

If so, you need at minimum a valve capable of the relieving rate for reaction without external fire (which you stated was about the same as the fire rate) with only 10% accumulation, and one for combined fire/chemical reaction with 21% accumulation. Depending on the details it could be either case which dominates.

Obviously 10% accumulation for the combined fire/reaction case is more conservative. Is that conservatism warranted? None of us here are involved enough to advise you properly.

Do you intend to attempt to re-use this vessel after such an event? If so, the 10% accumulation for the combined case is probably warranted.

 

[Chance17]

So -- what is the allowable accumulation for Fire plus two phase venting ??? 10% or 21%