Relieving Condition Exceed Design Temperature

Equipment design temperature is usually determined based on the maximum temperature plus a certain margin. The margin could be different for each project, say 20F, 25 F or 30F. Conducting rigorous simulation for fire case, the temperature in each stage simulation is set higher than the previous stage by certain interval. Let say, using 10F in interval temperature will result in temperature about 30F higher than the operating temperature at third stage simulation. That is why; relieving temperature for fire case is very often higher than the design temperature. Whether or not increasing rating pipe is required?.

PSV Doesn't Provide Adequate Protection of Fire Case

See the picture below

A Separator has operating temperature of 120 F with design temperature of 145F. Looking at simulation above, in only two stages of temperature increase, the temperature will reach 150F.

After fire, the temperature will increase commonly above the design temperature. And even in some cases, the vessel could rupture before increasing pressure reaches PSV’s setting pressure. In other word, the vessel will have ruptured before the PSV open. Thus, the PSV doesn’t provide adequate protection for the vessel in case of fire.

For example, the following picture show that the metal plate temperature reaches 1200 F in only 15 minutes after fire.

That is, actually, the PSV doesn’t provide adequate protection for the vessel in case of fire. Therefore, some other options for vessel protection from fire case beside the PSV should be considered, such as: 1. Depressuring 2. External cooling 3. External insulation 4. Provide proper drainage

My friend, for now, we already know the fact that PSV will not give adequate protection for vessel in fire case. But, why do we always provide PSV as protection devise of the vessel? Why do we consider for fire case too? That could be a joke, right? In my opinion, that is all to meet the CODE requirement

My friend, a note shall be made, although the relieving temperature is higher than the design temperature, and the PSV is not efficient to protect the vessel against fire, the PSV remain to be designed based on the relieving temperature, since the PSV is installed as a safe guard

Example Case

I still remember, one of our friends has ever asked me; at that time, he did calculation of fire case for instrument air receiver, the relieving temperature was exceeding the maximum allowable for 150# rating pipe. The question was, whether the 150# pipe rating of inlet PSV would be needed to be increased to 300# or not?

Of course NOT, the pipe rating class of 150# does not need to be increased to 300#, since the determination of pipe rating shall based on maximum condition without consider fire case.

Besides that, it is not common to design PSV for instrument air system with fire case. Personally, I prefer calculate the PSV load of instrument air receiver based on blocked outlet case (if it is possible). Furthermore, In some cases, it might not be applicable to size PSV of instrument air receiver based on fire case. Actually, it shall be based on FERA (Fire and Explosion Risk Assessment) justification whether the instrument air receiver included in fire zone or not.

The block outlet case has more possibility occurred than the fire case. Based on my experience, the load of block outlet is also smaller than un-wetted fire case. Therefore, it is better to design PSV of instrument air based on block outlet case than fire case. Moreover, if you still design PSV based on fire case to consider worst case load, that PSV will not provide adequate protection for the vessel: D.

Do you agree with me, don't you?

Talk back in comment section below and let me know your opinion !