Fire case audit of existing PSV

[omsava]

This is my first time at doing such an audit to ensure adequate orifice area per a HazOp study recommendation. This PSV is on a fractionator originally designed as a deethanizer but is now being used as a depropanizer. The feed to the column is mostly butanes with some propane. It is operated on total reflux and overhead propane ends up in our fuel system while the bottom butanes in LPG. The PSV is set at 550 psig per its original function.

1. In a fire scenario, assuming 100% n-butane the relief conditions become very close to its critical poperties ~ 565 psig and 305 F.
2. Ignoring the above and performing the calculations per Consolidated's API Fire Sizing for liquid hydracarbons the required orifice area is less than the actual.

How should I proceed from here to determine the adequacy of the PSV?

Thanks.

 

[Latexman]

Look at thread1203-145851 to the article on Google.

Good luck,
Latexman

 

[dcasto]

Once you read the article, did't you already answer your own question? You stated the required area is less than whats in place. I would expect your results because the relief valve on a deethanizer is always bigger than on a similar sized depropanizer, its a density thing.

 

[gene2007]

If you are concerned about discharge near or above the critical point, you may want to determine whether or not it is necessary to have the relief valve set so high. Lowering the set pressure of the relief valve would make sense if the set pressure is significantly above the maximum operating pressure of the column.

You may want to examine the loss of power scenario and, also, the loss of cooling scenario, especially since you indicate that the required orifice size for the external fire scenario is less than what is required. I've found in many columns that the worst credible overpressure scenario is not the external fire scenario, but either the loss of distillate cooling or the loss of electric power.

 

[omsava]

Thanks all,

Latexman - I am trying to download the article.

dcasto - I was concerned about the validity of API method at supercritical conditions.

gene2007 - You are right, I was planning to evaluate lowering the set point if I did not get guidelines for sizing relief valves for supercritical fluids (The column normally operates @ 180 psig). At this point, I don't plan on examining the loss of power/cooling scenarios since the recommendation came from a HazOp study specifically recommending audit of the fire case.

Regards.

 

[pipehead]

If you use an EOS to find the density instaed of a generalized supercompressibility the API equations will work. The mass rate is what you are really inputting into the equations.

 

[Tweec]

API 521 Section 3.15.2.1.2 states that discharge areas for pressure relief on vessels containing "super-critical fluids, gases, or vapors" can be estimated using the equation provided. A) What is meant by super-critical? B) What sizing equation is used when the fluid is NOT super-critical? C) Why doesn't the equation provided take into account any of the properties of the fluid? D)The previous section states "the heat absorption equations listed in the following (?) are for..." Since there are no such equations in the following section, what is being referred to here?