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[Proceed123]

My post is related to the following former post by fellow 'teedot': http://www.cheresources.com/invision/topic/12135-requirement-for-depressurization-system/ , only now from a somewhat different angle. (It seems  we are dealing with the same company in both cases. In my case, the question is being posted in the context of a grass-root HYDROCRACKER unit).

 

This company's design standard requires an emergency depressuring system (EDP), designed as per API 521, to be installed on PROCESS VESSELS which: 

1. Contain toxic or flammables fluids with True Vapor Pressure (TVP) higher than 2 bar @ 54 dec C, and 
2. are designed for pressures equal or greater than 30 barg.
 

What starts calling my attention is that, as far as I remember, the 'TVP' concept entails that the liquid of interest, though may include volatile components, is devoid of any dissolved fixed gases, as per ASTM D2879.  Since in my case the liquid of interest will come from the hydrocracker's reaction section at about 30 bars, it will surely contain dissolved gases in it. My thinking is that at the moment of the EDP system being eventually activated (either because a  Fire Case, an Adiabatic Case or other), most of the light ends (C4- components) and gas in the liquid effluent from the reactor will instantly pass to the vapor phase and be released to the flare system. Yet a tiny (but certain) amount of gas will remain solubilized in the remaining heavier liquid, and since the notion of TVP excludes any gas foreing to the pure liquid being dissolved into it, I believe that the TVP concept does not apply here, rigorously speaking, I mean.

 

Also, this is the first time that I find a company's criteria for determining the need of a EDP system includes checking the liquid's TVP at a reference temperature (certainly, in former designs, I have dealt with other companies' criteria, but never before I had seen the TVP aspect involved).

 

My questions follow: 

 

First, rigorously speaking, is it appropriate to invoke the notion of TVP in cases which, like this, the liquid of interest contains fixed dissolved gases in it? Or in other words, should the process simulation reflect the effect of the dissolved gases in the hydrocraker reaction system's effluent ACTUAL vapor pressure or not?

 

Second, can you make sense of such a 'TVP criterion' and how it actually relates to the need (or not) of providing an EDP system for protecting process vessels?

 

Third and last, are you familiar with published sources of information (articles, handbooks, etc.) that document the referred 'TVP criterion' or similar?

 

Many thanks in advance to all.