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Fire protection - heat exchangers- petroleum

Fire protection - heat exchangers- petroleum

Fire protection - heat exchangers- petroleum

(OP)
Dear all,

I would like to listen your opinion on how the relief flow through a PSV can be estimated in the case below :

Equipment : heat exchanger full filled with petroleum in case of fire ( in the shell and/or tube side ).

Petroleum is a broad range boiling point liquid. How can I calculate the relief temperature and the latent heat to estimate the relie  flow?

Thanks in advance

Ney

RE: Fire protection - heat exchangers- petroleum

Ney,

I've seen different approaches taken by different companies so you should probably review other systems in your plant and try to follow past practices.  But here are a few considerations.

I presume you have already determined that you need to provide separate relief protection for your exchanger.  In the case of fire exposure and depending on whether you follow API or NFPA the exchanger is located within 25 or 35 ft of a surface that could support a pool fire.  But for reference you may want to look at some of the discussions in API RP-521 Technical Inquiries
http://www.api.org/techinq/PRS/521/521_inquiries.htm#top
Specifically look at the question regarding "the need, or lack of need, for relief protection for heat exchange equipment".

As far as determining the heat load on the exchanger due to fire exposure, refer to API RP-521 equations for heat absorbed by a vessel exposed to an open fire.  Don't think there is much question about what is considered wetted area for the shell side but the tube side wetted area exposed to the fire is limited to the heads of the exchanger (this of course would be different for air cooled exchangers).  However, I've known some companies to add the shell area to the tube side evaluation since one could argue that you do get some heat transfer to the shell & shell fluid to the tubes.

The liquid full aspect of the exchanger can be a problem since it could involve 2 phase relief sizing, however, from API RP-521 (1997), Section 3.15 External Fire -
"There is an interim time period between the liquid expansion and the boiling vapor relief in which mixtures of both phases need to be relieved simultaneously, either as flashing, bubble, slug, froth, or mist flow until sufficient vapor space is available inside the vessel for phase separation. This mixed-phase condition is usually neglected during sizing and selecting of the pressure relief device. However, the impact of two-phase flow should be considered for the design of the piping system, liquid knockout facilities, and metallurgy (for example, potential for brittle fracture)." and

"Should a pressure relief device be located below the liquid level of a vessel exposed to fire conditions, the pressure relief device should be able to pass a volume of liquid equivalent to the displacement caused by vapor generated by the fire."

My experience has been if the point where the inlet line to the PSV is connected to the system is located at a high point in the system and the PSV is elevated above the liquid level such that the PSV will see vapor at the earliest possible moment, then sizing for vapor only for exchangers is reasonable.  

Otherwise, if the point where the inlet line to the PSV is connected to the system is not located at a high point in the system or the PSV is not elevated above the liquid level, then you should consider sizing the PSV for a liquid rate displacement equal to the rate of vapor formation.  This liquid should be considered at its boiling point and would likely flash through the relief valve.

As far as the thermophyiscal properties of the petroleum, I don't have much experience in the refining industry or working with petroleum fractions.  But some process simulators allow you to characterize petroleum fractions as pseudocomponents and you can generate temperature, vapor pressure, latent heat, etc from that.  Of course you have to know some basic characteristics about your petroleum fraction in order to characterize it as a pseudocomponent.  

If you don't have access to a process simulator, the Gas Processors Suppliers Association, Engineering Data Book has a section that covers calculations of key properties of petroleum liquids.  I'm sure there are other references but I haven't worked much with petroleum fractions outside process simulators.

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