Fire Relief Sizing for Heavy Petroleum with Cracking

Fire Relief Sizing for Heavy Petroleum with Cracking

Fire
relief sizing on equipment processing heavy petroleum resids (e.g. refinery atmospheric
columns, vacuum columns and other equipment) has historically been
problematic.  If a fire occurs on the columns, the column may experience
temperatures exceeding the design temperature with large relief rates.  API
Standard 521 “Pressure Relieving and Depressuring” has been changeable, by
adding and then removing guidance.  In actuality the resid may experience mild
thermal cracking into lighter components thus lowering the vaporization
temperature. Further, the heat utilized to crack the resid will not be used for
vaporization and thus reduces the fire relief rate.  Determination of the heat
of the cracking reaction is key but has been historically difficult due to the
large amount of computations.  However, a proven model utilizing recent simulators
can handle the computations.  This paper will provide guidance on pressure relief sizing.
It will outline a method to characterize the resid and products and use them to
determine the heat of cracking at different temperatures. The relief sizing
will be illustrated for several cases.  The method is practical yet
theoretically rigorous,  utilizes publicly available information, and is sufficiently
accurate for pressure relief determination.    

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