At the heart of any relief system design basis are the “scenarios” chosen to base the sizing of the relief system. The two most common scenarios used are the “fire scenario” and the “blocked-in scenario”. It is the fire scenario I wish to review in this article and what has to go along with this scenario.
An external fire impinging directly on the shell of a pressure vessel is an EXTREMELY DANGEROUS situation! Ensuring that the relief system on this pressure vessel is sized adequately is a critical path in safety of that pressure vessel. But when does a facility need to consider a “fire scenario” in their relief system design basis? Does this only apply to processes that contain combustible and flammable liquids and gases? Absolutely not! If we have the three components to the fire triangle (Heat, Fuel, and Oxygen) and the fire can impact our vessel, either with direct flame impingement or just a large heat load, we need to consider a fire scenario in our design basis.
We recently came across some fancy software that had a check box in the “design basis” section asking if there were any combustible/flammable liquids stored within 20’ of the vessel. The question is being asked because the engineer wants to know if they should consider a fire scenario when doing their sizing calculations. But the more critical failure was that each relief valve design basis stated that the fire scenario was not one considered because there were no combustibles/flammables within 20’ of the vessel. As we toured the process we noticed large quantities of both ordinary combustibles and combustible and flammable liquids stored in very close proximity and sometimes directly under the vessels in question. This is in NO WAY the fault of the contractor engineer who did the sizing calculations, as I am sure the question was asked of the facility and the answer was “we never have flammables or combustibles in our process areas”.
If we are going to assume that we will never have a fire load within 20’ of our vessels (especially the un-insulated vessels) then we MUST have some type of administrative controls (or maybe even engineering controls) to prevent this from occurring! Of course it would be MUCH EASIER and SAFER to just consider a fire scenario and size the relief system for such an event. When we make the assumption that there will never be any type of fire load that could impact the vessel we are making one hell of a HUGE assumption. And without some type of controls in place, I can assure you that at some point, someone will “store” something, either permanently or temporarily, right up against or under our pressure vessel(s).
It seems that Turnarounds/Shutdowns are the worse times for these types of failures! It seems real-estate becomes very lacking so workers get creative on where they store their boxes of filters, pallets of bags, portable fuel tanks, etc. We have even come across a tote storage area that actually blocked the view of the vessels the totes were stacked so high!!! And yes, that tank too had a relief system that stated “no fire credible scenario”. And yes the tank was actually 90% full of a high vapor pressure materials and the safe upper limit for this tank was set at 85% capacity (DOUBLE WHAMMY!!).
So… is there ever a case when a relief system would not need to consider a “fire scenario”? I do not know, as I have never been associated with a process that made this assumption as we never wanted to increase our risks! But I am sure there is an engineer out there who could convince me that a vessel somewhere in this world would not need a relief system designed for a fire scenario – I have just never been in that position!
So for you internal and external auditors out there looking to take your PSM/RMP audits to the next level, just remember when you are reviewing the relief system design basis to look for the “relief scenarios”. If the facility states “no fire scenario credible”, be on the lookout for ordinary combustibles and combustible/flammable liquids in close proximity to their vessels! If you find these materials in place – then we know their relief system design basis is INCORRECT. Keeping in mind that the storage needs to be more than a single trash can before it could have an impact on the vessel.
PLEASE NOTE: a professional engineer with specific training in relief valve sizing should be used when designing a relief system! An insulated vessel can get “credit” for its insulation in some cases and not in others. A fire scenario should be considered BEFORE the construction or purchase of a vessel as there have been occasions where the purchased/built vessel did not have a large enough nozzle for the proper sized relief valve for a fire scenario, merely because the original design basis did NOT include a fire scenario!