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Fire Case for Small Plant

Fire Case for Small Plant

(OP)
Hi All,

I'm doing the process design for a small new build facility, a near replica of an existing plant we have. The existing plant was designed by another party who just sized everything for the fire case without, I suspect, thinking too hard about what our credible worst case actually is. In the main processing part of the facility, the worst case liquid volumes that could credibly be released are very small (max 2m3 if the distillation column floods which it never has done to date) therefore I don't think it is credible that this type of release could sustain a fire under a vessel leading to a relief event.

Does anyone know of any resource I can use to perhaps work out the fire area for the plant, or anything else I could use to assess whether or not the fire case is actually credible for each vessel? API 521 states any vessel at a height of less that 9m could be exposed to fire but I think that's based on large refineries where liquid volumes are significant throughout the plant.

Cheers,
PR

The scientist describes what is; the engineer creates what never was.

RE: Fire Case for Small Plant

API 521 provides some guidance on fire zones. Another way is to perform fire risk assessment that will identify potential releases, which is typically followed by consequence modeling for the identified/credible releases.

This is job for a specialist company so unless your team has experience in performing these types of studies, you should engage a 3rd party for fire risk assessment.

Dejan IVANOVIC
Process Engineer, MSChE

RE: Fire Case for Small Plant

(OP)
Thank you for your response Dejan. I was hoping to find reference to some guidance that I could use to conduct the fire risk assessment myself, as I fear that the cost of an expert would exceed the cost of us just going with the bigger valves :)

PR

The scientist describes what is; the engineer creates what never was.

RE: Fire Case for Small Plant

ProcessRookie,

It's not too bad to do yourself, I've done it a few times since the plants our company builds are usually in the same category as yours. You can take your total inventory (2m3) and spread that out over your plant layout. Assuming no drainage, take the footprint of the area that the liquid could spread. There are equations to determine the spread rate of a liquid that is spilled, as well as equations to determine the spill rate of a liquid given a hole size in the container if you really want to get into detail. However for your case, just assume the distillation column ruptures instantly or something, or a pipe breaks loose, and it all just spreads onto the floor.

You have an area, and you have a volume of liquid, so you can figure out the depth of the liquid spill. If the area is entirely open, then you can assume a liquid film thickness after it spreads. If it's a normal hydrocarbon, it will spread pretty quick (just imagine when you throw a bucket of water onto the ground, it's a thin layer almost instantly) For something like diesel, it would be in the neighbourhood of 2 mm thick (got this from some AICHE Publication in 1999, "RELEASE - A Model with Data to Predict Aerosol Rainout in Accidental Releases).

For a pool fire, the liquid would burn at a rate of approximately 4 mm/minute (I think this is from the SEPE Handbook of Fire Protection Engineering). So now you have a liquid thickness, and a burn rate. So you can get a fire duration. Take every vessel in the plant at its normal operating pressure and temperature, and use the API equations to find the fire heat input to each vessel. See if in that period of time, there is enough heat to actually raise the liquid to its bubble point, and also boil off enough liquid to fill the headspace and bring the contents to the relief valve set pressure. If the fire doesn't last long enough, I think your case is closed.

Just make sure your inventory is correct. If you have a large storage tank anywhere, any spill or broken flange or whatever could give you much more than 2 m3 of liquid.

RE: Fire Case for Small Plant

Although they may not consciously recognize it, engineers are performing a risk assessment each time they do a relief design. The scenario evaluation task, regardless of whether fire is a potential risk, is a textbook risk assessment task. The decision of whether or not fire is a credible scenario is just part of this risk assessment that engineers routinely perform. By default, engineers tend to count fire as a credible scenario whenever there’s doubt, and in many time they will include fire even when fire isn’t a realistic risk, or in cases where the PSV is incapable of providing any meaningful protection (e.g. vapor filled vessel, or a vessel that will be quickly emptied of the little liquid that is present).
All design codes and standards intentionally leave it to the user to determine whether or not to consider fire as a credible scenario. Standards such as API 521 provide helpful guidance for evaluating most types of scenarios (instrumentation failure, loss of cooling, check valve failure, etc.) but there’s little to no guidance for deciding whether or not to include fire exposure in the first place. That’s a case-specific decision (frequently a very subjective one) that is left to the user. Once you’ve decided to include fire exposure as a sizing scenario you’ll find plenty of guidance on how to proceed, but that initial decision is intentionally left to you.

Factors to consider when deciding whether or not to size for fire exposure:
  • Are there flammable materials in the vessel or in nearby vessels/piping?
  • Does the vessel contain at least some amount of liquid, and will that liquid boil at the relieving pressure without exceeding the yield temperature of the vessel
  • Is a sufficient portion of the vessel’s internal surface area wetted by this liquid?
  • Will the vessel be isolated (inlet and outlet paths blocked)?
  • Will the fire duration last long enough to cause overpressure in the vessel?
When we size PSVs for fire we’re assuming that the answer to these questions is “yes”. If the answer to any of these questions is clearly not “yes”, then there’s justification to focus on other layers of protection, other than a PSV. Engineers should especially be sensitive to recognizing those cases in which a PSV is incapable of providing meaningful protection, despite a real risk of fire exposure. In those cases we do a disservice to the client by proceeding with sizing the PSV for fire. We give the client the misimpression that the PSV is actually providing protection when it’s really not, and this results in other protective measures being ignored.

In the specific case we're discussing, 2m3 of liquid can sustain a fire for a pretty long time if that fluid is leaking from the vessel, and the fire can't be easily extinguished.

RE: Fire Case for Small Plant

(OP)
Thank you very much for your responses. I like the sound of your suggested calculation method met11 and will proceed on that basis to check the credibility.

Regards,
PR

The scientist describes what is; the engineer creates what never was.

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