Overpressure Protection on Steam Jackets

[ggordil]

During recent PHAs, it became clear that we were lacking protection on steam jackets for the scenario of external fire. Several clients have equiped their jackets with PSVs, however, the concern has been raised, that the relief valve may not provide adequate protection for a jacket as there is very little heat transfer from the jacket to the steam, therefore, the presseure is not expected to rise very much. This presents a problem that the jacket may fail very quickly while still under pressure, as the walls of the jacket would heat up and fail. We have determined that the installation of fusible plugs would provide a means of relieving the internal pressure of the jacket before equipment failure due to exsesive wall temeprature.

My questions are as follows:
Does the above make sense? It makes sense to me and other engineers here but I just want to check

Are there other refineries/chemical plants/engineering firms/etc. following this same logic?

Also, has anyone ever heard of or seen a jacket fail (or other vapor filled vessel) fail during an external fire?

Thanks in advance for the help.

 

[don1980]

That makes perfect sense to me. When people consult with me about pressure protection for jacketed equipment exposed to fire, I explain that a fusible plug is about the only way to prevent the jacket from failing. That true whether the jacket has liquid or vapor.

If it contains vapor, then you have the same problem as you have with any other vapor-filled vessel which is exposed to fire. Regardless of size, a PSV won't prevent the vessel from failing during fire exposure unless there's a boiling liquid inside.

If the jacket contains liquid, then you have a classic 2-phase relief application. In most every case the jacket passages are too narrow to allow for liquid-vapor disengagement, meaning that 2-phase relief flow is inevitable. That requires a bigger PSV and you'll usually find that the inlet connection is too large for the jacket. Even if it's not, installing that PSV won't prevent the jacket from failing unless the fire is extinguished in less than ~10-15 minutes. That's because the 2-phase relief will very quickly empty all the liquid from the jacket. When that happens....well, refer to previous paragraph.

So, the only way to save a jacket from failing is to de-pressure it, and the most practical way to do that is with a fusible plug. That doesn't mean you don't need a PSV. You need to install a PSV just for code compliance. If you have other credible relief scenarios, besides fire, then size for the worst of those scenarios. If there are no other scenarios, then you get to pick any size PSV you want. I choose a small one, one with an inlet connection that matches the smallest nozzle allowed on the jacket.