Designing reliable fireproof insulation to achieve F = 0.1 to limit emergency pressure relief size

[David272]

Hi,

I have a question about using stainless steel jacketed mineral wool versus calcium silicate for fire protection of a process vessel. API 521 section 4.4.13.2.7.2 says that stainless steel jacketing and banding have demonstrated satisfactory performance in fire situations. A number of the threads on this site talk about using mineral wool that has been jacketed and banded with stainless steel for fire protection.

There is a 9000 litre process vessel, which cannot be realistically protected against two phase flow in the fire case, in part because it vents at very high level, above roof level, and because its worst case reactant has a very low latent heat per mole. There is a preference to have a pressure safety valve in the emergency vent rather than just a bursting disc, which also limits relief rates unless you install a relief valve the size of Kilamonjaro and then some poor shmuck has to remove it down to the workshop for testing once a year. My company will not want to reduce the level in the vessel a lot because that would reduce the throughput that is available from that vessel. Avoiding the risk of fire entirely does not seem to be an option either since there is hot oil and boiling flammable reactants present.

The obvious solution is to make use of a low environmental factor, to limit the amount of heat that can go into the vessel in the event of a fire. In order to maintain full usage of the vessel, it would be necessary to achieve an environmental factor of 0.15. I would aim for an environmental factor of 0.1 or better to be sure, and then go slightly over the top on the insulation/cladding reliability. I am aware that some design houses will not use an environmental factor less than 0.3 because they say that it is difficult to be sure that insulation will all stay on in the event of a fire, whereas others have no such rule, and the codes do permit the use of an environmental factor less than 0.3. A safety advisor from a previous company told me that you couldn’t rely on stainless steel cladding not to warp, buckle and fall off at the high temperatures in a fire, although this does not seem to chime with API 521. My company is a small one with just one site with old vessels from the 1960s and will want to do things well and professionally and economically. Can anyone offer any advice on how to go about designing insulation / cladding to make it reliably fire-proof without breaking the bank, or recommend a book? Alternatively, tell me why API521 is demonstrably wrong and stainless steel clad mineral wool cannot reliably resist a fire.

We also have some vessels with limpet coils that need occasional repair welds. I have no idea how to deal with those because it seems impossible to provide fire proof lagging that can easily be removed for maintenance inspection. The best plan I’ve managed so far involves throwing them away and starting with new vessels, so any views on those would also be welcome.

Thank you!

 

[met11]

David272, have you considered relocating the relief valve to the bottom of the vessel? It could cut down on the size dramatically (liquid service but flashing in nozzle/tailpipe), and make access easier. It's quite a big decision to use an environmental factor so low, I personally wouldn't be comfortable doing that.

 

[David272]

Met,
Thank you for replying. As it happens, this vessel does not have a bottom run-off, so for this particular case, it would not be possible to have a pressure relief valve at the base of the vessel. I have never heard of having emergency pressure relief from the base of a vessel before, but thinking it through, the idea may have some merit. For vessels with limpet coils, having a liquid emergency relief line at the base in addition to the one at the top could be helpful because it could be designed to drop the level and stop two phase flow from taking place out of the top vent before the MAWP is reached. This would rely on the relief pressure being set well below the design pressure and the base relief being big enough to drop the level sufficiently before the vessel MAWP was reached. I am curious whether other readers have experience with using base relief as well as or instead of top relief.
Thanks
David

 

[don1980]

Personally, I'm comfortable using a 0.10 F factor, if that's the calculated value based on the insulation material and its thickness. Within different companies, there's a range of different practices for minimum allowable F value. In my experience, EPCs commonly don't go below 0.3 without authorization to do so by the client. For an EPC, that's a reasonable default practice because they don't want to be accused of presuming to know the client's risk tolerance, or the integrity of their safety management system. For operating companies, I find that most are tolerant of allowing F values that are lower. API 521 allows it, and it's been experimentally tested. Regarding the claim by you former safety advisor that SS cladding and banding isn't sufficient, I think the burden of proof is on him to support that claim, rather than being on API 521. You can see an extensive explanation of the API 521 methodology in Appendix A and in Appendix C (see C.6).

Protecting external cooling/heating coils from overpressure due to fire exposure is very problematic. These coils have a high heat flux (high surface to volume ratio) and they have narrow passages that render liquid-vapor disengagement impossible. Therefore, if one chooses to protect the coil from fire using a PSV, it's hard to justify not sizing that PSV for 2-phase relief. The common outcome from that calculation is that the PSV inlet connection is larger than the nominal size of the coil. Additionally, one should consider the fact that this PSV is going to very quickly empty the liquid from the coil. Once that happens, you have a vapor filled vessel which will fail due to excessive temperature. After considering these things, the logical conclusion is that external coils can't be adequately protected from fire exposure by using a PSV. The conclusion I draw from that is that there's no point in sizing the PSV for fire. Instead, install a small PSV for liquid thermal, or just for code compliance if the coil contains vapor rather than liquid.

 

[georgeverghese]

In some vessels with large volatile hydrocarbon liquid inventories at high pressures, we've gone the extra mile to provide Chartek III fire rated passive insulation. See if this will be necessary in this applicaton.

Often, fire case relief loads derived manually are much larger than that produced by a process simulator which can replicate the fire case operation and account for incremental sensible heat absorption also. It may be some what more difficult to model this in this application where you have a chemical reaction during the fire incident.

 

[David272]

Don, George,

Thank you for your replies. Thankfully the reaction going on in the vessel is reversible and not significantly exothermic, so it's possible to ignore that. I had only been thinking about protecting the contents of the vessel and not the coils or the jackets. However, it's an interesting point that the volume of hot oil in the hot oil system is greater than the volume of flammable liquid in any one individual vessel. George, fire rated passive insulation sounds worth looking into, thanks for the idea. And yes, we are considering looking at getting hold of a package because the excel based simplified methods will be over-conservative, or take a long time to write if you want to make them less conservative.

Thanks

David

 

[MortenA]

You have to remember that if you for some reason must remove the PFP (e.g. vessel inspection) then you are looking at a total plant shut-down.

Best regards, Morten