Fire contingency for gas only vessels

[MarkkraM]

1. Does fire need to be considered a contingency for pressure relief if the vessel is gas only since failure of the shell from overheating would occur even if a PR valve were provided?

2. If there are no over pressure contingencies, do codes really ask for a relief device? If they do, how do they expect us to size it?

 

[KenA]

MarkkraM,
I'm no relief expert and I'm sure that you'll get plenty of replies to your post, but here's my thoughts:
(1) YES, there is a case for overpressure protection for external fire on a dry gas vessel. Fire will heat gas, which will expand and could overpressure vessel. Refer to API 521 Appendix A for method (Heat Absoprtion Across The Unwetted Surface Of A Vessel)
(2) If there a no overpressure cases for the vessel then ASME vessel code does not require a relief valve. But you must be satisfied that you have looked at all scenarios. Local codes may differ from ASME.

Look forward to reading other opinions on this subject!
K

 

[MJCronin]

mark,

OSHA 1910.10 requires that an evaluation be made of the plant facility and various scenarios considered. This evaluation must be by a group of professionals familiar with the plant operation and design. The evaluation must be updated and reviewed periodically.

For gas-filled vessels subject to a fire case, rated fireproofing can be added to the exterior and/or an acitive sprinkler system can be considered. The design fire case does not last forever.

Pressure relief valves are only one method used to accomplish the ends required by OSHA in 1910. Check with your local authority.......it could be that the vessel must be protected simply because it falls under state statute.

My opinions only

MJC

 

[pleckner]

KenA's point #2 is WAY OFF!!!

All pressure vessels that fall under ASME Section VIII, Div 1 MUST, READ MUST be provided with a relief device, no exceptions but one (and even this may not be an exception).

This exception has to do with invoking ASME Code Case 2211 which allows the engineer to design for containment. Without getting into detail here, invoking Code Case 2211 requires you to do everything that you would normally do but buy the relief device itself. And, it requires that you get pre-approval from the local authorities before the vessel manufacturer will even accept the Code Case; the manufacturer must include in his vessel report that the owner has accepted Code Case 2211. Many localities won't even accept Code Case 2211.

So, even if you don't have what looks like a credible scenario, many people either size for a ficticious fire scenario or use a themal expansion scenario.

The best way to guard against a fire scenario for a gas filled vessel is to set up a depressurization system. The system activates on elevated temperature, rapidly reducing the pressure in the vessel and emptying the contents. However, this will not prevent the need for the relief device but will help protect the vessel from catastrophic failure due to excessive metal temperature.

MJCronin is also giving good advice.

 

[MarkkraM]

Thanks for the feedback so far.

This concept of creating a fictitious fire scenario is something I was trying to get at in the discussion. For many of the fire relief cases given on the gas vessels I work with, I am struggling to work our where the fire is coming from. Perhaps this is another thread altogether, but what constitutes a credible fire scenario? Does it require more than just potential pooling of liquid hydrocarbons? or are things such as rupture of a gas line credible? Do I need to consider someone piling a bunch of twigs together under the vessel and lighting it ... ?

 

[KenA]

In my defence - I should have been more specific. The system must be protected from overpressure. This may not necessarily require each individual ASME pressure vessel to have a relief valve. For example the vessel overpressure protection could be provided by relief on an upstream vessel so long as there always is an open path between the vessels (i.e. just pipe). Totally agree with pleckner that you must consider fire case. It is not for us to speculate how a fire may or may not start - just assume that a fire exists! Hence my answer #1.
k

 

[EGT01]

You may want to look at the discussions in this thread regarding relief valves on pressure vessels
thread794-40785

But let's consider the first part of your question - Does fire need to be considered a contingency?

I think it is reasonable to say that fire can be excluded if there is no flammable material in the area. Actually, API RP521 suggests that equipment higher than 25 feet above a surface that could support a pool fire are normally excluded. NFPA standards I believe use 35 feet.

Also, consider NFPA standard 30, Flammable and Combustible Liquids Code. In their section Emergency Relief Venting for Fire Exposure for Aboveground Tanks, they have an exception "Tanks storing Class IIIB liquids that are larger than 285 bbl (45,306 L) capacity and are not within the diked area or the drainage path of tanks storing Class I or Class II liquids do not need to meet this requirement."

Now, ASHRAE Standard 15 for refrigeration equipment gives a method for determining the minimum required discharge capacity of the relief device for each pressure vessel that they say is based on fire conditions. Their basic formula is used even if there is no flammables in the area, if flammables are within 20 ft, then you use a 2.5 multiplier.

Flame impingement from a gas line rupture may be a different story. Seems I've seen some studies/standards relating to this but I'm at a loss to recall at the moment.