Fire and Non-Fire cases in Relief Valves? 3

[mucour]

Firstly, I must state that I understand the calculation process of sizing relief valves on a fire case and non-fire case conditions.

My question is trying to understand the case to select, i.e fire or non fire case

While I was reviewing the relief valve used for protecting a vessel installed on an un-manned wellhead offshore platform, I was wondering whether the sizing was based on a fire case or non-fire case sizing scenario. I do not have the calculation sheets to make it clearer.

Could somebody explain to me what are the basis for using a fire or non-fire case in relief valve sizing?

Is it based on the likelihood of fire on the plaform no matter how remote?

If on the basis that the plant is used for processing oil and gas which are flammable in nature and there is possibility of a lightning strike, is that enough to conclude that the vessel relief valve must be sized using the fire case sizing procedure?

If the platform is un-manned, does that influence the case to be used: fire vs non fire case sizing.

The fact that the platform is offshore and un-manned, does that influence the sizing condition to be used?

Any clarification/guidance on the above would be appreciated.

 

[PeterPiper]

mucour,

I'm not sure I understand your question fully but we use a guide word based system to determine which case to use for relief valve sizing.We would work down the list and perform the appropriate calculation where there is a reasonable probability that the situation could arise. The case that generates the largest valve size is, obviously, the one installed. In your case I would imagine it would be the fire case, but if you have some high pressure utilities or assosiated process it could be from there. Examples when the non fire case would be used are "Guillotine" rupture of a steam heating coil, but basically any form of overpressure not related to fire. I'm affraid I have no experience of off-shore applications.
Relief Cases Guide Words
1 Electrical failure
2 Cooling water failure
3 Steam failure
4 Inst. air failure
5 Fire
6 CV failure
7 Valve closed/opened
8 Blockage
9 Restriction
10 Excess flow from
11 Overfilling
12 Coil failure
13 Pump failure
14 Vacuum
15 Exothermic reaction
16 Coolant failure
17 Overheating
18 Shock waves
19 Check valve failure
20 Cold into hot
21 Outbreathing
22 Inbreathing
23 Thermal relief
24 Other

 

[TD2K]

I think I've misunderstood your question but the selection of a fire case versus non-fire case is based on the load(s) the PSV must handle. Essentially, you go through the procedure of looking at the resulting relief load, if any (there may not be) for the various cases PeteRJ has listed. The relief case with the largest required area for providing at least the required capacity is therefore designated as the sizing case.

If you are asking when to consider a fire relief, it depends on the system being protected, the system and the companies specifications. Blocked in piping, for example, is rarely provided with relief valves for a fire case. Vessels typically consider a fire relief case but not always. Some companies deem their procedures calling for blocked in equipment to be promptly drained of liquids sufficient protection.

 

[mucour]

Thanks PeteRJ and TD2k,

But from experience, there is always possibility of fire on a platform used for process flammable hydrocarbon. That is the more reason why fusible fire detection plugs are recommended in line with API-14C.

The fact that the amount of load (from both liquid and gas) during fire condition means that the relief area will be much higher in a fire case than in a non-fire case.

Based on above, I am of the opinion that the relief valves installed on pressure vessels used in the oil and gas industry are sized based on fire case condition.

What do you think?

 

[mucour]

TD2k and PeteRJ,

In addition, don't forget that it is expected that during fire situation, an ESD is trigerred. Meaning that the hydrocarbon fluid are trapped in the plant and a depressurisation is expected to take place.

But since the relief valve is expected to protect the vessel and it is a last resort, we can assume that the plant is not depressurised and the fluid containment is still in place.

 

[CHD01]

You must first determine if the possibility of an external fire exits around the vessel or equipment being protected. This involves the flammability of what is around the vessel. If the concern is the flammable liquid inside the vessel leaking out, then the issue is will the liquid be retained realistically under the vessel and serve as a heat sink or not. These are the issues to consider. Note that whether the liquid inside the vessel is flammable or not does not determine whether the fire case applies; the fire case could apply for either flammable or non-flammable liquids. Once you know the fire case applies, then the isssue is what can you do to limit the size of the relief valve by providing drainage, sprinkler systems, insulation, etc. This involves setting the fire Kp factor to 1.0 or 0.5 or 0.3 or even for some plants 0.15!!!

Finally, to answer your direct questions, I would say NO - the remote-ness of the platform and whether it is manned or not has no bearing on using the fire case; it only limits your potential consequences of a fire. Lightning strikes I view as a separate safety issue that must be guarded against by provining adequate grounding, etc. If a hazards analysis results in considering the fire case for lightening then I would do so.
The more you learn, the less you are certain of.

 

[grt12]

The fire case relief is not always the determining relief case for a pressure vessel. In a fire, unless you have a vessel extremely full (which is rare on a platform) the relief is of vapour not liquid therefore you do not have liquid and gas being relieved during the fire relief. To determine the relief valve size you must go through all credible scenarios for causes of overpressure in the vessel, including fire, and find the case that gives the biggest relief valve - this valve will then cater for all other relief cases.

 

[25362]

In addition to what has been said, fires may be tricky in the sense that well calculated RVs may not help, because the metals of the container under fire soften by heat and lose their mechanical strength. Quick depressurizing arrangements should be provided to avoid this occurrence. Draining may also be impossible because of debris or of hydrates choking valves.

There are a wealth of design practices intended to prevent fires from escalating and to mitigate their effects.

Trevor Kletz has written a series of books on safety matters that I heartily recommend for reading.

 

[1951101]

Mr Mucor by this time I think it is already clear about the governing case for PSV design,

The following is my opinion on the subject

1. If there is a pressure reduction by a control valve (or by other means), the down stream equipment has to be checked for block discharge case. If the liquid level in the down stream equipment is low or if there is no liquid Block discharge may govern the design.

2. If there is no liquid in the vessel, Fire case design is expected to give very high (unreasonable) relieving temperature. For such vessels containing only gases Fire case design may not be required.

3. If there is already a PSV on an upstream equipment with same set pressure (design pressure) as the down stream equipment PSV set pressure (design pressure) then block discharge is generally not applicable. Fire case or thermal relief is to be checked.

I think the details discussed in API 14C and API 520 are adequate to take a decision. Other applicable standards like API 2000 (for atmospheric storage Tanks) shall be referred, which is already known to you.

Regards

P. Bala Brahmam