PSV sizing for fire scenario on a PIG trap

[DanielB79]

Hello,
I've come across a quite mangy PSV sizing. I didn't find any similar argument across the forum, so I've started a new topic. I have a PSV that is mounted on a pig trap (receiver) that must be sized for Fire Scenario but that has a Set Pressure of 211.2 barg (relief at 255.6 barg according to API-520): I've tried to simulate the crude oil conditions at that pressure but obviously HYSYS fails in doing that (supercritical conditions). The pig trap is essentially a piece of 16" pipe with a total length of 9000 mm, so the volume of trapped crude is not much. The crude inside is in liquid state (dead oil) and is possibly trapped inside in case of emergency.
How would you proceed? Any hint or help is very appreciated. Thanks!!

 

[LittleInch]

I've designed a lot of pig traps, but not come across a fire case before so that's the first line, why is there a fire case? Where is the fluid going to come from to create a fire under the pig trap?

Second, pig traps are part of the pipeline code and essentially a bit of pipe, how are other locked in bits of pipe being dealt with?

If forced down this line I would calculate liquid expansion of your crude from ambient to some suitable flame temperature to get your volume to relieve, working out first if the pipe can actually get to 210 barg before it leaks or ruptures due to loss of strength in your fire. My guess is that the pipe/trap will fail first thus making a psv irrelevant.

Last, as a receiver, you would normally expect it to be empty, so having all combinations working against you looks too conservative.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

Adding to the comments from Littleinch, which I agree with, there are important points you should understand:

1: It's not accurate to say that a PSV "must" be sized for fire. Pressure vessel codes don't mandate how to size the valve. They intentionally leave that to the user. They recognize that each case is different, and prescriptive requirements would just make a mess of things, resulting in relief designs that are inappropriate for the specific risks of specific applications. It's inherently hazardous for a relief designer to whip out a design based on prescriptive requirements without fully engaging their mind to assess the case at hand. That's why codes leave this to the user. Fire is one of the potential scenarios that relief designers should always consider, but there are no cases in which a pressure vessel PSV "must" be sized for fire. It's up to the user to determine whther a fire-sized PSV makes sense.

2: By default, PSVs are not installed on piping to protect it from fire exposure. Is this pigtrap piping (built to ASME B31.3) or is it a pressure vessel (built to ASME Sec VIII)?

3: The user is always free to install a relief device to guard against any scenario they regard as an intolerable risk. For example, the owner if free to size this PSV for fire if they perceive the need to do so. But, they should always assess whether the relief device will be effective at defending from that scenario. Unless you have a significant amount of liquid in the vessel, and that liquid will boil at the relieving pressure, then the relief valve isn't providing any real protection from fire. At the pressure you mention (211-255 barg) I doubt this liquid will boil (assuming there's even any liquid in the trap - usually there's not). Above the critical point, fluids don't boil (there's no Hvap), therefore a PSV can't protect from fire. If the owner really wants to defend this pigtrap from fire, then they need to look at other layers of protection (other than a PSV). Proceeding with sizing this PSV for fire is a waste of time/money and it misleads the owner into thinking that they're protected. Explain that a PSV won't do the job.

 

[MortenA]

Daniel - don't know where you live - or where this application is for. But if its within the EU (and it may be since you quote bar) you may be a little careful here. I think that pig traps are covered by PED. I could think of fire cases e.g. on platforms where there is less space. On onshore you usually place them away from the rest of the plant.

Im not a PED expert, but you could look here

http://ec.europa.eu/enterprise/sectors/pressure-and-gas/files/ped/ped-guidelines_en.pdf

(its guideline 1/17 I'm thinking of)

Wrt to boiling or not - i don't know. But thermal expansion will occur. But if its only for thermal relief you don't need to size it - D is sufficient.

One thing: You are just on the edge of being able to generate steam. Are you 100% sure that the pigging operation can't move water to the pig trap (water can accumulate in low points etc).

Best regards, Morten

 

[LittleInch]

The pig trap should be built to a pipeline code, or at the last a piping code, though that is not the best choice. It should not be designed to a pressure vessel code because it isn't one. Pipelines are not subject to the PED reefs as far as I know.

This is a glorified bit of pipe /pipeline. I know it has more bells and whistles than most bits of pipe, but that's what it is. Don't apply pressure vessel logic to it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MortenA]

@Littleinch Why dont you check the reference i gave - of course its onlyt relevant if the project is within the EU. On the other hand if it is.

Quote from the reference:
Question: What is the meaning of the expression "standard pressure
equipment" in article 1 § 3.1 on pipelines
Answer : A standard pressure equipment is not specially designed and manufactured
for a specific conveyance pipeline, but is intended for use in a number of
applications, including other conveyance pipelines or, for example,
industrial piping.
Typical examples of standard pressure equipment annexed with pipelines,
pressure reduction stations or compression stations may include:
measuring devices, valves, pressure regulators, safety valves, filters, heat
exchangers, vessels.
Such equipment is covered by the directive.

 

[LittleInch]

Yes, but a pig trap is none of this things, it's part of the pipeline and just a bit of pipe, which is not covered by the PED.

When the PED first came out there was a worry that pipelines would fall into it, but it has been established that they don't. The issue would seem to be where you place the "outlet isolation device", see 1:29 of your attached document. If this is downstream of the pig trap then it's outside, if you have this on the pipeline then it might be covered.

I don't think it makes any difference in this case.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

This discussion about PED isn't relevant to original question and has no impact on the answer. Like ASME, PED codes leave it to the user to determine the scenario. There's no requirement to consider fire exposure. The answer to the question is the same, regardless of whether this pigtrap is in Texas or Germany, or anywhere else in the world.

 

[DanielB79]

Hello All,
thanks for the answers. I'll reply with some other informations: the pig trap must be designed as per ASME VIII (even if it has the same design pressure of the pipeline that ends in it), it's considered as a pressure vessel and the event of fire is possible since the vessel is also protected by Fire Fighting System. The problem is that the discharge temperature at relieving pressure (even if Hysys isn't able to calculate it) would be so high that the vessel would fail before the valve opens. In general, PED Regulation is not applicable to my case: the Company for which we are verifying the PSV sizing requires the application of API Regulations.

Daniele

 

[LittleInch]

Who "considered it as a pressure vessel" and why?? Pig traps are specifically included in pipeline codes to stop this sort of thing and also allow the ID of your pig trap to be the same as the pipeline - please check this. I'll take bets this was determined by a plant/mechanical engineer who has never used pipeline codes before.

There are many posts here that talk about failure of the Pressure Vessel before relieving pressure for fire cases. I think the result of that (melt before PSV goes off) is that a fire case is not an applicable case to size anything and nothing needs to be done, but I'm sure I'll be corrected....

API are not regulations as far as I know, but are a publication or standard.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[tickle]

I am not sure that you need to know the code that the pig vessel is covered by or designed to. Just work through the scenario. If the vessel can be "locked" in full of liquid and there is a heat source (solar or fire) then overpressure protection is required. There are numerous bits of pipework and pipeline out there that can be trapped in and have thermal PSVs on them. I would consider the worst case fluid (crude or water), I would check for flashing over the valve, but the rate is going to be very small. The answer will be C or D orifice.
API 520/521 provide thermal expansion properties of crude oil and water based on API gravity

 

[LittleInch]

Pressure vessels and piping are dealt with differently and tend to come under different legislation and inspection, that's why you need know. The issue is how to spec a fire case. A fire case for a pig trap is not normal and the OP is saying you don't get to flash the crude at the set pressure, therefore no boiling, no gas and very little volume to relieve before the pig trap melts....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DanielB79]

The worst case, in my opinion, is when the pig trap has the pig inside with liquid crude oil behind it and a fire starts on the outside before the trap can be drained or vented. If so, the first shots of the PSV would be made only of liquid crude, but as a free space creates inside the pig trap, the released fluid would turn into supercritical fluid with very different flowrates.

 

[LittleInch]

Errr, what does "as a free space

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

Errr, what does "as a free space creates inside the pig trap" mean?

You said earlier that the temperature would be so high that it would melt before it got to the relief pressure.

I still don't see a fire case being a realistic scenario.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DanielB79]

If the vessel is full of liquid, the initial thermal expansion causes a liquid release probably at very low temperature due to the initial volumetric expansion that will pressurize the vessel quickly but the subsequent valve releases (as the contained fluid mass decreases) tend to be made of supercritical fluid at high temperature.

 

[LittleInch]

Still no space left inside the vessel until it cools down?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DanielB79]

Remains the problem to determine the relieving temperature/rate or, otherwise, to justify the choice of a D size orifice.

 

[Duwe6]

You are overthinking it. What is the boiling point of crude at the PRV pressure? And how much heat - BTU's - does the boiling crude 'take away' [latent heat of vaporization]?

You can only get supercritical fluid *if* you set the PRV to a pressure above the fluid's critical point. After that first spurt of thermal expansion [which is so small it can be ignored in the calc's], you have a boiling [vaporizing] fluid that is relieving as a vapor and removing heat. The PRV only needs to be big enough to remove the heat input -- pool fire, in this case -- fast enough to keep the internal pressure rise to something safe [ASME Sect VIII Fire Case allows 21% overpressure]. So you calc the heat removal, in SCFM of vapor, at 121% of the initial PRV setting.

 

[don1980]

Daniel, here's the direct answer to your questions. I'm not trying to be a smart-a**. Instead, just pointing out the futility of what you're proceeding to do....relying on a PSV for protecting this super-critical filled pig-trap from fire exposure.

For this specific application:
Relieving T: No limit...it'll rise to the yield temperature of the vessel.
Relief rate: Insignificant...rounds off to zero.
Orifice: Doesn't matter. You'll get the same result with one D orifice, or two T orifices.