PSV sizing for fire scenario on a PIG trap
PSV sizing for fire scenario on a PIG trap
(OP)
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!!
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!!





RE: PSV sizing for fire scenario on a 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
RE: PSV sizing for fire scenario on a PIG trap
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.
RE: PSV sizing for fire scenario on a PIG trap
Im not a PED expert, but you could look here http://ec.europa.eu/enterprise/sectors/pressure-an... (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
RE: PSV sizing for fire scenario on a PIG trap
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
RE: PSV sizing for fire scenario on a PIG trap
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.
RE: PSV sizing for fire scenario on a PIG trap
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
RE: PSV sizing for fire scenario on a PIG trap
RE: PSV sizing for fire scenario on a PIG trap
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
RE: PSV sizing for fire scenario on a PIG trap
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
RE: PSV sizing for fire scenario on a PIG trap
API 520/521 provide thermal expansion properties of crude oil and water based on API gravity
RE: PSV sizing for fire scenario on a PIG trap
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: PSV sizing for fire scenario on a PIG trap
RE: PSV sizing for fire scenario on a PIG trap
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: PSV sizing for fire scenario on a PIG trap
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
RE: PSV sizing for fire scenario on a PIG trap
RE: PSV sizing for fire scenario on a PIG trap
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: PSV sizing for fire scenario on a PIG trap
RE: PSV sizing for fire scenario on a PIG trap
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.
RE: PSV sizing for fire scenario on a PIG trap
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.