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# Relieving pressure when MAWP and Vessel Code are not known

## Relieving pressure when MAWP and Vessel Code are not known

(OP)
Dear all

I have a vessel with design pressure of 10barg.

MAWP (max allowable working pressure) is not known. Therefore I will use design pressure for relief valve set pressure.

Also, vessel code is not known and hence I don't know what is accumulation pressure or % accumulation is allowable.

As this is the case, am right that maximum relieving pressure should be restricted to 10barg only? I.e.

If process upset is the worst case,

Set pressure = 9barg
Over pressure = 10%
Relieving pressure ~ 10barg

If fire case is the worst case,

Set pressure = 8.25barg
Over pressure = 21%
Relieving pressure ~ 10barg

KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

This vessel could not have appeared out of nowhere. There must be a (local) regulation/code based on which the vessel was fabricated and tested.

There are ways to calculate MAWP for non-standard vessels, e.g. see https://leg1.state.va.us/cgi-bin/legp504.exe?000+r...

Dejan IVANOVIC
Process Engineer, MSChE

### RE: Relieving pressure when MAWP and Vessel Code are not known

KS,

As long as the MAWP of the vessel also the vessel code are unknown, your approach appears to be logical; provided that lowering the PSV set pressure than vessel design pressure which lowers the maximum operating pressure as well, wouldn't be a concern...

### RE: Relieving pressure when MAWP and Vessel Code are not known

(OP)
Thanks Dejan

e43u8

Thanks very much for your input.

Regards,
KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

(OP)
Hi everyone

In past, I have come across vessel code : PD5500 where allowable accumulation is 10% of MAWP. Could somebody let me know which are other vessel codes and % accumulation please?

KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

EU: Pressure Equipment Directive 97/23/EC (PED) - maximum 10% accumulation regardless of the relief scenario.
PD(BS)5500 is withdrawn from British BS standards and replaced with EN 13445.

Dejan IVANOVIC
Process Engineer, MSChE

### RE: Relieving pressure when MAWP and Vessel Code are not known

(OP)
Thanks Dejan

Is that the only pressure vessel code applicable in EU please?

Thanks

KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

I assume that you have checked the vessel file and couldn't find documentation from the vessel fabricator. Have someone in the field check the vessel to locate the nameplate - all pressure vessels should have one, and it will identify the fabrication code.

If there are no vendor documents in the file, and no nameplate, then this is a pretty sad situation for the vessel owner.

We have to know the vessel code in order to know the allowable accumulation. It's highly probable that pressure accumulation is allowed for this vessel, but we need the vessel code in order to say that for sure. I would explain this to the owner and let them decide whether to proceed with allowing the usual accumation limits (10% and 21%).

BTW, EU PED doesn't limit the accumulation to 10% for fire cases. Refer to PED Guideline 5/2.

### RE: Relieving pressure when MAWP and Vessel Code are not known

If you haven't got a nameplate or design details how do you know the design pressure is 10 barg?

Is the vessel still the thickness it was designed for?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Relieving pressure when MAWP and Vessel Code are not known

(OP)
Thanks Dejan

don1980

No info on vessel file or name plate. I am very surprised though.

Please read my original post where I have allowed 21% for the fire case. Is that not ok for fire case assuming PED vessel code?

Littleink

That info on 10barg, I received from GA drawing. No info on MAWP or vessel code on it.

Thanks
KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

The first link in my previous posts opens PED Guidelines. See the section 5.2 as referred by don1980. The second link leads to the PED Directive. Read the section 2.12 of Annex I, which is referenced in the section 5.2 of PED Guidelines.

"Where necessary, pressure equipment must be so designed and, where appropriate, fitted with suitable accessories, or provision made for their fitting, to meet damage-limitation requirements in the event of external fire, having particular regard to its intended use."

PED does not specify maximum accumulation for exposures to external fire (unless these requirements were changed/updated afterwards - I don't have any information on that). The way I read these paragraphs is that, regardless if you set the maximum relieving pressure at 110% or 121% or at 100%, unless the design will provide for effective protection against possible damage caused by fire (i.e. vessel rupture), it doesn't make any sense to discuss about allowable accumulation. PED seems to be more strict with regards to this subject than API 521 because it uses the word "must", obliging the designer to "meet damage-limitation requirements".

Some designers I have been discussing with, use 110% for PED vessels and fire exposure. The others use 121%.

If this vessel has no history, no files in the dossier, not any usable information on the actual condition, I would hesitate to use even the design pressure as the PSV set pressure. If the vessel is corroded and been in service for 20-30 years without any inspection and testing, you (and the asset owner) do have a situation there, like others have said.

Dejan IVANOVIC
Process Engineer, MSChE

### RE: Relieving pressure when MAWP and Vessel Code are not known

(OP)
Hi Dejan,

Thanks very much for your help, much appreciated.

Just wondered, PED guideline says 1.1PS (110%) is not applicable to fire case but it does not say what is applicable instead? Also, why still people use 110% for fire while PED clearly says 'NO' to 110% for fire case?

We have instrumentation loop for primary protection but I will vessel checked eventually.

Thanks
KS

### RE: Relieving pressure when MAWP and Vessel Code are not known

Would suggest we ask an NDT specialist and a vessels design engineer to assist.

### RE: Relieving pressure when MAWP and Vessel Code are not known

Ummmm,

"That info on 10barg, I received from GA drawing. No info on MAWP or vessel code on it."

Then I believe, what you have isn't a pressure vessel, its a lump of scrap metal unless you can come up with something much better than this.

Sort that out first before worrying about relief valve settings.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Relieving pressure when MAWP and Vessel Code are not known

PED doesn't say you cannot use 110% or any other figure. But it also does not specify which % accumulation to be used for the fire case. The reasoning behind is that each fire case is unique, and specifying a "prescribed" max % accumulation does not provide warranty that vessel integrity will be ensured during exposure to external fire.

To me, this makes a perfect sense. If you recall the recent discussion on relief temperature, you'll remember that one may end up with unreasonably high relief temperature calculated at the PSV set pressure, for gas filled vessels and some other cases. This simply throws away the entire PSV concept as vessel overpressure protection. What PED says, is that the designer must made provisions in vessel design which will meet damage limitation requirements - whether it is a sprinkler/deluge system, fire resistant insulation, or vapor depressuring. PSV is not a protection device for the fire case.

Dejan IVANOVIC
Process Engineer, MSChE

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