15 replies to this topic

[fallah]

Hereunder you can see a statement included in the page 47 of "Safety Relief Valve Handbook" which its cover is attached:

"Note:To emphasize the difference between overpressure (safety relief valve characteristic) and accumulation (code limitation on pressure vessel),safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves,even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII."

IMO,it can not be correct because in fire case when PSV set pressure is equal to MAWP,allowable overpressure would be 21% and if its set pressure is less than MAWP,allowable overpressure even could be higher.

What is your opinion in this regard?

Attached Files

•  77122_fm.pdf   176.91KB   245 downloads

[Lowflo]

Hereunder you can see a statement included in the page 47 of "Safety Relief Valve Handbook" which its cover is attached:

"Note:To emphasize the difference between overpressure (safety relief valve characteristic) and accumulation (code limitation on pressure vessel),safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves,even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII."

IMO,it can not be correct because in fire case when PSV set pressure is equal to MAWP,allowable overpressure would be 21% and if its set pressure is less than MAWP,allowable overpressure even could be higher.

What is your opinion in this regard?

That paragraph isn't wrong, but it is poorly explained. The author is expressing an opinion that PSVs designed for fire protection should be set at 110% MAWP. ASME Sec VIII allows the user to do that, but it's not a practice that I follow, and in my experience I don't see this done by many others either. Regardless, it is an acceptable practice. If you set the PSV at 110% MAWP then a 10% overpressure results in 121% accumulation on the vessel. The author over-reached when he said that PSV installed for fire cases "will" have an overpressure of 10%. That's only true if you set the PSV at 110% MAWP.

[fallah]

That paragraph isn't wrong, but it is poorly explained.
Yes,very poor explanation!
The author is expressing an opinion that PSVs designed for fire protection should be set at 110% MAWP.
It can be considered as a personal opinion.
ASME Sec VIII allows the user to do that, but it's not a practice that I follow, and in my experience I don't see this done by many others either. Regardless, it is an acceptable practice.
Yes,it can be an acceptable practice.As far as i know,the only case that a PSV set pressure can be specified at 110% MAWP is for supplemental PSV in fire contingency that even in this case first valve being set at MAWP.Thus,it isn't a common practice that the set pressure of all PSVs in fire case would be considered at 110% MAWP.
If you set the PSV at 110% MAWP then a 10% overpressure results in 121% accumulation on the vessel. The author over-reached when he said that PSV installed for fire cases "will" have an overpressure of 10%.
That's only true if you set the PSV at 110% MAWP.
Agreed.

[sheiko]

Hereunder you can see a statement included in the page 47 of "Safety Relief Valve Handbook" which its cover is attached:

"Note:To emphasize the difference between overpressure (safety relief valve characteristic) and accumulation (code limitation on pressure vessel),safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves,even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII."

IMO,it can not be correct because in fire case when PSV set pressure is equal to MAWP,allowable overpressure would be 21% and if its set pressure is less than MAWP,allowable overpressure even could be higher.

What is your opinion in this regard?

Fallah,
Not taking the risk to misintepret the author´s comment, i would tend to agree with you. Overpressure is relative to set pressure and accumulation is relative to MAWP. dot. Moreover i have never heard of this practice to set the opening pressure at 110% of MAWP in case of fire...Seems odd to me.

[fallah]

Moreover i have never heard of this practice to set the opening pressure at 110% of MAWP in case of fire...Seems odd to me.

Sheiko,

As i mentioned the only case that a PSV set pressure can be specified at 110% MAWP is for supplemental PSV in fire contingency.

[paulhorth]

I would agree with Lowflo's interpretation. I have not read this handbook, but it would appear that it was not thoroughly checked, which is disappointing for an IChemE publication.

I would add that under the British vessel design code PD5500 ( which I believe is harmonised with European codes), the maximum accumulation is 110% of MAWP even in a fire case. This is an important difference from ASME VIII. Thus, for equipment designed to this code, a single relief valve installed for a fire case cannot have a set pressure higher than 100% of MAWP.
If the relief load is spread across multiple relief valves, then I believe you are allowed to set the second valve at 105% of MAWP to stagger the opening. But multiple valves are not often required for fire relief.

Paul

[Lowflo]

I would add that under the British vessel design code PD5500 ( which I believe is harmonised with European codes), the maximum accumulation is 110% of MAWP even in a fire case. This is an important difference from ASME VIII.
Paul

Paul, I used to think that too. Unlike ASME Sec VIII, the EU rules state the limit of 110% accumulation without any mention of an exception for fire cases. Later, I came across Guideline 5/2.(http://ec.europa.eu/...sign_all_en.pdf)

It says the 110% accumulation limit doesn't apply to fire, but it doesn't say what that fire limitation is. Absent any specified valve, I assume that it's OK to use the 121% ASME limit that we're all accustomed to. That's what I've been doing.

[CMA010]

21% allowable accumulation doesn't mean the valve will actually go to 21%

[fallah]

Absent any specified valve, I assume that it's OK to use the 121% ASME limit...........

Valve or value?

[paulhorth]

Thank you, Lowflo, I didn't know that. I can reradily believe that I am not up to date with the latest guidelines.

Paul

[fallah]

Paul, I used to think that too. Unlike ASME Sec VIII, the EU rules state the limit of 110% accumulation without any mention of an exception for fire cases. Later, I came across Guideline 5/2.(http://ec.europa.eu/...sign_all_en.pdf)

It says the 110% accumulation limit doesn't apply to fire, but it doesn't say what that fire limitation is. Absent any specified valve, I assume that it's OK to use the 121% ASME limit that we're all accustomed to. That's what I've been doing.

Seems attached has been stated that limitation of accumulation for fire case in EU rules is 10%.

Attached Files

•  Pages from BS-PD-5500-2006 (2).pdf   90.37KB   202 downloads

[Lowflo]

Fallah,

Pressure Equipment Directive (PED) set the governance requirements for EU countries. PED Guidelines, including the one I cited (Guideline 5/2), can't be superseded by any harmonized EU standard.

Your point is not up for debate. That guideline states that 10% accumulation does not apply to fire.

[fallah]

That guideline states that 10% accumulation does not apply to fire.

Lowflo

Then which value for fire case should be used?

You mentioned in one of your previous posts:

"It says the 110% accumulation limit doesn't apply to fire, but it doesn't say what that fire limitation is."

Then the guideline is incomplete one on fire case point of view!

[Lowflo]

That guideline states that 10% accumulation does not apply to fire.

Lowflo

Then which value for fire case should be used?

You mentioned in one of your previous posts:

"It says the 110% accumulation limit doesn't apply to fire, but it doesn't say what that fire limitation is."

Then the guideline is incomplete one on fire case point of view!

Fallah, PED does not specify a maximum allowable acculation for fire exposure. They just say that you're not limited to 10%. As I mentioned in a previous post, in the absense of any specified value I use the ASME limit of 21%.

[fallah]

They just say that you're not limited to 10%. As I mentioned in a previous post, in the absense of any specified value I use the ASME limit of 21%.

In one of projects i did work on,the wall thickness of some ASME pressure vessels due to high design pressure (more than 20 barg) were very high and after agreement with the client their governing standard changed to PD5500 in order to decrease the plate thickness and following that their total weght.

In mentioned case we were not allowed to keep the 21% accumulation for PSV sizing in fire case and we did size them based on maximum 10% accumulation in vessels.

[fallah]

gvdlans,

Would you please submit your viewpoint on this discussion.

Regards