API 6D 24th edition :- Cavity Relief Test Pressure

[al1979]

Hi All,

Can anybody throw some light on clause 5.8 of API 6D 24th edition.

[highlight #F57900]"Valve cavity relief pressure when added to the valve pressure rating shall not exceed 133 % of the pressure rating of the valve at its maximum specified design temperature"[/highlight]

1) "when added" I am unable to exactly interpret this (though looks simple math ) How much shall be minimum and maximum cavity relief pressure exactly?
2) Valve pressure rating at what temperature?
3) What is meant by maximum specified design temperature? Is it manufacturer design temperature or purchaser specified design temperature?
4) This means cavity relief pressure will change with temperature?

Also,

Clause H.8 Cavity relief testing says

[highlight #F57900]"Failure to relieve at a pressure less than the valve cavity relief pressure per 5.8 shall be cause for rejection"[/highlight]

In above sentence "Less than" means I have to maintain "some pressure" in cavity all the time?

Cheers !!!

 

[SPDL310]

1.) Cavity relief pressure can be set above the valve MAWP but can't be larger than 133% of MAWP. For example you have a 10,000 PSIG valve relief pressure can't exceed 13,300 psig.

2.) The temperature would depend on the max temp class you want top operate at.

3.) Maximum design temperature by the manufacturer. API usually designates products by temperature class. Temperature class X is 350 degrees F. If you are below 250F there is usually no consideration on temperature effects. API products are designed to operate at rated pressure throughout their full temperature range.

4.) No. From what you have posted cavity relief pressure is the maximum over pressure that your valve is allowed to take. Whether you are operating at 100F or 350F if you have a max design temp of 350F and a pressure rating of 10,000 psig at 350F the cavity relief pressure will always be 13,300 psig.

5.) you don't have to maintain pressure at all times but in order for you to accept your valve it must relieve at below 13,300 psig.

 

[al1979]

Hi SPDL310,

What you said is 'CORRECT' and followed everywhere in industry.
But the standard is saying something else (may be) and I want its correct interpretation.

23rd edition was more clear about this clause.

Cheers !!!

 

[SPDL310]

That is the way that I interpreted the language that you posted. I can't see any drastic change to the standard suddenly appearing as API steering comities tend to shy away from invalidating existing products. Changes to the standard that would place requirements on a product usually get included as a informative annex such as 6A Annex F for temperature cycle validation testing. I have seen required safety factors actually decrease in the latest edition of 16C for example.

 

[bcd]

To get a "CORRECT" interpretation, go to the API website and submit an inquiry. You will get an official response. A lot of changes were made in the 24th edition and they may have unintentionally screwed-up and caused confusion. Responses in this forum should never be considered fully correct and valid until you verify.

 

[al1979]

@ bcd :- CORRECT, already initiated the process for clarification from API.

 

[Ehzin]

I'll offer my opinion on the matter which mainly has to do with clarifying some of the wording of the 23th edition although it seems to add more confusion. The purpose, in both editions, of cavity pressure relief is to ensure that when a valve is partially open and exposed to the full pressure rating, once closed, a feature such as self-relieving seats prevents overpressure beyond 1.33 times the pressure rating. This is the ultimate worst case situation that is intended to be avoided as line pressure will decrease the effectiveness of the self-relieving function. If the seats are not designed appropriately then the seats may not relieve before the cavity pressure exceeds this point.

The procedure in B.8.2 of the 23th edition provides no effective means to factor in line pressure as it requires the flange side of the valve to be exposed to atmospheric pressure. When tested, it's generally anticipated that self-relieving seats will "lift" anywhere from 10-50 psig. Section (c) of B.8.2 simply states to record the relief pressure, which will be low and ensure it doesn't exceed 1.33 times the pressure rating. As stated, this procedure is inadequate to ensure the cavity pressure would not build beyond 1.33 times the pressure rating.

The wording was changed in the 24th edition to separately define a "valve cavity relief pressure" as only the added value to the valve pressure rating (when added to the pressure rating it is not to exceed 1.33 times the pressure rating). The recorded value, according to the test of H.8.2 of the 24th edition, is to be compared only to this added value.

To simplify, they defined a new term to be directly compared to what's measured in the supplemental testing. Again, this is just my understanding of why they changed it. I may have missed something but hopefully this provides some clarity.

Thanks,
Breaux