ASME Section VIII Div 1 & 2 with API 6AX Stress Allowables 1

[BurnieC]

Hi everybody,

API 6AX states that stress intensity allowables are 2/3 yield at operating and 5/6 yield at hydrostatic test.

When designing using ASME Div 1 & Div 2 it states to check stresses against allowables in ASME Section II Part D. Taking Div 2 as an example these are the minimum of UTS/2.4 and yield/1.5.

My question is can API 6A stress allowables be used on ASME calculations?

My feeling is they cannot. Taking a Div 2 4.16 flange stress section, you check the principal hoop, radial and tangential stress against the Part D allowables. Unlike 6AX which stipulates a stress intensity (Tresca) check, it seems ASME calculations and allowables are linked for a principal stress check. Hence the two code methods are not interchangeable.

Two further observations which strengthens the above thought:
- Section VIII Div 1 and Div 2 allowables differ. Div 2 allowables are higher but calculations more stringent suggesting they are tied and that Div 2 is optimised and more analysis based.
- ASME takes the lower of factored UTS or yield, API 6AX neglects this. Values in Part D show that the UTS criteria often govern so it seems wrong to neglect it and follow a 6AX stress intensity based on yield. This leads to lower allowables compared to 6AX but perhaps this is to cover that ASME check are made against principal stress.

Am I right or missing something here?

Thanks,
Calvin

 

[TGS4]

Why are you mixing standards like this?

 

[BurnieC]

Personally I wouldn't but I'm reviewing a whole set of calculations which I do not think are correct. I believe it is done because a valve is supplied to API 6A standards but the calculations are mostly ASME Div 1 or Div 2, hence the 'justification' for using 6A allowables on ASME calcs.

Still this is not acceptable, no?

 

[jmec87]

I agree that mixing codes is usually not acceptable. This is one of the rare exceptions where it is acceptable, because API 6A gives you allowable stresses to use and tells you to combine this with the "ASME method", so doing this is complying with the API 6A requirements. I recommend reading API 6A section 4.3. It is not very detailed nor very clear, but it does tell you to use these higher allowable stresses with the ASME BPVC Section VIII Div 2 (2004 edition w/ 2005+6 addenda) methodology.

Further clarification/history: API 6AX does not exist. There are API 6A and API 6X. API 6A references the 2004 version (with 2005 and 2006 addenda) of ASME BPVC Section VIII Div 2. This is because the API 6A committee members did not like the changes in the 2007 edition, so they referenced the earlier edition. Because they did not want to continue to reference an outdated standard, and because they wanted to unify the design methodology for API 6A, 16A, and 17D, they came up with API 6X. The next edition of API 6A will reference API 6X, but the current edition of API 6A does not. API 6X does not apply to any valves built to the current edition of API 6A.

 

[BurnieC]

Hi jmec87,

Thank you very much for that informative post. I did mean 6X but did not realise it was not applicable to 6A - I should be using API 6A section 4.3.

My main problem then comes from it being an allowable stress intensity.

Some calculations such as the pressurised cylinder thickness I can see are derived based on stress intensity. But take for example the flange design in 2004 Div 2 Section 3-340. Here it independently gives longitudinal, radial and tangential principle stresses and then checks each against the allowable stress. If using 6A allowables should I not be checking the maximum difference, i.e. intensity of these stresses? This is where I feel ASME Part D allowables often being limited by UTS covers their use as a check on principal stresses...

 

[jmec87]

Your point about stress intensity is a good one and is worth consideration. Per Div 2 AD-130 (2004), the Section II Part D requirements are also design stress intensity values. Therefore, using the 6A allowables in place of Part D allowables for the individual 3-340 criteria is appropriate.

Many materials that are commonly used in API 6A valves are not included in Section II Part D. API 6A will give higher allowables than ASME and doesn't consider ultimate strength as long as your materials meet or exceed the requirements of API 6A standard materials (see 4.3.3.1, 4.3.3.6, and Table 6).