New allowable stresses in ASME Sec.II

[SHG]

1- How does know the reason or philosophy of changing - or better say - increasing of the allowable stress since 1999.
2- Is it based on improving of steel making , vessel fabrication or maintenance systems.
3- If we have some modification or repairing ( new nozzle connection ) in old vessel what should it be done regarding to allowable stress for recaculation.

This matter may be concerned with every body in this field.

 

[ijzer]

The background for changing the allowable stresses is to be more compatative compaired to the more "modern" European Codes like BS5500, AD-2000,Stoomwezen ,CODAP.
ASME vessels were usually thicker , so more expensive.

 

[jte]

SHG-

I'd say that the *incentive* for changing the allowable stresses for many of the materials allowed to be used for Div. 1 was economics and international competitiveness.

The *justification* was a combination of better engineering, material quality, fabrication quality, and inspection techniques. There was a paper published in the PVP volumes in roughly 1998 which explained the justification in detail.

If you are repairing a vessel, you must use your judgement combined with any opinions your jurisdicional authorities (State / govt folks) may have. I might design the entirely new parts of your additional nozzle to the new allowables while calculating required reinforcement in the old shell to the old allowables. Basically, use the allowable stress for each part when it was placed into service. On the other hand, it may be easier (and not much more expensive) to use the old stresses in order to simplify the paperwork and minimize engineering mistakes on future repairs. Are you confident that 10 years from now an engineer looking at the vessel will know that parts were designed to one set of allowable stresses while other parts were designed to another set of allowables?

I consulted with my State authorities on a related issue: I'm replacing a channel on an exchanger. Physically identical material and dimensions - but with the higher allowable stresses, I can have a higher corrosion allowance. Is this a repair or an alteration? The State replied that this is an alteration since I'm using different allowable stresses. If I had specified to the fabricator to design using the old stresses, it would be a repair.

jt

 

[ctmfab]

If you want to rerate the vessel using the new code allowables, you should look into API-510. In particular, check into Section 7.3 "Rerating". If the vessel was built after 1968 with ASME materials then you stand a very good chance of being able to rerate it for higher allowable stresses.

 

[GenB]

Use Code year and addenda when the vessel was designed and built.
ER

 

[bvi]

Both API 510 and NB-23 permit you to use the current code (with its potentially higher allowable stress) to rerate a pressure vessel built with an older code and lower allowable stress. However, amoung other things, it must comply with all of the requirements of the current code. One possible sticking point is impact test requirements.

 

[JoeTank]

I would like to add a comment to cb4's post. Although NBIC and API-510 both permit re-rate per the current Code, there is a long list of about ten prerequisites that must be met before re-rating is performed. Notwithstanding the above, one must always confirm acceptance by the Jurisdiction. Not all states will accept the re-rate with higher allowable stresses. Hope this helps.

 

[davefitz]

another fine point: If a piping system has installed in it a new pressure vessel built with the new allowable stresses , and the allowable stress is based on yield stress ( ie, not designed for the creep regime), then one needs to limit the hydrotest pressure of the entire piping system to the new reduced level of 1.3 times design pressure. If you used the old value of 1.5 times design pressure yu may be reaching yield stress at some stress raisers in the new component.