New allowable Stress In ASME Sec. II

[SHG]

There are some question regarding to ASME new allowable stress.
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

 

[hokie96]

I believe the changes to Section II were made based on improved design capabilities, manufacturing practices and quality control in fabrication, however, those in this forum that may have been directly involved with the changes can provide a better response to your questions 1 and 2.

For repairs or alterations to vessels built pre-1999, you may generally use the more recent allowables for new material (such as a new nozzle sub-assembly) and the allowable from the original code of construction for any existing material. However, there are times where using the new allowables may not make sense from a practical standpoint.

A word of caution: Your local jurisdiction may have an opinion on this matter. I've seen some companies go so far as to put together methods for rerating older vessels using the new allowable stresses based on fitness for service concepts. Most of these procedures have a checklist of items that must be confirmed before proceeding with such a rerate. However, some jurisdictions outright prohibit this practice. You may also want to consult with your local Authorized Inspector (or equivalent).

 

[welding1]

It’s not just ASME, you will find most codes have increased allowable stresses over the years. I suspect that ASME are trying to make their code competitive, so as to keep its lions share of the International market.

In fact, pressure vessel design stresses in Europe are fixed by law! under the pressure equipment directive. And for most materials it’s based on 2/3 of the yield stress.

Regards
John

http://www.gowelding.com

 

[lasrado]

I think ASME has a better understanding & database of vessels in service all over the world & based on this information & knowledge, they decided to lower the devisor used on yield from 4 to 3.5( if my memory goes right) .

On the issue of repair, National Board has a detailed procedure which outlines requirements for testing & repair. A lot depends on the condition of the vessel repaired. Calculations should be based on year the unit was built, & if thickness of nozzles & shells are within required.

 

[ExAI]

My understanding is that the ASME STARTED to reduce there factor of Safety in 1999 as the result of foreign code competion. Code around the world have been producing safe vessels to a less strict factor of safety for a long time. ASME is trying to keep up. If I recall the disscussion correctly, you can expect another increase in stress values (reduction in safety factor) in the relatively near future since they wanted to realign slowly. Lane Baker
Baker Quality Services
LaneBQS@att.net

 

[Halket]

The allowable stress in the pressure vessel code has changed for Section VIII, Div 1 Pressure Vessels and Section I Boilers. The change is already in effect for most materials via Code Cases, and will be changed in Section II of the Code for all materials soon. This creates the opportunity for thinner wall vessels and rerating of existing pressure vessels for increased design conditions or greater corrosion allowance.

The factor on the tensile strength of the material that is used in establishing the allowable stress has been changed from 1/4 tensile strength to 1/3.5. This change brings the ASME Boiler and Pressure Vessel Code design allowable stresses closer to those of most European Pressure Vessel Codes, and was done after a careful study concluded that this change could be made without impacting the safety of the affected pressure equipment. Note that the last change was made over forty years ago, and was from 1/5 to 1/4.

Not all materials will be affected since some are limited by the existing 2/3 yield strength limit which remains unchanged. Also, the allowable stress is not changed at elevated temperatures where it is controlled by creep properties of the material.

 

[MJCronin]

SHG

ASME sells a very well written and informative book that discusses this topic on allowable stresses.

"Guidebook for the Design of ASME Section VIII Pressure Vessels" by: Farr & Jawad

About US$75 (plus or minus) ISBN: 0791800555

Hope that this helps.....

MJC

 

[davefitz]

The reason for the change was to be more competitive with European codes, and recognizing better quality control technology and NDT methods.

But watch Out!!! Associated with the new higher allowable stresses is the implied need to hydrotest at the lower test pressure multiplier of 1.3 instead of 1.5!! Do not design using the higher allowables and then test at teh 1.5 multiplier!!.