Re-rating of pressure vessel 2

[Alwynb]

Hi there,

I am investigating the possible re-rating of an existing A240 304L pressure vessel from 12barg to 16barg. If someone could comment on the issues below it would be appreciated.

1) The original material certificates in the databook for A240 304L indicate UTS of 610MPa which is well above the ASME value of 482MPa. It is even higher than 304 which is 517MPa. Is there merit in trying to use the 610MPa value to recalculate the allowable tensile stress by the ASME factor of 3.5? Is 3.5 valid for 304L?

2) Is there merit in trying to get the vessel material dual certified 304/304L in order to use the higher allowable stress of 304?

Comments would be appreciated.
Regards, Alwyn

 

[TBP]

Are there any flanges on this unit? Very often, the flanges are the limiting factor.

 

[deanc]

Talk to your Inspector. Follow the gudelines in The NBIC Part RC. Do you have the original design paperwork? If it is a used vessel you need to consider its present state.
If not,recalculate as a new vessel and consider every item as required.

 

[jte]

Alwynb-

Nope. This material is limited not by UTS/3.5 but by 2/3 yield. According to the 2001 edition of II-D, SA-240 304L plate has a minimum tensile of 70 ksi. 70/3.5 = 20. The minimum yield is 25 ksi. 25*2/3 = 16.66666. The allowable stress given in the Code up to 300°F is 16.7 ksi.

If your MTR states the actual yield strength, you might have something to work with, but the higher than required tensile strengths won't help unless stresses at higher temperatures happen to be governed by tensile...

As TBP and deanc commented, even if you do manage to work the material properties issue, you still need to deal with every item on the vessel, including flanges, nozzles and repads. Chances are that some part of the vessel was not overdesigned by 30% or have its material properties 30% greater than required.

jt

 

[Alwynb]

Thanks TBP, deanc and jte.
Next question if you don't mind:

The 0.2% proof stress (yield stress) as indicated on my material certificates is 310MPa which gives 2/3*310=206.7MPa allowable yield stress. This is significantly higher than the 114.9MPa used in the original calcs.

The question is where in the ASME code does it state that one can re-rate a vessel by using the actual yield stress as per material certificates. I cannot find it.

The flanges on the vessel however is limited to 15.8barg (ANSI B16.5 Class 150 A182 F304L).

Regards
Alwyn

 

[dig1]

ASME does not discuss re-rating of vessels.

DeanC is correct when he directs you to NBIC. Section RC-3022 discusses re-rating. There is an important point made in this section. It states re-rating "...shall be done only after the following requirements have been met to the satisfaction of the jurisdiction at the location of the installation."

What you have to do is present a plan for the re-rating including your proposed structural analysis of the vessel (ie what allowables) to your local authorities and have them concur prior to starting the work. Otherwise, the effort will likely be wasted as you generate round 2 to satisfy the authorities.

Good luck