acceptance criteria for corroded weld neck flange.

[Yasser Abu-elhassab]

Hi all

We have a pressure vessel with attached nozzle connected to WN flange, the flange is externally corroded tha the thickess of the flange reduced by about 4mm.

What is the acceptance criteria of the flange dimensions reduction by corrosion?

Regards,

 

[mk3223]

Instead of knowing the reduced thickness, you need to know the remaining UT thickness of the nozzle as well as the vessel body, and verify it with the Code calculation based on the MAWP and CA for the required wall thickness in order to determine if the vessel is safe to continue in operation.

 

[Yasser Abu-elhassab]

If all components of the vessel is accepted, but the flange is severely corroded, how can i evaluate its acceptance?

 

[SnTMan]

Yasser Abu-elhassab, I am not aware of any accepted criteria for corrosion of WN flanges like B16.5.

External corrosion is unusual, normal internal CA's would not be considered to affect flange thickness.

If I had to pick a number I'd go with the vessel CA as limiting.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

@ Yasser Abu-elhassab
Did you investigate the cause of the corrosion?
What are you waiting for to stop the advance of corrosion?
How much longer do you want to continue like this?

Regards

 

[GD2]

Yasser,
You are in a situation where you got a reduced thickness flange and wondering-What will go wrong?

With reduced thickness, the flange will loose stiffness and contribute to higher bending stress. If the flange is bend, you know the consequences- the joint might leak. Then you will use higher torque to tighten the bolts to stop the leak. Your field maintenance crew might use all their muscle power until the bolts yield and snaps. This eventually might trigger an emergency situation leading to an unscheduled plant shutdown and even injury to personals if the process was a toxic/corrosive fluid.

From engineering perspective you can check couple of things yourself:
1. ASME B16.5 flange tolerance gives you +3.00 mm tolerance for flanges less or equal to NPS 18 and +5.00 mm for flanges equal or greater than NPS 20. Check the flange thickness where it falls.
2. If it is within thickness tolerance, you are all good and take preventative measure to stop from external corrosion.
3. If not, and if the joint is not leaking, establish the Bolt Stress Limit. You will find some good material in ASME PCC-2 Tables that has beeen established using FEA.
3. Otherwise, calculate the Target Bolt Stress Limit.
4. Convert the Bolt Stress to Torque and tell your maintenance supervisor/crews NOT to exceed the torque. Usually, bolt stress is kept between 40-70% of the bolt material SMYS.
5. While all this is done, keep ready a new flange and replace it in the next opportunity window.

GDD
Canada

 

[LittleInch]

I agree - look at this as though it's a new flange and would it pass.

SO there are tables of dimensions and tolerances.

Also section 6.1.2 allows for some small deviations.

The tolerances as noted by GD2 are all positive tolerances. There is no under tolerance.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

Listen to r6155, because Rust Never Sleeps.



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