Code requirements around using 309L weld overlay as part of pressure boundary thickness

[Andrew63411]

I am planning the overlay of 2 shell courses in a continuous digester. My boss and I are in disagreement over how we can treat the added thickness from the 309L overlay. He believes that we do not need to do any base metal restoration BEFORE the overlay while I believe we cannot use the thickness of the 309L in code requirements for tmin or original metal thickness. Vessel thicknesses are below original but above tmin, but approaching tmin fairly quickly. My interpretation of UCL-23 is that if we are going to consider the thickness of the 309L then we must re-analyze the vessel to make sure it holds it pressure and temperature ratings, whereas if we ignore it for tmin and do some base metal restoration, then it is simply for corrosion resistance (which is the goal). I also have read PCC-2 and cannot find a definitive enough statement that says it is not allowed to use the thickness of the corrosion overlay in tmin as PCC-2 states the weld metal has to have a higher tensile strength than the base metal. I am going to reach out to our AI but i just want a better understanding of what im getting into before calling him. Thanks!

 

[Andrew ONeill]

1) General good practice is to ignore the weld overlay in calcs. I've seen plenty of pipes and vessels with weld overlays and have never seen anyone take credit for its thickness in the pressure boundary calcs, it has always been treated as an extra corrosion allowance. *There maybe some section of a code somewhere that allows it, but I've never seen anyone do it.

2) Given the vessel is already above tmin, the decision to restore the material to original thickness is really a judgement call, based on expected corrosion rates / required life / required inspection interval.

3) Side note, if using a stainless overlay on a carbon steel vessel at higher temps, you might run into differential thermal expansion issues (thermal expansion coeff of stainless is generally higher than carbon steel), something worth considering as well (also another reason to not consider the overlay in the tmin calcs)

Andrew O'Neill
Specialist Mechanical Engineer
Australia

 

[EdStainless]

They don't even take credit for clad thickness in explosively clad plate.
The process of trying to do this results in very little actual allowance, there are many derates.

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P.E. Metallurgy, consulting work welcomed

 

[Andrew63411]

Ed, can you explain what the derate cause might be, if 309 has a higher tensile strength than the base material (212b)?

Andrew, im not super familiar with corrosion causes. If the base metal is not below tmin and we do an overlay, would the thickness tend to stay the same over the years as it wouldnt see the corrosive environment that it does now. (assuming now material loss from vessel exterior) Would we see the base metal continue thinning even with an overlay?

Greeat info guys, thanks for the responses!

 

[SnTMan]

Andrew63411, see UCL-23(c), by my reading credit may not be taken for WOL with higher allowable than the base metal. You must treat the WOL thickness as having 212(b) allowables.

Having said that I'd think that would meet your need as to restoring the base matal.

Regards,

Mike

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

 

[Geoff13]

I am a little confused if some of your shell is already below tmin or not. I'll address both possibilities.

The most common manufacturing process is roll-bond clad. This starts with very thick base metal as well as very thick cladding metal. The repeated rolling thins both parts and causes them to bond at the interface. However this is not an exact science and the actual finished material WILL NOT have the exact specified base and clad thicknesses. May be thicker or thinner, and will vary from one plate to the next.

The Code writers seem very aware of this thickness variation and thus UCL-23(b) says "A reasonable excess thickness either of the actual cladding or of the same thickness of corrosion resistant weld metal may be included in the design calculations as an equal thickness of base material." This covers your weld overlay question. The Code not not define "reasonable". Note that this statement does not require the weld overlay to be stronger than the base metal.

If you have thicknesses below tmin, you know your measured thicknesses and must judge if the remaining base metal thickness is close enough below tmin that the weld overlay thickness within tmin will be "reasonable".

However if all your thickness readings are greater than tmin, do the weld overlay without any further thought. Presumably the weld overlay will protect the base metal from further corrosion. Continuous and batch digesters are one of the most corrosive services I've worked on.

 

[EdStainless]

You can't claim higher strengths, you have to take a reduction for the weld overlay, and a few other details.

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P.E. Metallurgy, consulting work welcomed

 

[r6155]

Are you planning to WOL with manual welding?

Regards

 

[Andrew63411]

@r6155, it will be automatic welding.

 

[jt1234]

Andrew63411,
Forget about what vessel code code says as now you are doing repair, you need to follow NBIC,PCC2 or whatever. I have done a lot of repairs in the past. Again, my experience in real world.
In your situation, you need to check the original code calculation. If MAWP is based on calculated tmin, and thinnest base metal has not reached tmin, you don't need to restore base metal. You just perform 309L WOL to whatever thickness you want. Anyone asking you to do analysis is just faulty. You are not violating the original code stamp.
If any area already under tmin, you got to do weld restoration using the weld metal or equivalent used for the base metal, then perform 309L WOL. Check original WPS/PQR to see what was used. The vessel will need R stamp.
If you want to restore base metal even it is not under tmin, it is not necessary but your choice.

 

[jt1234]

Just a correction to my comment above. Even if the actual thickness is below tmin, you don't need to do base metal restoration if meeting thin out criteria per Div. 1 Appendix 32. We have the case 3 years ago.

 

[r6155]

Appendix 32 ASME VIII Div 1 is for new pressure vessel.

Regards

 

[jt1234]

If doing a repair, even if the existing base metal is less than tmin, it might already qualify Appendix 32 when new, such that no need to restore the base metal. The post saying actual thickness is more than tmin, definitely there is no need to restore the base metal, Andrew Oneill says it well. 309L WOL in most cases is just for corrosion allowance purpose, don't get yourself into too much trouble to consider it as the pressure part unless reading the code carefully.