Corrosion Allowance - Effect on butt-weld joint

[marty007]

Code of Construction: ASME VIII-1

I'm probably overthinking things, but I'm wondering if there are any interpretations that discuss this topic (I may have to convince another engineer, and interpretations are often handy...)

We have two sections of a pressure vessel that will be butt welded together, each with different specified corrosion allowances as follows (customer specs - long story):
Plate 1: 1/8" internal / 0" external
Plate 2: 0" internal / 1/16" external

At this point we are planning for the vessel to be 3/4" thk plate / 60" ID.

Based on these corrosion allowances, the corroded thickness of the two plates would be:
Plate 1: 0.625" thk
Plate 2: 0.6875" thk

Right at the weld joint however, the minimum thickness would be: 61.375" OD x 60.25" ID = 0.5625" thk (under code required thickness). Am I overthinking this small region corrosion allowance step change, or do I really need to consider a 9/16" thickness for the vessel design?

In reality, there will be some excess weld material deposited in this area that would provide additional material for corrosion...

 

[weldstan]

You are over thinking. You still have a but weld of 3/4" thickness plus reinforcement.

 

[LittleInch]

But not as much thinking as the corrosion will have to do when it has to stop at the weld line either inside the vessel or outside.

It must have been a long and painful story to get into that mess.

Also if the customer is stupid enough to come up with a spec like that they won't pick up any issues at the connection point.

A wholly artificial problem IMHO.

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

 

[SnTMan]

Well, it doesn't hurt to consider it, just to be prepared in case someone questions it.

I'd not consider a step change in the thickness, but some reasonable transition between surfaces, 1:3, 45 deg, whatever, and check that the resulting thickness including weld reinforcement along the transition is not less than tmin.

If I had to. I guess.

Regards,

Mike


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

 

[JStephen]

You specify a corrosion allowance, assuming it's going to be uniform all over, etc. In reality, you'll have three times as much corrosion here as what you have over there, you'll have pits here and bands over there, etc. Basically, you're throwing some extra metal in there to help out, but that doesn't imply that the corrosion will behave the way you're planning for it to behave. So yes, overthinking it.

 

[GenB]

Is this a puzzle or what! Straight 3/4 lined up plates ,just be sure the welds have the proper reinforcement to cover both sides required corrosion.

General Blr. CA,USA