Allowable thickness for restoring wasted wall thickness of pressure vessel

We are in process of repairing a pressure vessel which the wall thinning has happened because of corrosion. We decided to restore the corroded area using weld buildup.
I have refereed to NBC and API 510 to find the maximum thickness is allowable to be restored by buildup welding. In other words under which circumstances we must use flush patch or lap patch?
Does anybody knows the limit based on industry practices, regulatory association like ABSA, BCSA, TSSA or code and standard for in service pressure equipment.

  • PV thickness: 05"
  • Area to be restored to original thickness: 0.4"
  • How many sq units (inches or cm) need to be repaired: Around 5" by 5"
  • Material: A516-70N Sour services

We have certified welders, and a certified welding process already in place.


There is ALWAYS a risk of burn through (by an unskilled worker trying to exceed his competence level, or by a good welder attempting to work too fast, or by a skilled welder trying to use too high a heat rate (amperage) or too larger diameter rod or too large a filler wire …

However, a skilled welder can join or repair almost any thickness metal by choosing the right diameter wire or rod and the right amperage and the right speed. Your “thumb rule” is both correct and incorrect. Do not let it become a “Thumb rule of law” rather than mere guidance.

Other things that they can change: “time” of the current => small, short “taps” of the current let the thin metal cool off before the next little bit is melted on. Longer arc distances help. Lower voltages help. Good gas flow helps cool the metal. Applying shield gas as a coolant to the back side of the base metal helps suck heat away from the joint.

On your PV, 1/2 thick wall is a modest thickness that is workable - even for a significantly deep repair over a modestly small repair area. Check that the weld area is preheated carefully, and that the irregular - and probably very dirty ! - bottom and sides of the repair area are ground completely clear of pits and tears and valleys, and are both chemically and mechanically kept clean. Clean about 1-1/2 inch to 2 inches (35 to 50 mm) around the repair area as well.

Above is a snippet.

@RACookPE1978, About how many years does it take for a welder to turn into a “skilled” welder?