Edge of Dent Definition API RP 579-1 / ASME FFS-1

[TehMightyEngineer]

We're performing a Level 1 assessment per API RP 579-1 / ASME FFS-1 of a dented pressure vessel and are meeting all criteria except the edge of dent to other structural discontinuity distance (12.4.2.1(c)).

I've found no definition in API 579 (or elsewhere) of what the "edge of dent" actually is? If it's to the beginning of major deformation of the "dent" we're acceptable, however, the deformation of the shell spreads out ever so slightly for a good distance beyond the edge of major deformation, and this is what I would imagine is the "edge of dent", and thus we're not acceptable by Level 1 or 2. We're really trying to avoid having to go all the way to Level 3 assessment for this dent...

Anyone got a concrete definition of edge of dent that can help us out?

Maine EIT, Civil/Structural.

 

[Duwe6]

It is the same as determining the length of "Straight Shell" on a pressure vessel. Tangent point-to-tangent point. Use a straightedge for straight sections, have a radius 'board' cut for curved sections. "Dent" is not the limiting case in evaluating Fitness-for-Service; is the vessel out of tolerance? I have a number of tanks that look like hammered feces -- dented and 'wavy', but when a proper radius board is applied, they all make tolerance. Figure where the start and stop of the out-of-tolerance areas are; that will be the 'effective dent size'.

Are these dents 'innies' or 'outies'? Inward deformations get better [closer match to the theoretical curve] as pressure is applied. Are any of the areas creased? All creases have to be removed. And have you tried flame-straightening? There are some genius 'blacksmiths' that can heat a circle in the dent and cause the metal to shrink enough to 'pop' the dent out. And this heating, if done by somebody good stays 100's of degrees below the Lower Transformation Temperature.

 

[TehMightyEngineer]

Project is completed at this point, we went with the conservative definition of extreme edge of dent where it measurably started to change the radius of the shell. We had a mold made of the dent to find the extents.

The dent was made by improper use of a bottle jack causing a 1/2" indentation on a high pressure heat exchanger. It was well out of tolerance. It was curved inward uniformly around the dent with no creases or major discontinuities. Flame straightening was not considered as it would be easier to just install a blind nozzle as an acceptable repair.

In the end we were able to find an acceptable solution but also found the strain during deformation was just short of the ultimate strain. Thus, the section has little ductility left and we made an engineering judgement that it be repaired despite being acceptable by a fitness for service analysis.

Maine EIT, Civil/Structural.