So in my boredom this evening as I await test results on a project I'm working on, I figured I'd dig into the MAWP formula as I do tend to like the basic equations and spreadsheets... But here's where I run into a question. When a boiler is ultrasounded, it's done in a grid. How does that factor into the calculation? Do you take the thickness measurement at each grid and use that to come to some sort of average, or do you look at it as a point of that's your lowest safe pressure?

I would expect that the thinest measurement of the boiler shell material would be the basis of that strength calculation.

Brian

How is is used in the calcs is a different question than how it should be used in the calcs. IT is important to remember when doing the measurements how does your boiler material going to fail. A thin spot is a high stress spot like a tensile sample and so it should be the effective thickness of the boiler. Corrosion pits or thin fillet welds results in this high stress.

Robby Peartree

"I would expect that the thinest measurement of the boiler shell material would be the basis of that strength calculation."



The problem with assuming that is that if you inadvertently UT'ed a pit that was very thin you would condemn the boiler. If you refer to NBIC RB-5510 first paragraph and RB-9130, pitting: "widely scattered pits may be disregarded provided that:...", you will find that a thin reading is not the ruling dimension.

What we need to consider is that boiler calculations are a statistical undertaking. You can take two different sets of UT readings and arrive at two widely differing values for the statistical strength.

UT should be used with internal and external visual inspection, the NBIC, and input from your AI. Relying on UT alone isn't conclusive.

An example would be a set of great UT readings and a longitudinal crack at the boiler stop check. The crack would condemn even if UT were great.

I'm always interested in learning. Please elaborate.

Mr. DeWitt started to address the issue. You really need to look at the effective thickness as far as the material sees it. Shallow scattered surface pitting is not a problem, Areas that result in high stress risers such as machined sharp edges, or deep heavy pitting where the average thickness may be higher than an area like near the area in the boiler where the steam water interface occurs, and other shapes such as cracks limit the area forces can be exerted need to be recognized and make sure your modeling them correctly. After all when pulling a nail out of a board with a claw hammer how much force does the hitting surface see versus the prying end?

Robby Peartree