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Why do we inspect?

By Greg Alvarado at Inspectioneering Journal. March 3, 2013

This is a question with which I frequently like to start the API RBI 580/581 training course when I am instructing. It is meant to provoke the attendees to really think hard about why they order an inspection or really think about inspection strategies.

Inspectors, even senior inspectors, have become so busy over the years that the pressures of managing resources, outside inspectors, politics, and everything else, leaves little time to devote to analyzing data and really doing what they like to do. So why do we inspect? Answer: To fine tune the reliability prediction. To uncover information that may confirm if the equipment is degrading at the rate we predicted, or in other words, to confirm what the true damage state of the equipment is. Risk levels dictate how accurate we need to be based on the amount of risk we are willing to accept.

A true appreciation and understanding of practices like fracture mechanics, fitness for service (FFS), and risk-based inspection (RBI) helped evolve and enhance my level of thinking. Things like failure assessment diagrams and proactive FFS analyses really provided me with a better understanding of the role of FFS and NDE in managing the lifecycle of equipment. Let me give you an example:

As most of you know, pressure swing absorbers (PSA vessels) are subject to fatigue cracking. When selecting an inspection strategy, which might include UT or AUT, and checking for peaking, one of the first questions I would want to ask myself and answer is, "what is the limiting flaw or crack size?", then “where do I need to look?”, and these questions and answers are interrelated. “What is the maximum tolerable flaw size?”, one that, if it exists and if I don't find it, will not go critical or through wall before the next scheduled outage. The answers should have a huge impact on the inspection methods I select, because they need to be sensitive enough to find flaws of varying size, and certain methods are contingent upon where I need to look. Logical, right? This was a little over 20 years ago, when we really started to hear more and more about FFS.

Then I started to use this logic to some level, kind of a quick check mentally, and utilizing it in other applications. For example, an UG pipe inspection and considering LRUT such as GUL or Teletest, realizing that on a good day, with a great operator, we could miss a 5% volumetric through wall hole. It was important to understand the limitations so I could recognize if that was good enough, giving thought to how the pipe could fail and understanding the damage mechanisms. If not, what other practices could I compliment it with? This line of thought led me to appreciate the practice of using inspection effectiveness tables in RP 581, where an ‘A’ level strategy finds the potential damage 80-100% of the time, ‘B’ level is 6-80% of the time, and so forth.

RBI helped me understand how the expense of confidence in the reliability of your equipment (for instance performing an ‘A’ level inspection, versus a ‘B’ level inspection or ‘C’ level of inspection using WFMT or AUT for HIC/SOHIC costs money) is dictated by risk, of which health, safety, and economics play a huge role. The more effective the strategy, the more expensive the inspection, however the more certain you will be about the condition of the equipment.

Truly the appreciation of practices like FFS and RBI have improved my understanding of using risk to direct inspection strategies in order to identify, measure, and manage uncertainty. No doubt that RBI and FFS were the products of precursors.

I must say it has been a very interesting journey...


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