FFS Forum: The Problem with Inspection of Older Vessels

Editor’s Note:

This is the second edition of a new column dedicated to all things fitness-for-service (FFS). This is a forum format, so readers are encouraged to send us suggestions for future FFS related topics, comments on the current article, and raise FFS issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those that submit.

The author, Greg Garic, P.E., has over 35 years of experience in FFS and mechanical integrity assessment of pressure systems. He began his career at NASA’s Stennis Space Center, where his tenure spanned 15 years. After rising to the position of Senior Engineer, he joined Stress Engineering Services and has been there for over 20 years. As a Managing Principal with Stress, Greg focuses most of his work on stress analysis, fracture mechanics, pressure systems engineering, and FFS across the process industries. He also teaches FFS training courses, is qualified as a stress analysis expert in federal court, and leads Stress’ New Orleans office. Suffice it to say, he’s been around the block more than a few times and knows FFS.

A well maintained pressure vessel can last a long time. Many vessels from the 1940s and 50s remain in service and I occasionally still run across pre-World War II vessels. This is a testament to the good work of our predecessors and the safety and value delivered by the ASME boiler and pressure vessel codes. But… it’s also true that, generally, older steels and older welds just aren’t as clean as they are today. Advancements in steel making practices and welding technology have greatly reduced non-metallic inclusions, porosity, slag, incomplete penetration and a host of other problems, which were much more pervasive in older steels. Even vessels from the 70s show a significant difference in quality compared to current vessels.

Inspection technology has also significantly improved in the last 50 years – or even in the last 20 years. Digital X-ray (RT), phased array ultrasonics (PAUT), and automated ultrasonic testing (AUT) can identify defects at far smaller sizes and with far higher reliability than older methods.

If you were to take a handful of code stamped 1950-era pressure vessels that had successfully passed all inspections and other QA requirements and thoroughly inspect them with today’s inspection technology and criteria, you’d very likely have a big problem. There is a good chance that you would find numerous rejectable defects.

Of course, not all indications necessarily need to be evaluated by fitness-for-service (FFS) methods. For example, indications that are acceptable by either the original code of construction or the current version of the code, would not usually require evaluation. API 579 points out that:

“[The] key decision that needs to be made is whether the flaw and associated deterioration (regardless of its origin) is likely to progress in the future based on the material, stress, service conditions, and flaw size.”
(API 579, ¶2B.3.2.2)