API 653 FFS Evaluation for Hydrotest

[jnleroux]

We were recently considering a hydrostatic test exemption, following API-653. The repair of the annular plate was considered major and did not fall under the shell repair or minor shell jacking sections. This left the fitness for service evaluation for hydrotest exemption. Although the ultimate decision was made to hydro the tank, I was having difficulty with the concept that a fitness for service evaluation could be used instead of a hydrotest.

A hydrotest stresses the system and is primarily used for detecting quality issues with welds that were not detected using NDE. Presumably, a flaw found with NDE that was large enough to result in a leak during a hydro would have been repaired.

FFS assessments are used to determine if degraded or flawed equipment is suitable for continued operation.

Has anyone had experience with using FFS to exempt a hydrotest on a tank?

I appreciate any info.

 

[Duwe6]

I'll sidestep your exact question, and go to hydro's after repair. If I had my annular ring [or any other major component] repaired, I'd put a good Inspector on the job, too. Waaay cheaper than a hydro.

Tank was originally hydro'd after construction
Annular ring degraded, without the tank failing
Bad sections of ring were removed and replaced
My API-653 & CWI Inspector evaluated the removed areas, and had any further questionable areas replaced.
Inspector observed 100% of the welding & welders, and assured that no sloppy or marginally competent welders worked on my tank.
Inspector 'signed off' on the Visual Examination of the welding, and the proper and complete replacement of all the degraded areas.
Inspector specified the type and extent of NDT testing required to further assure that the replaced areas are as good [or better] than the original construction. NDT passed.

Why would I not want to waive hydro? What will it tell me that my Inspector hasn't?

Keep in mind that to have a hydro failure, the tank has to be below the strength of the *actual* material -- below *all* the combined Safety Factors. This means that my tank is now less than 1/4th of the Design Strength. Remember also that the tank did not fail with the degraded ring in place. There is no credible scenario that would cause my tank to fail during hydro after witnessed and tested repair/replacements.