API 579 FFS assessment for Tee Knuckle region

[schaali1]

ASME B16.9 fittings conform to the same wall thickness schedules as for B31.3 pipe and this code states that the "allowable pressure ratings.....may be calculated as for straight seamless pipe". So for example an 10" standard wall equal Tee would have a nominal wall thickness 9.27 mm which is the same as for 10" pipe of this schedule. It has been stated by someone that the wall thickness at the knuckle region of this type of fitting would be thicker than this. My question is "Is this by design code or as a consequence of the manufacturing process?". We deal in in-service inspection and it was further stated that the minimum allowable wall thickness of this knuckle region would require a Level 3 API 579 FFS assessment which seems to be contrary to the original design code. Anyone have any thoughts?

 

[jte]

The silence is deafening... Any other thoughts out there?

thread292-252769

jt

 

[DSB123]

schaali1,
Forged tees are generally of greater thickness than the equivalent piping schedule. Only at the ends where the fitting is to be welded to the adjacent pipe will the thickness be as per the schedule of the pipe. This is generally because the design of forged fittings are based on "proof" testing such that the fitting is of equivalent strength to straight pipe. Thats why you do not need to perform pressure thickness calcs. However if you have defects in a fitting such as corrosion then the fitting needs to be checked for further service. The only way to do this is by a level 3 assessment where the supplemental loadings are also included as well as the internal pressure. You cannot do this using simple formulae and must tend towards FEA.