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?
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Allowable thickness of a Tee fitting at knuckle region per ASME B16.9 1
- Thread starter schaali1
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schaali-
You need to take a closer look at B16.9. There are only four pages of text, so not too involved. The design intent is that the fittings, in this specific case a tee, be stronger than the pipe to which they are attached. Thus, a spool with standard wall pipes welded to a standard wall tee which is then subjected to a hydrotest to failure… The intent is that the pipe fail prior to the tee failing.
The only place the thickness of the tee is defined is at the weld ends. At other locations, it is not. Look at several tees from different manufacturers and you’ll see various shapes – some relatively cylindrical, drawn from a pipe, and others with a more spherical shape at the branch area. The radius between the branch and run will vary. These fittings are qualified either through testing to failure or FEA or other involved analysis techniques.
Again, pull out your copy of B16.9 and read it closely, in particular 2.2, Design of Fittings. It includes phrases such as:
…established by mathematical analysis…
…manufacturer’s option by proof testing…
…it is expected that some portion of formed fittings may have to be thicker than the pipe wall…
So, do you need a Level 3 FFS? Well, probably. If you didn’t establish the thickness measurement locations and initial thicknesses, then you don’t know how much corrosion has occurred. I won’t complicate the subject by mentioning that at the branch you have bending going on and the stiffness and strength are thickness cubed or squared functions so linearly subtracting a CA may not result in a conservative result. While you’re at it, be sure to get a good estimate of the piping loads.
jt
You need to take a closer look at B16.9. There are only four pages of text, so not too involved. The design intent is that the fittings, in this specific case a tee, be stronger than the pipe to which they are attached. Thus, a spool with standard wall pipes welded to a standard wall tee which is then subjected to a hydrotest to failure… The intent is that the pipe fail prior to the tee failing.
The only place the thickness of the tee is defined is at the weld ends. At other locations, it is not. Look at several tees from different manufacturers and you’ll see various shapes – some relatively cylindrical, drawn from a pipe, and others with a more spherical shape at the branch area. The radius between the branch and run will vary. These fittings are qualified either through testing to failure or FEA or other involved analysis techniques.
Again, pull out your copy of B16.9 and read it closely, in particular 2.2, Design of Fittings. It includes phrases such as:
…established by mathematical analysis…
…manufacturer’s option by proof testing…
…it is expected that some portion of formed fittings may have to be thicker than the pipe wall…
So, do you need a Level 3 FFS? Well, probably. If you didn’t establish the thickness measurement locations and initial thicknesses, then you don’t know how much corrosion has occurred. I won’t complicate the subject by mentioning that at the branch you have bending going on and the stiffness and strength are thickness cubed or squared functions so linearly subtracting a CA may not result in a conservative result. While you’re at it, be sure to get a good estimate of the piping loads.
jt
- Thread starter
- #3
Thanks jte..
“I understand the portion of B16.9 you mention but am still unclear. If I buy a 10” std wall tee the minimum thickness would be 9.27 mm at the weld ends but how would I know that the Tee was within code if there is no requirement for a thickness at the knuckle region?
The statement ...it is expected that some portion of formed fittings may have to be thicker than the pipe wall... is hardly a requirement as it contains the words expected and may. In other words it also may not be expected.
Even if I was to measure a brand new one there would be no way of “knowing” that the measured thickness was sufficient to make sure the pipe failed before the fitting. Is the expectation that the manufacturer is trusted to have produced an acceptable fitting even though the minimum required thickness of the knuckle region is unknown? Incidentally, even knowing the initial thickness would be of limited use in determining the fitness for purpose as the number we need to know, the minimum thickness at the knuckle, is not code defined. I find that very strange.”
“I understand the portion of B16.9 you mention but am still unclear. If I buy a 10” std wall tee the minimum thickness would be 9.27 mm at the weld ends but how would I know that the Tee was within code if there is no requirement for a thickness at the knuckle region?
The statement ...it is expected that some portion of formed fittings may have to be thicker than the pipe wall... is hardly a requirement as it contains the words expected and may. In other words it also may not be expected.
Even if I was to measure a brand new one there would be no way of “knowing” that the measured thickness was sufficient to make sure the pipe failed before the fitting. Is the expectation that the manufacturer is trusted to have produced an acceptable fitting even though the minimum required thickness of the knuckle region is unknown? Incidentally, even knowing the initial thickness would be of limited use in determining the fitness for purpose as the number we need to know, the minimum thickness at the knuckle, is not code defined. I find that very strange.”
schaali-
Well, I agree… It is strange, though I understand how it came about. The problem is that the B16.9 is a new construction code. The fabricator develops a product which passes the requirement of a new pipe failing prior to the fitting failing. The only way you know that the fitting will fail prior to the pipe failing is the manufacturer’s certification of the design. This makes more sense in a mass produced product such as a fitting where a proof test approach may be economical. You wouldn’t typically take such an approach with a one-off vessel, though in theory you could with some swaged opening which does not have a code defined formula. Doesn’t make sense to build a prototype just to break it if you aren’t going to build thousands of ‘em.
The “expected” statement is, I imagine, worded in a way to provide guidance without restricting a fabricator who may come up with a design which may at some point on the fitting be thinner than the corresponding pipe. Based on the simple observation that openings in pipes and vessels are commonly reinforced with repads or (more commonly) ‘olet type reinforcement, it is not unexpected that a fitting such as a tee might also benefit from the reinforcement near the opening. But the standard leaves open the possibility that some other part of the tee might be thinner.
No information is provided to consider corrosion allowances. A similar situation comes up with B16.5 flanges: What is the allowable CA on a B16.5 blind?
Bottom line – that I think you already know but are struggling with accepting… If the minimum thickness must be known, some analysis must be done.
jt
Well, I agree… It is strange, though I understand how it came about. The problem is that the B16.9 is a new construction code. The fabricator develops a product which passes the requirement of a new pipe failing prior to the fitting failing. The only way you know that the fitting will fail prior to the pipe failing is the manufacturer’s certification of the design. This makes more sense in a mass produced product such as a fitting where a proof test approach may be economical. You wouldn’t typically take such an approach with a one-off vessel, though in theory you could with some swaged opening which does not have a code defined formula. Doesn’t make sense to build a prototype just to break it if you aren’t going to build thousands of ‘em.
The “expected” statement is, I imagine, worded in a way to provide guidance without restricting a fabricator who may come up with a design which may at some point on the fitting be thinner than the corresponding pipe. Based on the simple observation that openings in pipes and vessels are commonly reinforced with repads or (more commonly) ‘olet type reinforcement, it is not unexpected that a fitting such as a tee might also benefit from the reinforcement near the opening. But the standard leaves open the possibility that some other part of the tee might be thinner.
No information is provided to consider corrosion allowances. A similar situation comes up with B16.5 flanges: What is the allowable CA on a B16.5 blind?
Bottom line – that I think you already know but are struggling with accepting… If the minimum thickness must be known, some analysis must be done.
jt
I agree with jte. Most of these designs are justified by proof tests. Each manufacturer will have a different design and I suspect that the same manufacturer might have different designs through time. So long as all are justified by a proof test, then they all pass the Standard.
Your problem is not unique. jte has highlighted some of the issues with trying to determine "acceptable" from an FFS perspective. If it were me, I'd perform a complete pipe stress analysis on the attached pipe to calculate the piping loads on the tee. I'd also want to determine the "self-limiting" nature of those loads, because there are different design margins (load factors) depending on whether the loads are deadload or self-limiting. Regardless of whether or not an identified thickness is in the Standard or not, all of these issues need to be determined for a proper FFS assessment. Not an easy task.
Are you capable of performing a Level 3 assessment? In my experience, most people try to avoid a Level 3 because they can't do it. Do you have access to a firm that can perform these assessments?
Your problem is not unique. jte has highlighted some of the issues with trying to determine "acceptable" from an FFS perspective. If it were me, I'd perform a complete pipe stress analysis on the attached pipe to calculate the piping loads on the tee. I'd also want to determine the "self-limiting" nature of those loads, because there are different design margins (load factors) depending on whether the loads are deadload or self-limiting. Regardless of whether or not an identified thickness is in the Standard or not, all of these issues need to be determined for a proper FFS assessment. Not an easy task.
Are you capable of performing a Level 3 assessment? In my experience, most people try to avoid a Level 3 because they can't do it. Do you have access to a firm that can perform these assessments?
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