I know, plenty was said on SIFs in this forum, newsletters, manuals and Codes, but I would like to hear your opinions about the modeling technique of the long radius elbows (or tees) of significantly higher actual thickness then the matching pipe (tb>>tp).

According to ASME B31 codes, stress should be calculated based on the matching pipe thickness and SIFs based on the actual bend thickness.
Just as an example, Imagine simple L pipe (10” 9.3mm thick pipe and 18mm thick bend), anchored on one end and loaded on the other end with a force attempting to close the bend.

(A Model) To comply with the Code, the thickness in the input sheet for the pipe and bend remains the same (tp), but the fitting thickness (tb) is added in the bend input sheet for SIF calcs. The stress results in MPa are:
1. inlet weld – pipe side, SIF(1 / 1), Code Stress 80 (80)
2. inlet weld – bend side, SIF(1.666 / 1.388), Code Stress 100 (147)
3. inlet to the centre of the bend curve, SIF(1.666 / 1.388), Code Stress 95 (142)
4. outlet from the centre of the bend curve, SIF(1.666 / 1.388), Code Stress 95 (142)
5. outlet weld – bend side, SIF(1.666 / 1.388), Code Stress 88 (130)
6. outlet weld – pipe side, SIF(1 / 1), Code Stress 72 (72)
(For comparison, the values in the brackets are for the case if the bend was of the same thickness as pipe(tb=tp), SIF (in / out) = 2.564 / 2.12).

It seems to me that the calculated stress at nodes 2 & 5 is too conservative.

(B Model) Altough the Code has its requirements as per above, I believe that we should use the engineering judgement in this situation and model the bend as a separate element, with the actual thickness in the input sheet, and the SIFs user specified (same as per A). Here the calculated stress is significantly reduced!
1. inlet weld – pipe side, SIF(1 / 1), Code Stress 80
2. inlet weld – bend side, SIF(1.666 / 1.388), Code Stress 52
3. inlet to the centre of the bend curve, SIF(1.666 / 1.388), Code Stress 50
4. outlet from the centre of the bend curve, SIF(1.666 / 1.388), Code Stress 50
5. outlet weld – bend side, SIF(1.666 / 1.388), Code Stress 46
6. outlet weld – pipe side, SIF(1 / 1), Code Stress 72

In reality, considering the surface imperfections at the butt weld and its consequent stress risers, I would say that the pipe would either fail on the thin section of the pipe side of the weld, nodes 1 & 6, where the ovalization is not so prominent (???), or at the centre of the bend curve, nodes 3 & 4, where the effect of ovalization is at its maximum (if fitting is flexible enough). Actually, in case of very thick elbows (up to 3xtp for FRP), the pipe might have more tendency to ovalize (more flexible) then the bend, so the bend would make the 1,2 and 5,6 section more rigid then the straight pipe further away from the joint.
The basic two questions are:
- In reality, the extent of ovalization is not the same along the bend curve. What would be the actual stress causing the failure at the butt weld (thin side)? On which SIF value is it based; the actual butt weld's SIFs (unpredictable), or bend's SIFs, or SIF=1? If the first is the answer then we would have to account for it at every butt weld in the system to be safe.
- Are the SIFs calculated per ASME 31codes average values along a bend (tee) or max. values (at the centre of a fitting)? If the first is the correct answer, is the stress (B) calculated at nodes 3 & 4 insufficiently conservative?

With exotic piping materials or FRP piping, design to A or B would make major cost difference. We all know A is the safe and costly option. But I would like to hear from the more experianced engineers what they think about the B option, if it is safe enough?

Originally when S.I.F.'s were tested formulated and the code was written there was no such thing as FRP or GRP.

As such the use of code S.I.F.'s for the analysis of said systems is a choice made by the designer but not necessarily a correct choice.

Metallic pipe and butt welding fittings were the basis for the current requirements of the code for analysis....

Thanks for your reply. Actually, I was talking about the steel piping, FRP was just an example.

My main doubt is: Are ASME B31.3 & 31.1 Codes too conservative for the case when elbow or tee thickness is two or more times larger then the matching pipe thickness?

Hello,

What is to prevent the piping engineer from modeling the physical reality (e.g., wall thicknesses) of the elbows and TEEs in his/her analysis model?

It has been my experience that MANY manufacturers make B16.9 elbows one schedule thicker than specified and then end-bore the elbows such that the wall thickness will be closer to that of the matching pipes. Strictly speaking, this satisfies B16.9. However, since the analysis did not allow for it the model will be "approximate".

Regarding structural analyses of systems that include TEEs, the B31 Codes assign a flexibility factor of 1.0 to TEEs. We know that the TEE has much more flexibility than this. This is conservative in that it affects the overall stiffness matrix of the system. The SIF's were developed using one company's fittings and as we all know there is practically no guidance in B16.9 regarding the allowable geometry of these fittings. The critical geometric issue (as far as actual SIF's are concerned) is the TEEs "crotch radius". There are many vagaries in piping design that must be enveloped by assigning adequate design margins to the Codes. If you have more accurate SIF's and FFs for the fittings that you are using, you are permitted to use those data. There are very useful papers published that guide you in developing these data by modeling YOUR fitting via FEA.

The B31 Code rules have a 50 year "track record" of providing design rules that work. I don't think there is much practical sense in undermining the conservatism of the Codes just for the sake of making them more "theoretically accurate". At least not until we reduce the unknowns that we face in the "real world". We use beam theory and the branches are modeled as intersections (moment connections) of perfectly round (cross section) beams whose inertial data never changes (although we "know" that many loadings serve to ovalize the pipe and thereby change the section modulus). After reviewing countless piping stress analyses, I am left with the opinion that we are lucky if we can even get within 10 percent of the real loadings that the actual piping system will experience. I think that the "conservatism" that is perceived to have been codified into the design is really a lot less than the piping engineer believes. Addressing the question: "are the B31 Codes too conservative" - I think not.

Regards, John.

My opinion is that using N=7000 for fatigue has also saved a lot of bad work from failing due to fatigue in service.... This added bit of conservatism has protected us from ourselves....