Summary
The purpose of this document is to propose a piping tee dimension definition for standard tee- and custom designed tee calculation in EN 13480-3.
The previous proposal with a straight-line distance definition for the limits, from the run side and branch side, resulted in significant differences with the EN 10253-2/4 approach. The limits are the lines that define borders of the pressure area and the material area in the calculation in Clause 8.
Two alternative proposals have been investigated for this report:
1. Add a factor of 1.25 to the limit length calculation,
* 1.25
Update formula 8.4.1-2 reinforcement limit in run pipe direction
* 1.25
Update formula 8.4.3-1 reinforcement limit in the branch pipe direction
2. Start distance for limits at the 45 degrees point at the crotch outside arc (see red arrow):
Figure limits ls and lb start at 45 degrees on crotch
The alternative 2 gives the best results. The calculation results are close to the EN 10253-2/4 calculation method and do justice to the lower stress intensity in the tee crotch compared to a stub-on (set-on or pipe-pipe connection) with same run and branch sizes.
Description
The previous proposal was more conservative for the thin wall tees in comparison with the EN 10253-2/4 results. The maximum allowable working pressure would be lower according to the previous proposal. The reason for an update of the proposal is:
- the EN 10253-2/4 tee fittings are already in use for quite some time and do not give any problems.
- the ASME B16.9 rated tees have even less wall thickness than the EN 10253-2/4
- the DIN 2615-1 show a higher allowable pressure for the DIN tees in comparison with EN 10253-2/4 (source: known by editor)
- FEM calculations show that the stress concentration in the crotch of a tee is much less when compared with a stubon
Above arguments are the reason for the update of the proposal in this report for the tee area and limit definition in En 13480-3.
Results
The tables below show the result of the new limit definition for both type A and type B tees in the EN 10253-2/4. In the figures below, the values lower than 1.0 (and red color) mean that the EN 10253-2/4 allows a higher maximum allowable working pressure compared to the proposal.
Table: results of proposal for new limits Type A fitting
Table: results for proposed new limits Type B fitting
The proposed limited definition will give an average a slightly more favorable result (higher MAWP) than the EN 10253-2/4 calculations. The differences are very small which make the EN 10253-2/4 tee dimensions valid in relation to the pressure factor. The current proposed limit definition provides a good basis for custom made tees.
Proposed limit definition
The proposed limit definition result in the following images to be added to the EN 13480-3 Clause 8:
Figure 8.3.9-3 – Thick wall tee, reinforcement inside
Figure 8.3.9-4 – Detail of limit start or reference point
Figure 8.3.9-5 – Thin wall tee, Dm and dm diameter definition
Figure 8.3.9-6 – Thin wall tee, detail of analysis wall thickness definition
Figure 8.3.9-5 – Thick wall tee, reinforcement inside and outside
The reinforcement limit lengths, ls and lb, are depending of the equivalent diameter and the analysis thickness for both the run and the branch part. This is consistent with the formulas 8.4.1-2 and 8.4.3-1.
The limit length is measured in a straight line from the reference point. This is consistent with figure 8.6.1-1. The reference point in this case is on the outside arc of the crotch at 45 degrees angle.
The equivalent diameter is taken at the point of the limit length. This is consistent with figure 8.4.3-1, figure 8.4.3-2, figure 8.4.3-4 and figure 8.4.3-5.
The analysis thickness is defined at the point of the limit length. This is consistent with figure 8.4.3-1, figure 8.4.3-2, figure 8.4.3-4 and figure 8.4.3-5.
The advantage of this consistent approach is, that if the crotch radius is set to zero, the calculation areas and the calculation will show the same result as the isolated opening of a pipe-pipe branch in clause 8.4.
The Clause 8 will require additional text to define the ‘starting point’ for the limit:
“The limits ls and lb will be taken from the point that lies on the outside arc of the crotch at the 45-degree angle as illustrated in Figure 8.3.9-4.”
“Iteration is required to determine the ls and lb values”
“Goniometric- and other mathematical formulas are required to calculate the pressure area and material area with sufficient accuracy of +/- 1%.”
Observations to EN 10253-2/4
The l’s and l’b limits calculated in EN 10253 are sometimes outside the tee. As a result, the material and pressure area used in the calculation are extended outside the tee.
The tapering/beveling angle and length is not taken into consideration. Just a square related to the thickness of the tee Type B wall is deducted to cater for the material reduction. Tapering is not considered for Type A tee.
The calculation method for a tapered Type A fitting is unanswered. Does this fitting need to be calculated as per Type A or Type B?
We received new dimensions for the Type B fitting. The reinforcement or extra thickness is also to the outside of the tee and the crotch radius factor is now 0.5 instead of the 0.75 in the rc ≤ 0.75 × (G − D/2) formula. A NoBo will question the ‘0.5’ definition.
The dimensions for the DN15 x DN15 S8 tee result in overlapping material areas:
Figure DN15 x DN15 S8 as per EN 10253-2 Type B
With updated crotch radius and reinforcement to the outside of the pipe OD:
Figure DN15 x DN15 S8 as per EN 10253-2 Type B, crotch radius factor 0.5 and outside reinforcement
In both cases the material areas overlap and fluid can not enter via the run side
Attachments
(Not part of this article only with original report)
EN 10253-2 Type A calculation results with modified start point start at 45-degree of crotch, the middle tables are left with limits starting at the OD of run and branch and right with modified start point
EN 10253-2 Type B calculation results with modified start point start at 45-degree of crotch, the middle tables are left with limits starting at the OD of run and branch and right with modified start point
EN 10253-2 Type B calculation results with dimensions received from (name with editor), with reinforcement to the outside of the tee and the reduced crotch radius.
Document: EN 10253-2 vs EN 13480-3 Tee limit definition Type B DN800xDN500 S4 (discussion about limit lb being outside the Tee branch side dimension)