Introduction

This document provides the rationale (arguments) for the definition of the limit calculations as used in document ‘EN 10253-2 vs EN 13480-3 Tee limit definition Type B DN800xDN500 S4 rev.1’ of August 18, 2023.

The limits for the material and pressure area are calculated based on the thickness and diameter at the limit position. This is consistent with the current definition in Clause 8 of EN 13480-3, formula 8.4.1-2 (ls) and formula 8.4.3-1 (lb). The position and distance of the limit is depending the equivalent diameter and the analysis thickness.

The figures 8.4.3-2, 8.4.3-3, 8.4.3-4 and 8.4.3-5 in the EN 13480-3 show in detail what the definition is of the material/pressure areas and the positions of the limits. This is in contrary with the figures for the Tee, refer figure 8.3.9-1 and 8.3.9-2 of the EN 13480-3.

Purpose - custom design Tee

The purpose of this document and the previous document is to have the same level of detail for the Tee areas and limits. The last and current proposal includes the experience with the EN 10253 fitting pressure factor. The calculation results, with the limit starting at 45 degree point,  are almost the same as the EN 10253 calculation results.

Custom design Tee’s are used in the industry  for high pressure piping. The below images, of an heavy wall Tee example, show the results of the current proposal.

 Figure 01 Heavy wall Tee DN800 x DN500, Design pressure: 9.8 MPa Design temperature: 250 Celsius Material: A234 WPB MAWP: 9.812 MPa The run side tapering is at the end of the run side. Notice the position of the limit ‘ls’. The Tee is just strong enough for the design conditions. The loading is 99% of the allowed stress. Figure 02 The same Tee except that the tapering length is longer. The ls is now 213.126 mm instead of 234.17 mm. The material and pressure area are likewise changed, Design pressure: 9.8 MPa Design temperature: 250 Celsius Material: A234 WPB MAWP: 9.153 MPa The Tee is now not strong enough and the calculation is not valid. Figure 03 The same Tee except that the tapering is now moved close to the crotch. The ls is now 186.5 mm instead of 234.17 mm, refer Figure 01. The material and pressure area are likewise changed, Design pressure: 9.8 MPa Design temperature: 250 Celsius Material: A234 WPB MAWP: 8.412 MPa The Tee is not strong enough and the calculation is not valid.

The above results demonstrate that there is a lower MAWP when the limit ls decreases and material/pressure area change, this is logical and is what an engineer expects.

The limits in the proposal give our customers, which use custom designed Tees, a proper and workable definition for the calculation.

The ASME allows (prefers) a special type of tapering which, in the current proposal, give results as expected. The next figure 04 illustrates this special tapering with an offset.

 Figure 04 ASME type of tapering, the offset of the tapering is exaggerated only for illustration. The outside diameter is increased compared to the previous figures 01, 02  and 03. The offset is at run side only.

Conclusion

The proposed definition of the ls and lb limit, with the exact area calculations, provide a workable approach for both custom designed Tees and standard Tees of the EN 10253 standard.

Figure 05