EN 1591 and EN 13445 Annex G

Alternative Design Rules for Flanges.

The new rules, known as the Alternative Method (EN 1591/EN 13445 Annex G), are based on a method used successfully in East Germany for many years. While most aspects of flange design were considered afresh by the authors of this method, the most notable features must be the use of limit analysis, the recognition that the bolt load has its own tolerance, the calculation of the way the bolt load changes as pressure (or other loads) are applied and the detailed analysis of gasket behaviour.

Any method that wishes to supplant Taylor Forge must be complete and attractive economically and has to include taper hub flanges. It should assist understanding of what is happening in a flange assembly as an aid to good design and trouble-shooting. The Alternative Method does these things.

It is clear that the Taylor Forge method most often calculates the hub stresses of the flange incorrectly (Sawa, Int Journal of Pressure Vessel Technology, 2005). In those cases it is also clear that the Alternative Method is giving more economical solutions. The Alternative Method is the only method for taking into account the leak tightness requirements. By using the Alternative Method and careful bolt tensioning, the design becomes more reliable and economical than the by Taylor Forge method.

Some advantages by using the Alternative Method includes:
– Gives in many cases a more economical solution
– Ensures a leak free joint/leak tightness
– Allows the designer to analyse existing flanges to determine cause of un-tightness
– Includes effect of thermal expansion effects due to differences in bolt temperature and/or materials
– Determines the rotation/deformation of joint in all conditions
– Includes effect of external loading (moments and axial forces)
– Determines minimum required bolt torque based selected bolting up method
– The flange can be attached to a conical shell or a domed end, not only a cylinder.
– Considers the non-linear elastic behaviour of the gasket
– Considers the influence of the number of mounting cycles on the gasket
– The range of the allowable bolt forces for installation can be determined
– Determines the fluctuation of bolt force based on the bolting up method(e.g. with a wrench, impact wrench, torque controlled wrench with or without measuring equipment).

Even if the method itself is quite complex, the input and the use by the VVD software is quite simple. Below is a sample output showing the input section and the results summary section.

However, all intermediate calculation results are also available, the total number of pages then becomes 16 instead of the 4 pages shown in the sample calculation below.




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