EN 1591

Older calculation methods for flanged joints ask whether flange and bolts are strong enough. EN 1591-1 additionally asks whether the joint stays tight. To do so, the standard treats flanges, bolts and gasket as one elastically preloaded system – and follows how the forces in that system change from assembly through the pressure test into operation.

The result is therefore not only a strength check but also an assembly requirement: the bolt load that has to be reached when tightening.

Tightening compresses the gasket. In service, internal pressure, temperature differences, gasket relaxation and external loads relieve this compression again. The standard calculates the bolt load needed at assembly so that in every later condition the gasket remains compressed as strongly as the required tightness demands.

It then checks whether bolts, gasket and flanges withstand these forces in all conditions. The results are load ratios, each of which must not exceed 1. The scatter of the actual bolt load, which depends on the tightening method, is taken into account as well.

The calculation stands or falls with the gasket parameters. They are determined by testing to EN 13555 and published by the gasket manufacturer for the gasket type concerned: the minimum gasket stress for a given tightness at assembly and in service, the maximum permissible gasket stress, relaxation under temperature and stiffness.

Tightness itself is expressed in classes – L1.0, L0.1 and L0.01 – which describe a maximum permissible leakage rate related to the gasket circumference. In Germany, guideline VDI 2290 specifies class L0.01 for flanged joints within the scope of the clean-air regulation TA Luft and refers to the calculation in EN 1591-1.

A flanged joint in a pipe carries more than internal pressure. The pipe stress analysis supplies axial force and bending moment at each flange location; EN 1591-1 includes both as external loads. The bending moment unloads the gasket on the tension side – and that is exactly where a joint can leak although no component is overstressed.

For highly utilised joints, large diameters and fluids with a tightness requirement this calculation is therefore more meaningful than a comparison with the flange rating.

The result presupposes that the calculated bolt load is actually applied: with the assumed tightening method, the assumed lubrication and the intended gasket. If a different gasket is fitted on site or bolts are tightened without a defined procedure, the verification no longer applies. Part 4 of the series therefore deals with the qualification of assembly personnel.