Combined external load tests for standard and compact flanges

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The recognised standard method of gasketed flanged joint design contained within most pressure vessel codes is that based on the Taylor Forge procedure (Trans ASME 59 (1937) 161). This has, as its basis, bolt load calculations, which are designed to apply sufficient load to both seat and initialise the gasket, and to ensure sealing via a gasket when the operational pressure load is present. The flange ring and hub transmit the bolt load to the gasket and must therefore be stiff and flat. However, there are many real situations where additional loads arise through external pulling and bending. This is commonly seen in piping systems and other flanged pressure equipment. Although the codes do not specifically address the ‘combined load’ problem, the normal method for considering this additional load is to form an equivalent pressure. This over-pressure is calculated by making the stress generated in the pipe or vessel wall, by the external load, equal to a longitudinal pressure stress which may be tensile or compressive, depending on the nature of the load. This results in an over-pressure which can therefore be added to the operating pressure. For bending loads, no account is taken of the variation around the circumference, or the change in gasket seating width, which will vary as the flange faces rotate. In order to assess the effects of external loading on flanges, a combined load test rig has been constructed and a number of bolted flange assemblies examined including standard ANSI joints and compact VERAX VCF joints (Fig. 1a and b). These assemblies have been strain gauged and tested for a variety of load conditions. Tests have been carried out using hydraulic fluid as the main pressurising medium. The results of the individual tests and the combinations of load are presented and discussed.