Combined external load tests for standard and compact flanges

The recognised standard method of gasketed hanged joint design contained wi thin 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 t he 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 gas ket and must therefore be stiff and flat. However, there are many real situ ations where additional loads arise through external pulling and bending. T his is commonly seen in piping systems and other hanged 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 equiv alent pressure. This over-pressure is calculated by making the stress gener ated in the pipe or vessel wall, by the external load, equal to a longitudi nal pressure stress which may be tensile or compressive, depending on the n ature of the load. This results in an overpressure which can therefore be a dded to the operating pressure. For bending loads, no account is taken of t he variation around the circumference, or the change in gasket seating widt h, which will vary as the flange faces rotate. In order to assess the effects of external loading on flanges, a combined l oad 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 variet y of load conditions. Tests have been carried out using hydraulic fluid as the main pressurising medium. The results of the individual tests and the c ombinations of load are presented and discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.