Effect of Piping Loads on Bolted Flanged Joints in GRP Piping

Abstract Glass fibre reinforced plastic (GRP) or glass reinforced epoxy (GRE) piping is being increasingly used in firewater and cooling water duties where seawater is used. These composite materials are especially suited for these applications due to their excellent corrosion resistance to seawater, but they do have some special design requirements that are different to metallic piping. One of these requirements relates to the design of the bolted flange connections. Unlike metallic piping materials, GRP and GRE materials are not ductile and the strength properties can degrade over time when the material is under stress. Partly for this reason, piping flanges are generally specified as full face rather than raised face to reduce the bending stresses due to bolting loads. Consequently, relatively “soft” gasket materials are also generally specified as they do not require a high contact pressure that is more easily attained with a raised face flange to produce a good seal and prevent leakage. A common requirement in stress analysis of piping systems is to check the effect of bending moments and forces from the piping system on the ability of the flanged joint to prevent leakage. This assessment is built into pipe stress software using either the equivalent pressure method and comparing this to the flange rated pressure or to the “ASME III NC 3658 method”. However, neither method is directly applicable for full face flanges. This study investigates appropriate criteria for the assessment of bending moments and forces from the piping system on bolted flanged joints in GRP piping. Another important variable is the initial flange bolt tension. Higher bolt pretension reduces the likelihood of leakage but for GRP flanges, limits are required on bolt pretension to avoid damage such as cracking of the GRP flange material. Despite the lower bolt pretensions, GRP full face flanges together with soft gaskets can be shown to have good resistance to external piping loads.