Gasket characteristics for the design of bolted flange connections of metal-to-metal contact type

Abstract

Frequently used joints in industrial plants are bolted flange connections of metal-to-metal contact type (MMC) especially where dynamic loadings are to be expected. Contrary to floating type flanged joints, where the total bolt load is transmitted through the gasket, only that part of the bolt load is taken over by the gasket which is necessary for MMC. Two basically different types of MMC joints have to be distinguished. In the first case, the gasket — mostly spiral wound gaskets (SWG) without distance rings — is implemented in a groove in one of the flanges, and during assembly flange contact has to be reached. If this type is used the most important parameter is the clearance on the outer diameter. If the difference between the outer diameters of the groove and gasket is too small, excessive gasket stress for MMC results which cannot be reached in practice. This effect is enhanced with increasing oversize of the gasket (difference between gasket thickness and groove depth). The acceptable range of radial clearance is between 1.0 and 1.5 mm, depending on the gasket thickness. Gaskets with small width (about 6 mm) show rather large leakage rates due to collapse and/or inward buckling. In the second case, flat face flanges and a gasket with a sealing element and distance ring (e.g. SWG with outer distance ring) is used. Tightening of the bolts leads to contact between the flanges and the distance ring. Depending on the width of the gasket the required gasket stress for MMC can be too high. The optimum width with respect to adequate MMC stress and tightness is about 10 mm or less.