A leak tight design methodology for large diameter flanges based on non-linear modelling and analysis

Abstract

There is currently a need for large diameter flanges for the supply of water in South Africa. These large diameter pipe flanges are required to accommodate pipes with nominal bores of up to 4 m and should successfully withstand internal pressures of up to 8 MPa. No current relevant standard / code contains prescribed design values for flanges which either operate at such high pressures or have such large diameters. Due to this an alternative method of design, by means of non-linear finite element modelling, is proposed. Three types of integral flange designs are considered, namely: flat face, raised face, and a modified raised face with an O-ring groove. The effects of creep-relaxation, flange rotation, and the bolting sequence are considered. For each of these designs a finite element model was created and compared to a small scale experiment which included strain and contact pressure measurements. The proposed non-linear finite element models were capable of accurately predicting the strains in the flanges as well as the contact pressures between the faces of the flange and the surfaces of the packing material. Finally, a comparison between the ASME design method and the proposed non-linear finite element modelling design method was done for the large diameter flanges. It was found that the ASME design code did not have the ability to accurately predict the stresses in the flanges. It was also found that by using the maximum equivalent Von Mises stress as failure criteria for the flanges and fasteners, and contact pressure for the sealing ability, circular bolted flange connections which are lighter, safer, and leak tight could be designed by means of the proposed non-linear finite element models.