Improving the Integrity of Lap Joint Flange Connections

Bolted connections where proper alignment is critical usually require the use of lap joint flanges. The ability of lap joint flanges to swivel around the pipe enables the connection to be installed with no rotational stress applied to the attached piping and equipment. The majority of lap joint flanges on the Enbridge Pipelines system are ANSI 600 and are located at pump nozzles. In these applications, Enbridge Pipelines’ standard specifies the use of spiral wound gaskets with inner rings. Problems with lap joint flanges typically occur at the time of assembly. There are a number of factors that contribute to the challenges of properly seating a gasket in these joints: • Since the raised face is separate from the flange, there is some flex in the face that could unevenly load the gasket; • The flange may not be concentric with the raised face that is welded to the pipe because of clearance between the pipe OD and the flange ID. Using the bolt holes to align the flange does not necessarily ensure that the raised faces are aligned; • Line up pins are not recommended on pump nozzles in order to ensure that no stress is placed on the pump. Consequently, the bottom studs are used to center the gasket and it is possible for the gasket to sit in the stud threads, increasing the misalignment between the gasket windings and the flange raised faces; • Specified torque values for a given flange size may be too high for use in a lap joint application. The perfect bolt alignment in these joints may result in a lower nut factor and subsequently a higher bolt stress for a given torque value. All of these factors can cause damage to the windings on a spiral wound gasket, eventually resulting in a leak. This paper will discuss ways to mitigate these issues.