A lens gasket is a specific type of metallic ring gasket that is usually only deployed in high pressure gas applications where a high integrity bolted flange joint is required. Lens gaskets are not common in ASME design and there are no ASME rules that guide the design of flanges with lens gaskets, nor are there ASME standards to control lens gasket specifications.
Lens gaskets present a special challenge when determining target bolt tension values for flange assembly. For calculating target bolt tension, the gasket seating width is an important parameter. With lens gaskets, the gasket seating width depends on the applied bolt tension, therefore calculation of bolt tension is by nature an iterative process. In these flange joints, the lens gasket has spherical machined surfaces that are in contact with conical gasket seats in the flanges. At low bolt tensions, gasket contact is nearly equivalent to line contact. At high bolt tensions, finite contact widths are developed, sometimes involving plastic deformation of the gasket.
This paper will present a method that was developed to determine target bolt tension for a lens gasket bolted flange. Reference will be made to European standards that address lens gaskets, and the results of finite element analysis studies that were used to validate the calculation method will be presented. The successful deployment of the outcome of this work for all the lens gasket flange joints on a plant will be discussed.