Sealability of API R, RX, & BX Ring Gaskets

ABSTRACT This report presents work done to determine the sealability limits of API R, RX, and 8X gaskets. The work included finite element modeling and full scale testing. The research teaches that bolt stresses of 15 to 30 ksi are necessary to seat the gaskets. If these seating loads are not achieved, the gaskets will likely leak at design pressure, particularly for gas service. If the flanges are properly made up, the sealability limits in API Bulletins 6AF and 6AF2 are too conservative and can be eliminated for non cyclic service. INTRODUCTION API Spec. 6A (1)* give specifications for equipment with pressures to 20,000 psi and temperatures as high as 650º F. This equipment utilizes metal ring-type joints for all end connectors in the specification. These end seals are of three basic types, RX, R and BX. BX gaskets are generally used for the higher pressures. Recently API has sponsored a considerable amount of research into the load capacity of the various end connectors in Spec. 6A. This research has resulted in Bulletins (7) which provide the users a prediction of the combined pressure, moment, and tension load capacity of the various connectors, All of this research has been done, however, with the assumption that the gaskets will leak when the applied loads relieve the initialpre-load on the gaskets. The calculations for this were necessarily crude and the assumption has not beenverified. Accordingly, it was desired to perform a general research study on metal gaskets in order to provide users the background of the design of these gaskets to enable one to intelligently assess the degree of safety againstleakage in the end connectors. In practice, ail of these gasket types have apparently worked reasonably well. BX gaskets were explained in a 1964 paper (2) and were introduced to achieve the benefits of pressure energized gaskets in high-pressure flanges. R gaskets, which are not designed to be pressure energized, and RX gaskets, which are pressure energized, do not makeup with the flange faces face to face. The BX gaskets, however, do generally makeup face to face. Type R gaskets have been in service since the 1930's. (Reference 1, 13th Edition, 1981). ASME (3) publishes procedures for calculated gaskets seating and retaining codes for R type gaskets, but not for RX or BX gaskets. RESEARCH OBJECTIVE The objective of this research (13) was to develop general theories which could be used for design and prediction of gasket performance for R, RX, and BX gaskets. Fundamental to this is a prediction of leakage as a function of flange pre-load or flange face movement, Seating load prediction is also an important factor for designers. The philosophy of this research project was to use the finite element method and full-scale testing to help confirm and validate general theories of behavior. The general theory of behavior which was evaluated was that leakage is likely to occur when the contact pressure between the seat and the seal is less than the applied internal pressure that is being sealed.