Gasketed flange connections should be designed taking actual behavior of the connections under their operating conditions into consideration. However, such actual behavior as bolt load change, gasket load change and flange rotation were not clear because adequate calculation method was not developed due to difficulty and complicacy to solve statically indetermine problem among three bodies, bolt, flange and gasket. In this paper, authors develop a method to calculate load factor for gasketed flange connection. Load factor describes bolt load change when an external force is applied to the connection. Load factor represents flange rigidity including gasket stiffness that dominates not only the behavior of joint after pressurising but also its sealing performance. By using the load factor, bolt load change as well as gasket load change due to internal pressure can be obtained by simple equations. When the required gasket stress is given to achieve a prescribed sealing thightness, the required initial bolt preload can also be calculated. Authors also proposed a load equilibrium diagram for gasketed flange connection with internal pressure. The diagram helps us to understand schematically how bolt load and gasket load change under pressurized condition. In addition, experimental tests are performed using 3 inch and 20 inch flange connections with spiral wound gasket in order to demonstrate validity of the proposed calculation method based on load factor and load equilibrium diagram. In conclusion, it is found that the proposed calculation method can estimate bolt load change and gasket load change under pressurized condition.

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