[en] Pressure locking occurs when the fluid in the valve bonnet is at a higher pressure than the adjacent piping at the time of valve opening by pressure trapping and bonnet heatup. Pressure locking is a phenomena which can cause the unseating thrust for a gate valve to increase from its typically static unseating thrust. This can result in the valve actuator having insufficient capability to open the valve. In addition, this can result in valve damage in cases where the actuator capability exceeds the valve structural limits. This paper discusses the primary mechanisms which cause pressure locking. These include sudden depressurization of piping adjacent to the valve and depressurization of fluid trapped in the valve bonnet due to heat transfer. This paper provides a methodology to calculate the unseating thrust for a valve which is pressure locking and applies to motor operated isolation valves of Residual Heat Removal System(RHRS)