PURPOSE:To make it possible to control the valve-closing time characteristic easily, by forming a back pressure chamber on the back surface of the valve disc of a main steam insulating valve so as to be variable in its capacity. CONSTITUTION:A valve disc 4 is set in the valve disc supporting part 3, for closing a main steam flow route 2, and a back pressure chamber 5 is formed on the back surface of the valve disc 4, for receiving steam for back pressure. The opening end of a pipe 19 the other end of which is closed, is connected to said back pressure chamber 5, and closing valves 20-22 are set interposingly in this pipe 19. The back pressure chamber 5a can be enlarged in its capacity, by opening these closing valves 20-22 in order, and in this way, the valve- closing time characteristic can be controlled.
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