The series S&S 857 is a direct acting, spring opposed diaphragm actuator with a 4-inch maximum travel. Signal pressure supplied to the upper housing chamber forces the stem downward. When the signal pressure is reduced, the compressive force on the spring moves the stem upward. If the signal pressure should fail, the spring will force the stem in the upward direction providing “ fail - open action” for “push-down to open” valves.

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