| 摘要 |
A fluid flow control device such as a volume booster has an inlet, an outlet and an exhaust port all for an operating fluid. A pilot inlet receives a pilot pressure signal which operates a valve stem actuator via a diaphragm assembly. The valve stem is operable to open a supply valve between the inlet and outlet and, separately and independently, to open an exhaust valve disposed between the outlet and the exhaust. In a further position both the valves are closed. Movement of the stem in a reciprocal manner by the diaphragm assembly causes a respective annular shoulder to engage a respective valve and lift it off a valve seat against the force of a biasing spring. A first fluid leak path is provided between the valve stem and the supply valve for allowing fluid to leak through the supply valve chamber when the supply valve is open and a second fluid leak path is defined by the exhaust valve for allowing fluid to leak through the exhaust valve chamber. The leak paths provide for a force balancing arrangement that allows the valve stem to move easily and the diaphragm assembly to be reduced in size. The arrangement allows the exhaust port to be positioned on the same side of the diaphragm assembly as the inlet and outlet so that fluid can be vented through the exhaust at flow volume comparative to the operating fluid flow in the inlet and outlet.
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