A valve stem is mounted to the valve housing and is engageable with an external surface region of the diaphragm. The valve stem is selectively movable toward and away from the inlet passage and causes the diaphragm to selectively move into or out of sealing engagement with regions of the valve housing surrounding the inlet passage. Thus, the valve stem can be moved toward the valve housing so that the diaphragm is deflected into sealing engagement with portions of the valve housing surrounding the inlet passage for preventing fluid flow through the valve. Needle ValvesThe valve stem then can be moved away from the valve housing to permit the diaphragm to deflect away from the inlet passage.PLUG VALVES Thus, fluid is permitted to flow from the inlet passage into the valve chamber and subsequently from the valve chamber into the outlet passage.

Some diaphragm valves are designed specifically for controlling the flow of liquids, while others are designed specifically to control the flow of gases. Many manufacturing processes require controlled amounts of very pure gases. Even, small amounts of ambient gas or other contaminants can adversely affect the manufacturing process. The components of such gas flow systems are manufactured and assembled in ultra clean environments. Fittings and valves for such systems typically are made entirely from metal and must positively prevent seepage of ambient air into the very pure supply of gas accommodated by the system.

Diaphragm valves are well suited for fluid systems that are intended to accommodate pure gases. PLUG VALVESHowever, the metal diaphragm of a diaphragm valve is capable of generating only a relatively small amount of deflection. As a result, prior art diaphragm valves have not been well suited for high flow fluid systems. The flow of fluid through a diaphragm valve theoretically can be increased by merely increasing the cross-sectional dimensions of the inlet passage, the outlet passage and the valve chamber. However, a prior art diaphragm valve must be sufficiently deflectable to seal entirely around such a large inlet passage. The metal diaphragm of a prior art diaphragm valve would have to be positioned very close to such a large inlet passage to effectively seal the entire inlet passage with the limited amount of deflection that is possible with a diaphragm. Thus, even though a fairly large inlet passage can be provided, the actual flow through a prior art diaphragm valve would be restricted by the close proximity of the diaphragm to the inlet passage.

Some prior art diaphragm valves have secured a rigid valving member to the side of the diaphragm facing the valve chamber. The rigid valving member is selectively moved by the diaphragm into and out of sealing engagement with portions of the valve chamber surrounding the inlet passage. Prior art diaphragm valves of this type often are referred to as tied diaphragm valves and are intended to achieve both a high flow and a good quality seal around a fairly large inlet passage. In particular, it has been assumed that the valving member secured to and moved with the diaphragm can achieve better sealing around a large inlet passage than the metal diaphragm could achieve without the benefit of thee valving element. Tied diaphragm valves are used successfully in some fluid systems. However, fluid systems intended for highly pure gas flows often cannot tolerate the presence of a weld area in the gas stream. The weldment creates the potential for contamination and defines a potential weak point that can lead to a costly failure of the valve.

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