AXIAL FLOW VALVES

Basic valve operationBecause there is no mechanical connection to the control element, there’s noworry about shaft sealing. Except for the valve inlet and outlet, the onlyconnections to the Axial Flow Valve are three pneumatic lines to the manifold:inlet pressure, control pressure and exhaust / downstream bleed. The valveresponds to the difference in pressure between the inlet port and the controlport. The different functions of the valve) downstream regulation, relief, etc.)are determined by the type pf external pilot and the piping of the pilot. Theoperating characteristics of the valve (fast opening, slow closing, etc.) aredetermined by the setting of the adjustable restrictor in the manifold.Closed position Throttling position Full open positionInlet pressure is applied to the innersurface of the sleeve in theupstream section of the valve andcontrol pressure is applied to theexterior of the sleeve. Because thesleeve is slightly smaller in diameterthan the cage, when inlet andcontrol pressure are equal thesleeve preload keeps the valveclosed.As control pressure is reduced,the inlet pressure overcomes thepreload and begins to force thesleeve away from the inlet cage.As the sleeve continues toexpand, a portion of thedownstream cage is uncoveredand flow begins through thevalve. When downstreamdemand is satisfied, the balancedforces on either side of the sleevemaintain it in an equilibriumposition.When fully opened, thedownstream cage is completelyexposed and the sleeve isexpanded to the point where it issupported against the inner bodyof the valve. Since the controlpressure is usually aspiratedthrough the downstream bleedport, the control pressure issignificantly lower thandownstream pipeline pressure athigh flow rates. This minimizesthe differential between inlet andoutlet required for full valveopening.