Samson 3271 Mounting And Operating Instructions

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Samson 3271 Mounting And Operating Instructions Manual pdf 15 page image

EB 8052 EN 3-1Design and principle of operation3 Design and principle of oper-ationRefer to Fig. 3-1.The single-seated Type 3251 Globe Valve ispreferably combined with a SAMSONType 3271 or Type 3277 Pneumatic Actua-tor. It can also be combined with other actu-ators.The seat (4) and plug with plug stem (5) areinstalled in the body (1). The plug stem isconnected to the actuator stem (A7) by thestem connector clamps (A26) and is sealedby a spring-loaded V-ring packing (15). Thesprings in the pneumatic actuator (A) are lo-cated either above or below the diaphragm(A4) depending on the selected fail-safe ac-tion. A change in the signal pressure actingon the diaphragm causes the plug to move.The actuator size is determined by the dia-phragm area.The medium flows through the valve in thedirection indicated by the arrow. A rise insignal pressure causes the force acting onthe diaphragm in the actuator to increase.The springs are compressed. Depending onthe selected direction of action, the actuatorstem retracts or extends. As a result, the plugposition in the seat changes and determinesthe flow rate through the valve.We recommend the use of positioners withintegrated diagnostic firmware (see sec-tion 3.3) for valves used for on/off service.The partial stroke test included in this soft-ware helps prevent a shut-off valve normallyin its end position from seizing up or gettingjammed.Fail-safe positionsThe fail-safe position of the control valve up-on air supply or control signal failure de-pends on the actuator used (see associatedactuator documentation).Depending on how the compression springsare arranged in the SAMSON Type 3271and Type 3277 Pneumatic Actuator, thevalve has one of two different fail-safe posi-tions:− Actuator stem extends (FA)When the signal pressure is reduced orthe air supply fails, the springs move theactuator stem downward and close thevalve. The valve opens when the signalpressure is increased enough to over-come the force exerted by the springs.− Actuator stem retracts (FE)When the signal pressure is reduced orthe air supply fails, the springs move theactuator stem upwards and open thevalve. The valve closes when the signalpressure is increased enough to over-come the force exerted by the springs.The actuator's direction of action can be re-versed, if required. Refer to the mountingand operating instructions of the pneumaticactuator:u EB 8310-X for Type 3271 and Type 3277TipTip