The electro -pneumatic positioner is used in Control Valves with pneumatically operated actuators. The valve is operated by means of electrical controller or Control Systems With an control signal of 4 to 20 mA or split ranges of 4~12/ 12~20 mA. Electro-Pneumatic Positioner converts this control signal into a pneumatic output in proportion to the lift of… Read More »
Control valve selection is depend on below these points : Type of fluid to be controlled Temperature of fluid Viscosity of fluid Specific gravity of fluid Flow capacity required (maximum and minimum) Inlet pressure at valve (maximum and minimum) Outlet pressure (maximum and minimum) Pressure drop during normal flowing conditions Pressure drop… Read More »
The root cause of the problem – a varying pressure drop caused by frictional losses in the piping and other factors – generally cannot be eliminated. This means there is no way to regain maximum flow capacity short of replacing the control valve with one having a greater Cv rating. However, there is a clever way to flatten… Read More »
Hydraulic actuators use liquid pressure rather than gas pressure to move the valve mechanism. Nearly all hydraulic actuator designs use a piston rather than a diaphragm to convert fluid pressure into mechanical force. The high pressure rating of piston actuators lends itself well to typical hydraulic system pressures, and the lubricating nature of hydraulic oil helps to overcome… Read More »
Pneumatic actuators use air pressure pushing against either a flexible diaphragm or a piston to move a valve mechanism. The following photograph shows a cut-away control valve, with a pneumatic diaphragm actuator mounted above the valve body. You can see the large coil spring providing default positioning of the valve (air pressure acting against the diaphragm moves the… Read More »
Recall that the purpose of a valve positioner is to ensure the mechanical valve’s position matches the command signal at all times. Thus, a valve positioner is actually a closed-loop control system in its own right: applying as much or as little pressure to the actuator in order to achieve the commanded valve stem position at all times.… Read More »
Motion-balance pneumatic valve positioner designs also exist, whereby the motion of the valve stem counteracts motion (not force) from another element. The following illustration shows how a simple motion-balance positioner would work: In this mechanism, an increasing signal pressure causes the beam to advance toward the nozzle, generating increased nozzle backpressure which then causes the pneumatic amplifying relay… Read More »
Glob valve is so much used in current industries now a days. In glob valve flow is controling by movable plug and stationary seat. Glob valve design is one of the most popular design of sliding stem valve. Cut view of Glob valve photos are given below. … Read More »