Valve positioners

 Taken from: http://www.globalspec.com/learnmore/flow_transfer_control/valve_actuators_positioners/valve_positioners

Valve positioners compare a control signal to a valve actuator’s position and move the actuator accordingly. They are used with both linear valves and rotary valves. When a control signal differs from the valve actuator’s position, the valve positioner sends the necessary power to move the actuator until the correct position is reached. There are four basic types of valve positioners: pneumatic, electronic, electro-pneumatic, and digital. Pneumatic devices send and receive pneumatic signals. Single-acting or three-way pneumatic positioners send air to and exhaust air from only one side of a single-acting valve actuator that is opposed by a range spring. Double-acting or four-way pneumatic positioners send and exhaust air from both sides of the actuator. Electric valve positioners send and receive electrical signals. There are three electric actuation types: single-phase and three-phase alternating current (AC), and direct current (DC) voltage. Electro-pneumatic valve positioners convert current control signals to equivalent pneumatic signals. Digital or “smart” devices use a microprocessor to position the valve actuator and monitor and record data.

Performance specifications for valve positioners vary by device type and include pneumatic input signal range, maximum supply pressure, milliampere input signal range, split range, operating temperature, and output action. Pneumatic input signal range and maximum supply pressure are measured in pounds per square inch (psi). Common split ranges include 4 – 10 mA and 12 – 20 mA. Two-way, three-way, and four-way splits are available. There are three types of output actions: direct, reversible, and field reversible. Direct action devices increase the output signal as the input signal increases. Conversely, reversible action devices decrease the output signal as the input signal increases. With field reversible products, devices can be switched between direct and reversible action.

Valve positioners differ in terms of applications, features and approvals. Some products are designed for automotive, aerospace, marine, medical or military applications. Other products are suitable for food processing or pharmaceutical applications. General-purpose devices are commonly available. Intrinsically safe (IS) valve positioners do not produce sparks or other thermal effects that would ignite a specified gas mixture. Devices that are made from stainless steel are used in corrosive or high temperature environments. Common approvals for valve positioners include marks from the Canadian Standards Association (CSA) and Underwriters Laboratories (UL), an independent testing organization.

Taken from: http://www.maintenanceresources.com/referencelibrary/controlvalves/cashcopositioners.htm

A valve Positioner is a device used to increase or decrease the air pressure operating the actuator until the valve stem reaches the position called for by the instrument controller.      Positioners are generally mounted on the side or top of the actuator. They are connected mechanically to the valve stem so that stem position can be compared with the position dictated by the controller.      A positioner is a type of air relay which is used between the controller output and the valve diaphragm. The positioner acts to overcome hysteresis, packing box friction, and valve plug unbalance due to pressure drop. It assures exact positioning of the valve stem in accordance with the controller output. Reasons To Use Positioners       Increase control system resolution: i.e. fine control.       Allow use of characteristic cams.       Minimize packing friction effects: i.e. high-temperature packing.       Negate flow-induced reactions to higher pressure drops.       Increase speed of response to a change in process.       Allow split ranging.       Overcome seating friction in rotary valves.       Allow distances between controller and control valve.       Allow wide range of flow variation: i.e. operate at less than 10% travel under normal conditions.       Allow increased usage of 4-20 mA electronic signal.       Increase fast venting (unloading) capability.       Permit use of piston actuators.       Facilitate operation when the higher number in the bench-set range is greater than 15 psig: i.e. 10-30 psig, 6-30 psig, etc.