What Are Valve Positioners?

Digital Positioner (Valtek)

A valve positioner is used in combination with a valve actuator to precisely position a valve so that optimum flow accuracy may be achieved. The positioner does this by measuring a the process variable, comparing it to a desired condition (set point) and then pneumatically, electrically, or hydraulically changing the valve disc, globe, plug or ball position until the difference between the set point and actual position is zero.

Valve positioners are key elements in a closed loop control system where the final control element is a valve. They assist in maintaining tight control, overcoming the realities of imprecise calibration, differential pressure across the valve, valve wear and a host of other process control challenges.

A valve positioner responds to a signal from some type of master control system, typically a distributed control system (DCS), a programmable logic controller (PLC), or PID controller. The control system reads a signal from a process sensor (flowmeter, temperature sensor, pressure sensor, etc...) and compares that reading to the desired setpoint. A corrective signal, based on the difference,  is provided to the valve positioner which re-adjusts (if necessary) the valve position to bring the system in to equilibrium.

Valve positioners are available with pneumatic, electrical, electro-pneumatic, and digital operation. Here is a brief description of each:

Pneumatic

Pneumatic Positioner (Valtek)

Pneumatic positioners receive pneumatic signals (3-15 or 6-30 PSIG). The positioner then throttles supply air to the valve actuator to move the valve to the required position. Pneumatic positioners are intrinsically safe and can provide a large amount of force to close a valve.

Electric
Electric valve positioners receive an electric signal, usually 4-20 mA, 1-5 VDC, 2-10 VCD or 0-10 VDC and generally drive the motors in electric actuators. They perform the same function as pneumatic positioners do, but use electricity instead of air pressure as an input signal.

Electro-Pneumatic

Electro-pneumatic Positioner
(PMV)

Electro-pneumatic valve positioners contain internal I/P (current to pneumatic) modules that converts the electrical input signal to a pneumatic output (4-20 mA to 3-15 PSIG for instance). Very similar to a pneumatic positioner except that its input is electrical.

Valve positioners are used throughout the process industries including power, pharmaceutical, chemicals, oil and gas, food and beverage, pulp and paper, refining and petrochemicals, pipelines, and many other processes.

For more information on valve positioners, contact Swanson Flo by visiting https://swansonflo.com or by calling 800-288-7926.

Solenoid Valves - How They Work

Solenoid valve, 2-Way, Brass
(ASCO)

Solenoid valves are used throughout many commercial, municipal, industrial, and even residential settings to manage fluid flow. What we refer to as a solenoid valve is an integrated valve and actuator. The actuator, or solenoid, operates via electric current flowing through its helix shaped coil. Energizing the coil with a control signal produces a magnetic field, which then actuates the valve mechanism. Depending on the port configuration of the valve, solenoid valves can either function as two way flow controllers or as diverters in a process system, If the valve contains two ports, then the valve is an on/off valve. If the valve contains three or more ports, then the valve directs the flow of a fluid in the process system. Thanks to their flexibility, reliability, and need for only a small amount of control power, solenoid valves are a frequently used fluid process control device.

The solenoid used in a solenoid valve functions as a converter for electrical energy, using the supplied electrical energy to produce mechanical energy. Metal or elastomeric seals on solenoid valves can be coupled with electrical interfaces, allowing for relatively easy operation by the process controller. The valves typically use a metal plug to cover up a hole, and when pressure from the process fluid is applied to the valve, the pressure difference causes the solenoid valve to be in its normal position. Instead of referring to two directions of flow, the two-way solenoid valves are named two-way because these valves contain two valve ports which the fluid uses to travel.

Three way valves, similar to the name of the two-way valve, have three fluid ports. In an application example, these ports could correspond to pressure, exhaust, and cylinder. In a pneumatic system, these would be used for compressed air supply, vent, and the actuating mechanism. Regardless of the application, the valve function is the same, connecting the inlet port to one of two outlet ports. The selection array of solenoid valves for commercial and industrial use is vast, with variants suitable for a wide range of media, pressure, temperature, and operation sequence.

Pneumatic and hydraulic systems are typical applications for solenoid valves, as are processes such HVAC, where solenoid valves help control liquid refrigerant, as well as suction and hot gas lines. Solenoid valves are a popular fluid flow control options used in processing industries.

Share your fluid control requirements and challenges with application experts, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Watch the video below for more on how solenoid valves work.