Positioners are devices that contribute to the control valve in positioning the actuator with the help of a control signal. They respond according to the input signal they receive pneumatically or electrically. These positioners would provide output power to the actuator. Positioners move control valves to a specified position. A control valve positioner is an interpreter between the control valve assembly and the control system that translates the output signal from the control system and adjusts the air to the actuator which then moves the valve to the desired position requested by the control system. Therefore, the positioner will fight against any other forces acting on the valve stem to reach crisp and accurate stem positioning based on the command signal. A properly functioning positioner ensures the control valve will be well-behaved and obedient to the command signal.

Control valve positioner types

These positioners come in different types: pneumatic, electronic, electro-pneumatic, and digital.

Pneumatic devices use a pneumatic input signal to send pressure for opening or closing the valve. They use the flapper/nozzle system and require compressed air to make everything work correctly. A signal of 3-15 pounds per square inch is needed to receive the data from the controller. The input signal tells the bellows to expand or compress, depending on the signal. The bellows move the flapper assembly and the beam measuring the valve stem feedback through the cam. In this process, the flapper will move closer or further from the nozzle. Therefore, the relay will increase or decrease the air output to the valve actuator. When the valve stem moves, it sends feedback through the beam. Then the air output will decrease or increase again, to position the valve appropriately according to the input signal.

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 positioners receive an analog signal, usually 4-20 milliamps from a controller. The signal creates an electromotive force according to the electrical signal applied to a coil. This force moves the flapper and increases or decreases the air to the valve actuator. Here is also feedback from the valve stem and positioner to decrease or increase the output to the valve actuator when required.

Digital or “smart” positioners use a microprocessor to position the valve actuator and monitor and record data. Thus, they are very accurate. They bring a lot of benefits to the user. They use less air than analogue positioners and provide more flexibility for setup, plus they allow for online digital diagnostics which as a result increases the life of the valve, reducing maintenance and unscheduled downtime. Best of all, these smart positioners use loop power and offer a handful of field protocols for you to choose from.

Advantages of control valve positioner

As valve positioners know the valve’s exact position, they provide more precise control than can be achieved by an actuator on its own.

Positioners make control valves respond more rapidly to changes in the process variable, which as a result, minimizes the amount of time operating by the system above or below the set point.

Positioners can cope with large variations in forces acting on plug.

Varying differential pressure across the valve shows instability in the control loop. A positioner helps stabilizing valve position.

A positioner allows you to put distance between the controller and the control valve; therefore, more flexible configurations and functions will be provided.

They remove the effects of large distances between valve and positioner.

They minimize the effects of friction.

They eliminate hysteresis which reduces productivity.

It allows for split ranging. For instance, one controller for two valves.

It allows use of piston actuators with high instrument air supply pressures.

It allows a wide range of flow variation. For instance, operate at less than 10% travel under normal conditions.

It allows the use of characteristic cams in rotary valves.

When should a positioner be installed?

A positioner should be considered in the following circumstances:

To increase the pressure that a particular actuator and valve can close against.

When exact valve positioning is required.

To linearize a non-linear actuator.

To speed up the valve response. The positioner uses higher pressure and greater air flow to adjust the valve position.

When controlling with wide throttling range.

When valves are handling sludge or solids in suspension.

Resources

Top Categories

Product

Company