Basics of Control Valve Positioners

The control valve positioner is a field device that is used to increase or decrease the pressure of instrument air load driving the valve actuator of the control valve.

Until the stem of the control valve travels and reaches the balanced position as directed by the process controller.

Generally for the linear sliding stem control, the pneumatic positioners are mounted on the top casing or the side yoke of the pneumatic actuator.

And for rotary control valves, the pneumatic positioners are mounted near the end-of-shaft.

In the basic design type, the control valve positioner to the valve stem is connected mechanically

The position of the valve stem or valve shaft is compared with the position indicated by the controller. 

This mechanical feedback link connected to the valve operates in such a way that the process controller commands the valve positioner to change the position.

The feedback linkage will be acknowledged back to the valve positioner to confirm that the valve stem position has changed and delivers the magnitude of the change in position of the valve stem.

For controlling the entire stroke of the valve in case of continuous control, the valve positioner is required to follow the signal received by the controller.

However in closed-loop control systems that employ feedback control. Here the feedback signal from the process is quicker, and the valve opening is proportional to it.

The valve is driven directly by varying its actuating force through the controller, and the positioner doesn’t require any position feedback.

What are the main reasons for using the valve positioners?

The reasons for using the valve positioner are shown below.

1. To increase the resolution of the control system i.e. fine control.
2. To smoothen the operation when the actuator bench-set range is greater than 15 psig.
3. To allow the use of characteristic cams in rotary valves.
4. To minimize the friction effects of valve stem packing,
5. To minimize the effects of hysteresis, usually for higher temperature packing materials like graphite.
6. To nullify the flow-induced reactions to higher pressure drops. i.e. for compensating the internal force imbalances.
7. To allow faster loading and venting for increased speed of response to a change in the process.
8. To allow split ranging, i.e. only one controller for two valves.
9. To defeat or reduce the seating friction in rotary valves.
10. To allow distances between the controller and control valve.
11. To permit a wide range of flow variation. i.e. under normal conditions, it operates below 10% of valve travel.
12. To permit larger usage of 4mA to 20mA electronic signal.
13. To permit the use of piston actuators with higher pressure of instrument air.

What are the basic types of valve positioners?

The valve positioners are available in three basic configurations.

1. Pneumatic Positioners:

The pneumatic valve positioner translates the received pneumatic signal 3 -15 PSI to the proportional valve position.

The pneumatic valve positioner supplies the required air pressure to the valve actuator to move the valve stem to the desired position.

2. Analog I/P Positioners:

This analog I/P positioner implements a similar function to a pneumatic type positioner, but this I/P Positioner uses 4-20 mA electrical current instead of instrument air as the input signal.

3. Smart or Digital Positioners:

The smart or digital positioner will function like the analog I/P Positioner as said above, but these smart or digital positioners differ, in which the electronic signal conversion is either digital or analog.

Here the digital positioners include three categories namely.

> Digital Non-Communicating :

In this type of digital non-communicating positioner, the current signal of 4-20 mA is supplied to the valve positioner that powers both the electronics and controls the output signal.

> HART:

This is similar to a digital non-communicating positioner.

For the analog signal, the HART positioner is competent for two-way digital communication or duplex-type communication through a single wire or cable.

> Fieldbus:

The Fieldbus type positioner is a device that positions the valve using digital signals through digital electronic circuitry coupled to mechanical equipment.

In the field bus, all-analog control signals are replaced by digital control signals. Additionally, the two-way digital communication system is possible over the same cables.

The advantage of field bus technology is

• It enables improved controlled architecture.
• It enables improved product capability.
• It reduces the wiring.

Typical control valve configuration without Positioner:

1. The diagram shown above represents a linear sliding stem control valve without a positioner.
2. The control valve has an actuator bench set of 5 psig to 13 psig.
3. The control valve configuration is air to open, fail close (ATO –FC) type.
4. Here the pneumatic controller (PIC) sends a control signal called designated SIG as shown in the above diagram.
5. The signal on the diaphragm actuator acts as the load pressure for the valve changes resulting in valve stem travel.
6. The plot of Diaphragm Pressure against % travel of valve shown aboveindicates that there is significant hysteresis throughout the control valve stroke largely due to forces of inertia and friction within the control valve.
7. The controller signal SIG and LOAD pressure on the valve actuator have the same flow stream.
8. The actuator air flows through interconnected tubes and the pneumatic controller, finally, through the controller the instrument air is vented out.
9. There is no stem position feedback due to the absence of a valve positioner.

Typical Control Valve Configuration with Positioner:

1. The diagram shown above represents a linear sliding stem control valve with a positioner.
2. The valve has an actuator bench set of 5psig to 13psig.
3. The control valve configuration is air to open, fail close (ATO –FC) type.
4. In this configuration, the mechanical feedback linkage is provided between the valve stem and the positioner to provide the stem position feedback to the valve positioner.
5. As can be seen above, the pneumatic controller sends a signal, SIG, to the valve positioner.
6. But in this configuration, the controller signal SIG and LOAD pressure on the valve actuator have a separate flow stream.
7. The actuator air flows through interconnected tubes and the pneumatic controller and the tubing interconnecting and the positioner, it is known as SIG tubing.
8. The valve positioner now enables the stem position feedback signal to the control loop so that a variation in the controller SIG output forces a change in positioner LOAD that represents a valve stem travel.
9. The mechanical feedback linkage then confirms that the valve travel has occurred.
10. The positioner output will continue to change until stem travel is confirmed.
11. The plot of positioner output (LOAD) versus % Valve travel indicates that the hysteresis is very small compared to the control valve without a valve positioner.
12. Generally, the valve hysteresis with positioners is about 5% of valve stroke.