What is a control valve positioner and how does it work

What is a control valve positioner and can we use a control valve without positioner

Positioners are devices that help the control valve for positioning its actuator with the help of a control signal. Positioners would act according to the input signal it receives and this device would receive the signal either pneumatically or electrically. These positioners would provide output power to the actuator. Control valves are moved to a specified position with the help of a positioner. A control valve positioner is an interpreter between the control valve assembly and the control system. It translates the output signal from the control system and adjusts the air to the actuator, which moves the valve to the position requested by the control system. If there is no positioner then the control valve positioning would be affected. In a control valve, the positioner is used for motion control, proper stem positioning is achieved by using a positioner. Based on the control signal the positioner will operate. So if there is no positioner in the control valve then we won’t be able to position the stem.

What is the purpose of a valve positioner

  • This would help to improve the response time of the valve
  • It can characterize the valve response
  • In order to position the valve, the air is required and the volume of air output from pneumatic controllers won’t be enough to position the valve. So the positioner will convert the low volume control air signal to a higher volume air pressure and this is sent to the actuator to position the valve.

What are the parts of the control valve positioner

  • Bellow  – bellow would apply a force proportional to the pneumatic input signal to the balance beam
  • Spring – a positioning spring would apply variable load to balance the beam through a cam and a linkage
  • CAM – an interchangeable cam related to the stroke of the actuator and making possible any characteristics between actuator position and pneumatic input signal
  • Flapper
  • Nozzle
  • Adjusting screw
  • Bypass lever
  • Screened vent
  • Rotary shaft arm

Why do we need to use positioner in a control valve and what are the functions of a valve positioner

  • It can make a split range between valves
  • It can reverse the actuator action from air to open to the air to close
  • Hysteresis effect can be reduced
  • Valve would perform better
  • Valve friction can be reduced by using this
  • In case the actuator doesn’t have a spring then positioner can be used
  • It allows a wide range of process variation
  • The unloading capability would increase
  • It can reduce actuator sizing

What is a smart positioner

Smart positioners are loaded with integral sensors and a microprocessor and it would continuously check the control valve performance under normal operating conditions. The positioners feedback valve position without the need for linkages, levers, and rotary or linear seals. The position is sensed by non-contacting means. Smart positioners have capabilities beyond positioning, smart positioners does the positioning more accurate and reliable.

How to select a valve positioner

Design

If the positioner is simple then it would be better, a simple positioner is good for operation, maintenance, and installation

Performance

The positioner must be able to perform its functions properly like, it has certain characteristics such as gain, hysteresis, repeatability, and linearity must be proper. How the positioners can do the positioning in the valve determines its performance.

Reliability

It is the ability of the positioner to operate in difficult conditions, if the positioners are able to meet its performance under difficult conditions then they are reliable.

Durability

It is the ability of the positioner to operate under difficult conditions for a period of time, and also the ability of the positioner to last long.

Maintenance

Less maintenance would be better

How does a valve positioner work

How does a valve positioner work

There are many types of positioners but the operation is the same, the connection of the positioner to the stem is done mechanically. Mostly the valve positioner has an amplifier and it can detect the valve position. The signals from the controller and from the valve stem are detected with the help of bellows or diaphragms. When the signal is received from the controller then the stem position is compared with the position determined by the controller. So to set the position of the stem properly air supply will be bought into the positioner of the valve so that the valve can be positioned at the point that the controller needed.

What are the types of valve positioners

Pneumatic positioner

In this type of valve positioner, a pneumatic signal is supplied to the positioner. So the pneumatic positioner will receive a control signal and translates that into the appropriate pneumatic output signal to the control valve actuator. In pneumatics air or gas is used to operate a mechanical device. After receiving the signal from the controller the positioner outputs air pressure as directed. The actuator will be toggled by the air pressure and the pressure from the controller and the valve opening would align. The pneumatic positioner would continuously check the valve’s opening position and correct it as required.

Digital positioner

This positioner is also called as SMART positioner and it is comprised of sensors and microprocessors in order to do the proper positioning. These positioners would check the valve position and convert these details into digital. The digital positioners have high valve position accuracy, the control valve condition can be changed as needed, easy setup, and calibration. This valve positioner allows tighter control over the process variable by increasing the speed and accuracy of the actuator response.

Electro-pneumatic positioner

It does the same function as the pneumatic type. But it uses electrical current instead of air as an input signal. Mostly the electric current would be 4-20mA instead of air as the signal that uses the I/P converters

Electronic positioner

Electronic positioners use a sensor to detect the valve stem position. Instead of an I/P converter, this positioner uses a microprocessor inside it, and it does the exact function as the electro-pneumatic positioner.

How to calibrate valve positioner

Bench set confirmation

This calibration is done before the installation. By this calibration, proper actuator operation with air pressure would be achieved. This calibration is important for air to open and air to fail valves

Feedback linkage alignment, including cam alignment

This calibration is to make sure that the zero and span of the feedback mechanism position correspond to the fully open and fully closed valve travel position. In this calibration verification of installation will be done, positioner mounting and linkage will be checked, cam installation would be checked.

Zero and span adjustment

This calibration is done so that the valve position can be synchronized according to the positioner demand.

Inline or online calibration

This calibration is only for smart or digital positioners, this calibration is done in the field itself.

What are the advantages of using positioner in a control valve

  • Valve stem can be accurately positioned
  • Characteristics of the valve can be changed
  • A quick response can be achieved
  • Reverse the action of the valve
  • Tight shutoff

What are the applications of the control valve positioners

  • Limits the control valve dead, band
  • Change valve response
  • Mitigating stiction or stick-slip
  • Control double acting actuator
  • Split ranging

To get more info about the control valve check the following link

News Reporter
post-graduate in Electronics & communication