Electropneumatic positioner: operating principle and advantages

In modern industrial process automation systems, shut-off valves play a key role in controlling liquid and gas flows. Various devices are used to monitor valve positions – from a simple position sensor to a sophisticated electropneumatic positioner. Let us consider their features, operating principles, and advantages.

Position Sensors: A Basic Solution

Simple position and monitoring sensors, such as limit switch boxes, are often used for valves. For example, the SCBF-FIDA-ITS-100 series is installed on pneumatic actuators with a 90° rotation and provides visual indication of the pipeline valve status (open/closed). The housing of such units offers a high level of protection against external influences (NEMA4.4X/IP66/IP67).

The ALP-210N position sensor can operate with SC-RAT/RST series pneumatic actuators for controlling butterfly valves, disc valves, and other valve types. It features a visual indicator (yellow – open, red – closed) and provides dry contacts for integration into control systems.

This position sensor has the following technical specifications:

• Protection rating IP67
• Operating temperature range: -25°C to +80°C
• Mechanical switch with changeover contacts
• Rotation range: standard 0-90°, optional 0-180°
• Cable connections: 1/2NPT, PT1/2, PF1/2, M20, PG13/5
• However, this sensor has limited functionality – it only signals the extreme valve positions (fully open or closed) and does not allow precise control of intermediate positions.

Electropneumatic Positioners: An Advanced Solution

Unlike simple position sensors, electropneumatic positioners provide precise control of shut-off and control valves. They convert an electrical signal into a pneumatic action on the valve actuator.

Operating Principle

An electropneumatic positioner receives an electrical command (4-20 mA current or 0-5 V voltage) from the control system and converts it into a pneumatic signal to operate the pneumatic actuator. The process can be described in the following stages:

• The sensor receives an electrical signal of a specified value
• The electronic section converts this signal into a control action
• The pneumatic block regulates the air supply to the pneumatic actuator
• The actuator moves the shut-off element to the required valve position
• If feedback is available, the positioner monitors the actual position and adjusts control to achieve the required accuracy

Types of Positioners

Electropneumatic positioners are divided into two main types:

1. Rotary – for controlling rotary valves with a rotation angle of 0-90° or 0-180°. Example – SCBF-REP-1000RD series.
2. Linear – for controlling and monitoring valves with a linear stem movement. For example, as used with the SV-HS-AS-20NC-111-1000R valve.

Positioners also differ by the availability of feedback:

• With feedback – provide high positioning accuracy and automatic position correction
• Without feedback – simpler and more cost-effective solutions

Technical Specifications

The SCBF-REP-1000RD electropneumatic rotary positioner has the following parameters:

• Input/output signal: 4-20 mA DC
• Input resistance: 250±15 Ohm
• Air connection: G 1/4"
• Operating stroke range: 0-90°
• Hysteresis: ±1.0% of full scale
• Sensitivity: ±0.2% of full scale
• Repeatability: ±0.5% of full scale
• Flow capacity: 80 l/min (1.5 bar)
• Operating temperature: from -20 to +70°C
• Housing material: cast aluminum
• Protection rating: IP65
• Weight: 2.7 kg

Advantages of Electropneumatic Positioners

Positioning Accuracy – allow the shut-off element to be set at any position within the operating range with high precision (up to ±0.2%).

Flexibility – support standard signal interfaces (current or voltage), ensuring compatibility with most automation systems.

Reliability – robust construction and a high protection rating (IP65 and above) ensure long service life in demanding industrial environments.

Versatility – can operate with both single-acting and double-acting pneumatic actuators.

Diagnostics – advanced models can provide actuator and valve performance diagnostics.

Energy Efficiency – optimize compressed air consumption compared to simple control schemes.

Fast Response – ensure a quick system reaction to changes in the control signal.

Conclusion

Electropneumatic positioners are an important component of modern automation systems, providing precise control of shut-off and control valves. Unlike simple position sensors, which only indicate extreme positions, positioners allow the shut-off element to be set at any intermediate position with high accuracy.

The choice between simple position sensors and advanced positioners depends on the requirements of a particular system: where it is only necessary to monitor fully open or fully closed positions, limit switches are sufficient; for precise flow control, electropneumatic positioners with analog control and feedback are indispensable.