Positioners: automation of pneumatic control valves

Positioners by SAMSON
|   Know how

Positioners ensure a predetermined assignment of the valve position (controlled variable) to the input signal (reference variable). They compare the control signal issued by pneumatic or electric automation equipment (controller, control station, process control system) to the position or opening angle of the control valve and supply a corresponding output signal pressure. Positioners are often used as servo-booster as they convert low-energy signals into strong proportional signal pressures up to the maximum supply pressure (6 bar/90 psi). They can be used in standard and splitrange operation.

At SAMSON, we can draw on more than 100 years of experience and expertise in control valve engineering. In 1957, we launched the first pneumatic positioner onto the German market. Since then, we have enhanced and perfected our positioners, their attachment and their interaction with a wide variety of valve models and valve accessories.

Positioner attachment is a complex matter since various mounting interfaces, actuator motions and manufacturer-specific applications exist. The wide range of high-grade mounting kits allows our positioners to be mounted on many different kinds of linear and rotary actuators by various manufacturers. Besides the standard mounting kits (IEC 60534-1, VDI/VDE 3847, VDI/VDE 3845) and SAMSON's integral attachment, mounting kits for actuators by other manufacturers are available. They all share the same benefits: they are simple to mount, particularly rugged and durable. The mounting kits modified by SAMSON to meet customer requirements guarantee problem-free operation of the positioners.

Control valves are subject to numerous requirements that vary considerably depending on the application and site of installation. In cases where the standard positioner version cannot meet the control demands, SAMSON positioners can be fitted with an extensive range of accessories.

These switches signalize that the valve has reached one of the two adjustable limits, e.g. OPEN or CLOSED position or an intermediate position.

This switch indicates the end position even after a power failure in a safety-instrumented system.

Alarms generated by the EXPERTplus valve diagnostics are issued at the fault alarm output as a condensed state.

These options guarantee emergency venting of the actuator in compliance with IEC 61508 up to SIL 3. As a result, the control valve is moved to the fail-safe position determined by the actuator.

Certain diagnostic functions can be started by the binary input, e.g. data logging or partial stroke testing or the valve is moved to the fail-safe position.

The analog position transmitter provides a 4 to 20 mA signal for position feedback indicating the current travel or opening angle of the actuator.

This sensor enables positioners with integrated diagnostics to detect seat leakage or trigger an alarm after a limit violation.

This sensor allows the positioner to be mounted away from the valve (e.g. on a wall). It can be used when the external conditions are unsuitable for the positioner, e.g. extremely high temperatures or increased vibrations at the valve.

The analog input allows conventional linear or angle position sensors issuing a 4 to 20 mA signal to be connected to the positioners.

The integration of additional components into the positioner housing minimizes the need to mount accessories and the complexity of the required hook-up. As a result, the cost of installation is reduced while the operational reliability of the valve is increased.

The formation of an explosive atmosphere cannot be ruled out in most chemical and petrochemical processes. In this case, the plant components must meet special explosion protection requirements. This also applies to positioners used in hazardous areas. For the use in explosive atmospheres, we offer positioners in type of protection intrinsic safety, in non-sparking, protection by enclosure, and flameproof enclosures.
It is possible to operate SAMSON positioners on site using their rotary pushbutton without posing any risk.
Despite the increased requirements placed on the housing, this also applies to positioners in flameproof enclosures. They are designed to withstand an internal explosion without igniting the explosive atmosphere outside. Various installation methods make it easier to wire the positioner:

  • Direct cable entry with terminal compartment in flameproof enclosure (Ex d)
  • Indirect cable entry with terminal compartment designed for increased safety (Ex e)
  • Conduit system with factory-sealed terminal compartment

At SAMSON, we focus on the user: automatic start-up and a proven operating structure in all our digital positioners as well as numerous details, such as initialization at the push of a button, convenient operation using one rotary pushbutton, a display that is easy to read and whose reading direction can be turned by 180°, contribute to making start-up and operation easier for the user.
Our positioners are easily adapted to the control task at hand – either on site or, in Series 3730, 3731, and 3793 Positioners with diagnostics, on a computer using our TROVIS-VIEW software.

Full integration of the control valve into process control and asset management systems allows the full use of the positioner functions, such as status logging and diagnostics. In addition, this manufacturer-independent integration enables the control valve to be configured and started up from a central location.
Our Series 3730 and 3731 Positioners support standard digital interfaces and protocols, such as HART®, PROFIBUS® PA and FOUNDATION™ fieldbus. Numerous device integration files for engineering tools and systems are available for downloading.

Due to the increasing automation of plants, positioners are not only used on valves for throttling service but on valves for on/off applications as well. All SAMSON positioners can handle classic analog control loops. TROVIS SAFE positioners can also perform discrete analysis of the set point as well as partial stroke tests, which makes them particularly suitable for mounting on on/off valves. Special positioner versions can also take on the discrete analysis of the set point of mounted on/off valves and partial stroke testing The EXPERTplus valve diagnostics are integrated into all positioner versions. They record, save and analyze the data on the valve condition.

These tests form the basis for all diagnostics involving the signal pressure. Such tests include detection of air leakage in the pneumatics or an excessively high or low supply pressure.

This function allows an alternating zero point or a creeping zero shift due to seat and plug wear or dirt between the seat and plug to be detected.

The statistical analysis of process data pinpoints a changed manipulated variable range, external or internal leakage and an incorrect attachment of the positioner.

Process conditions can be monitored and documented by event-triggered data logging.

The use of an optional leakage sensor allows the detection of seat leakage inside the valve.

The diagnostics provide information on the valve end positions, transit times and breakaway times of the valve (TROVIS SAFE range).

This test determines the valve dead band and provides information on the friction hysteresis.

It assesses the dynamic response of the valve to a defined change in the set point or over the entire travel range.

These tests check the ability of shut-off valves installed in safety-instrumented systems to move and assesses their dynamic control response (TROVIS SAFE range).

Critical valve conditions can be detected by performing tests online while the process is running as well as offline to provide predictive maintenance. The condensed state is issued at the fault alarm output. Detailed test results and their analysis can be reviewed using our TROVIS-VIEW operator interface or using the protocols of HART®, PROFIBUS® PA or FOUNDATION™ fieldbus.

We have added the new TROVIS SAFE series to our range of positioners specially for use in safety-instrumented systems. The SIL-certified TROVIS SAFE positioners include partial stroke testing. All diagnostic parameters necessary for on/off valves are ready configured.
In safety-instrumented systems, the control valve acts as a safety valve by opening or shutting off the pipeline in an emergency. High demands are placed on functional safety as outlined in the IEC 61508 and IEC 61511 (SIL) standards.
Smart positioners with diagnostics can be used to enhance or even replace the functions of solenoid valves mounted on shut-off valve assemblies. They also increase the reliability, e.g. by performing regular partial stroke tests.
Use of TROVIS SAFE positioners is possible on observing the requirements of IEC 61511 and the required hardware fault tolerance in safetyinstrumented systems up to SIL 2 (single device/ HFT = 0) and SIL 3 (redundant configuration/ HFT = 1).

Available for all versions
οNot available for all versions
Only in combination with Type 3770 or Type 6116

SAMSON digital positioners are single-acting or double-acting positioners for attachment to pneumatic linear or rotary actuators. Due to their digital signal processing technology, these positioners have the following advantages over conventional positioners:

  • Simple operation
  • LCD with rotatable reading direction
  • Automatic zero and span calibration during initialization
  • Automatic detection of faults in the actuator
  • Direction of action independent of mounting position
  • Continuous monitoring of zero point
  • Low air consumption
  • Permanent storage of all parameters in EEPROM (protected against power failure)

TROVIS SAFE digital positioners with single-acting or double-acting function are SIL-certified devices for attachment to pneumatic control valves in safety-instrumented systems. In addition to the integrated valve diagnostics, they perform full stroke tests (FST) and partial stroke tests (PST) and contain ready-configured parameters for on/off valves and HART® communication.

Together with valve accessories, pneumatic control valves serve as engineered solutions for special applications. SAMSON has been developing and manufacturing high-quality valves, actuators and valve accessories for over 100 years. Thanks to our experience and expertise as well as customer requirements, we have continuously developed the individual components and optimized their interaction.

Click here for information on other valve accessories and hook-ups: