Valve Positioners

Valve positioners perform the function of determining the error between the optimum vale position and actual valve position and provide a corrective output. Specialized sensors on the valve stem or shaft correspond to the setpoint from the control system. The positioner's output is sent electrically or pneumatically to a valve actuator, which moves the valve in the corrective direction. 

As a control valve accessory and the interface between the control system and valves, positioners play a vital role in ensuring the process loop's performance. The valve positioner adjusts the valves' opening, thereby varying the valve's flow rates, from completely shut or wide open to anywhere in between. 

Flowserve Corporation is a leading provider of flow control products and services for the global infrastructure markets, which offers a broad selection of analog, digital, and SMART positioners. For more information about Flowserve positioners in Northern California, contact CTi Controltech. Call them at 925-208-4250 or visit their website at https://cti-ct.com.

The Flowserve Logix 3800 Digital Valve Positioner

The Logix 3800 positioner features powerful diagnostics that identify field problems and expedite corrective actions to ensure reduced return-to-operation times.

Compatible with linear and rotary valves and actuators, the robust design of the Logix 3800 delivers high reliability in tough environments, reduces inventory costs and contributes to a lower total cost of ownership, and installs with an innovative, one-button quick calibration feature.

Partial List of Features:

• Rugged, certified SIL 3 capable modular design delivers maximum reliability in the toughest environments. 
• Installation and operation ease from innovative quick calibration feature to simplify commissioning. 
• Broad application versatility with industry-leading communication technologies, including analog, HART, Foundation Fieldbus, 4-20 or discrete I/O signals.
• Intrinsically safe, non-incendive and explosion-proof design from -52°C to 185°C (-62°F to 365°F) ensures safe, reliable operation in hazardous conditions for increased plant and personnel safety.
• Compatible with a variety of valve and actuation configurations including: rotary or linear valves; double- or single-acting; air- to-open or air-to-close.

https://cti-ct.com
925-208-4250

Pneumatic Control Valve Positioners

Smart valve positioner
Courtesy Rotork

Valve positioners can provide process operators with a precise degree of valve position control across the valve movement range, as well as information about valve position. A relationship exists between applied pneumatic signal pressure and the position of the valve trim. The relationship between the two elements is dependent upon the valve actuator and the force of the return spring reacting to the signal pressure. In a perfect world, the spring and pneumatic forces would reach equilibrium and the valve would return to the same position in response to an applied signal pressure. There are other forces, however, which can act upon the mechanism, meaning the expected relationship between the original two elements of pressure and position may be offset. For example, the packing of the valve stem may result in friction, or the reactive force from a valve plug resulting from differential pressure across the area of the plug may be another.
While these elements may seem minor, and in some cases they are, process control is about reducing error and delivering a desired or planned output. Inclusion of a positioner in the valve assembly can ensure that the valve will be set in accordance with the controller commands.

Each positioner functions as a self-contained small scale control system. The first variable in the positioning process is the current valve position, read by a pickup device incorporated in the positioner. A signal which is sent to the positioner from the control system, indicating the desired degree of opening, is used as the setpoint. The controller section of the positioner compares the current valve position to the setpoint and generates a signal to the valve actuator as the output of the positioning process. The process controller delivers a signal to the valve, and then the positioner takes that signal and supplies air pressure required to accomplish the needed adjustment of the stem position. The job of the valve positioner is to provide compensatory force and to act as a counterbalance against any other variables which may impact valve stem position.

Magnetic sensors can be employed to determine the position of the valve stem. The magnetic sensor works by reading the position of a magnet attached to the stem of the valve. Other technologies can be employed, and all have differing ways of overcoming degrees of inaccuracy which may arise with wear, interference, and backlash. In addition to functioning as a positioner, control valve positioning devices can also function as volume boosters, meaning they can source and subsequently ventilate high air flow rates from sources other than their pneumatic input signal (setpoint). These devices can positively affect and correct positioning and velocity of the valve stem, resulting in faster performance than a valve actuator solely reliant on a transducer.

The inclusion of a positioner in a control valve assembly can provide extended performance and functionality that deliver predictable accurate valve and process operation. Share your valve automation requirements with a knowledgeable valve automation specialist and combine your process knowledge and experience with their product application expertise to develop an effective solution.

Digital Valve Positioner Delivers Extended Functionality

Digital Valve Positioner
Flowserve

A valve positioner works in concert with an actuator to regulate the movement of an industrial process control valve in a predictable manner. Evolving from mechanical based designs, current state of the art positioners no longer employ mechanical means, such as cams, to establish positioner output. The latest models are software based and provide additional information and functionality to the user.

• Loop powered device
• Accommodates 4-20 mA input, as well as HART protocol for two way remote communications
• Suitable for use with single or double acting actuators, linear or rotary motion
• Certificates of approval for use in wide range of hazardous industrial environments internationally

Either of the input signal paths, analog or digital, can be utilized as the command source. When analog is selected as the input source, the 4-20 mA signal is considered the command signal. Selecting the digital source causes the level of the 4-20 mA input to be ignored and the signal sent via HART serves as the input command signal.

The software based units deliver enhanced functionality for in-field setup and calibration, as well remote monitoring and setup via HART. Multiple operating modes are accessible to the user to properly characterize valve position to desired curves, one of which can be customized. An analog feedback signal of the stem position is provided as an output.

There is more to learn. The document below goes into more depth about the features, specifications and options, with a good illustration of the user interface and screen shots taken from the SoftTools software suite that works with the digital positioner. Reach out to a valve control specialist with your requirements and challenges. The combination of your process knowledge and their product application expertise will produce effective solutions.

Digital valve positioner for industrial process control valves from CTi Controltech