Positioner for a Control Valve: 6 Things You Should Know

1. What is the Role of a Positioner in a Control Valve?

A positioner for a control valve is designed to adjust the valve's position according to the desired set point of a process variable, such as pressure, temperature, or flow rate.

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In the sections below, we will explore the reasons for utilizing positioners, how the three primary types function, and the options available from Kimray.

2. How Does a Positioner Function?

Positioners are commonly attached to the yolk or top casing of pneumatic actuators for linear control valves. In the case of rotary control valves, positioners are typically mounted inline with the valve and actuator stems, either on top of or beside the actuator. The installation method is contingent upon the type of actuator being utilized.

Attaching the positioner to the actuator enables it to track the movement of the valve stem (for linear valves) or the rotation degree (for rotary valves). This mechanism allows the positioner to adjust the valve's position based on the input signal received from the instrument controller.

When a discrepancy arises between the process variable and the set point, the instrument controller transmits either an electrical or pneumatic signal to the positioner. The positioner, in turn, modifies its pneumatic output to the actuator, prompting the valve to open or close accordingly. This adjustment process continues until the desired process variable is achieved.

3. What Are the Different Types of Positioners?

There are three main categories of positioners:

• Pneumatic Valve Positioner
• Electro-Pneumatic (EP) Valve Positioner
• Digital Valve Positioner

4. What is a Pneumatic Valve Positioner?

A Pneumatic Valve Positioner functions by receiving a pneumatic signal, typically ranging from 3-15 or 6-30 psi, from a controller. It ensures that the valve actuator receives the appropriate air pressure needed to achieve an accurate valve stem or shaft position, proportional to the pneumatic signal input.

5. What is an Electro-Pneumatic Positioner?

An Electro-Pneumatic Positioner, commonly known as an "EP Positioner," receives an electric signal (generally 4-20mA or 0-10 VDC) from a controller. It then translates this electric signal into a corresponding pneumatic output that drives a pneumatic valve actuator.

These EP Valve Positioners may also be referred to as 'analog positioners' due to their reliance on analog input signals. Many processing units utilize a 4-20 mA DC signal to modulate control valves, necessitating the positioner's conversion of electronic current signals into pneumatic pressure outputs.

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This type of positioner is also known as an I/P positioner or transducer, exemplified by Kimray's YAK2.

6. What is a Digital Valve Positioner?

A Digital Valve Positioner similarly receives an electric signal from a controller (typically 4-20mA, HART, PROFIBUS, or Foundation Fieldbus), but utilizes digital technology as opposed to analog methods. It translates the electric signal into a corresponding pneumatic output for the valve actuator.

What sets this apart from an EP Positioner is the use of a microprocessor for control signal conversion and position regulation, rather than relying on a mechanical system like a beam, cam, and flapper assembly found in EP devices.

The digital input allows for enhanced capabilities, as these "smart positioners" possess the ability to gather data and automatically notify users regarding valve performance, diagnostics, and needed maintenance.

Siemens PS2

The Siemens PS2 serves as an exemplary model of a digital valve positioner. Capable of remotely monitoring and controlling any pneumatic valve, it offers various communication protocols and enclosure options tailored to meet customer specifications.

The PS2 can function either manually or electronically, featuring a user-friendly interface with three buttons and a two-line display, enabling swift calibration.

Notably, the PS2 is designed for low air consumption relative to industry standards, utilizing air only as necessary, thereby resulting in reduced costs associated with air generation, maintenance, and materials.

Electric Valve Controller

One innovative solution developed by Kimray is the Electric Valve Controller, which combines the positioning capabilities discussed earlier along with additional features.

The Electric Valve Controller is a compact and versatile option for oil and gas automation, offering the ability to monitor and control production remotely.

This device can manage back pressure control, suction control, pressure differentials, or hi-low gap control using a user-friendly 3-button interface.

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