From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who might have missed them will be able to catch up on the best of the best. This article, “Valve Positioners Offer Improved Control Valve Performance initially ran on February 16, 2016.
By closely controlling the pressure and temperature of both heating and cooling mediums within heat exchange systems, the use of an automated control valve can help assure the efficient transfer of heat.
Rising stem, linear modulating type control valve have been utilized within processes to precisely control the amount of flow and pressure of those fluids since the eighteenth century. Once reliable actuator designs and automatic controller were introduced, pneumatically or electrically actuating valves via a remote signal gained popularity. Automatically moving a valve’s position increased system accuracy, which greatly influenced product quality and reduced waste associated with manufacturing processes. When specific feed-back sensors were added, system accuracy was once again increased. The control loop was now closed between the controller, sensor and automatic valve.
Although performance was improved this way, control valve and actuator designs still determined total system accuracy and changes in actuator design did not significantly change their control accuracy. The addition of valve positioners has radically changed the way actuated control valves respond within a process control loop. By adding either a pneumatic or electro-pneumatic positioner to the valve, both single-acting and double-acting actuators can be precisely positioned and controlled. The valve can become an integral part of a digitally controlled system that relays both valve health and diagnostic information to assure that the loop maintains set-point within a desired accuracy. This article details many of the advantages of installing and applying a positioner to a basic, pneumatically actuated control valve package.
Control Valve Basics
The operation of a control valve involves positioning its plug relative to the stationary seat within the valve. An actuator is directly coupled to the valve plug via the stem and moves the valve plug to the desired control position. A pneumatic or electric actuator is normally used to control this valve plug position. Pneumatically actuated control valves function either air-to-open or air-to-close. Air-to-open valves are normally held closed by a spring and require an air pressure to open, and air-to-close valves are normally held open by a spring and require air pressure to close them. The mechanical design of the valve and actuator combination determines which way the actuator functions but the desired fail-safe condition determines which type is applied (in case of plant instrument air failure).
A computer, controller, PLC, thermostat or other electrical controlling device sends an analog electrical signal directly to an electric actuator or through a current/voltage-to-pneumatic converter, or electro-pneumatic positioner to a pneumatic actuator. The signal is based on the desired system set point and modulates the valve linearly more open or more closed. By automating valves in this manner they can be used to directly and/or indirectly control temperatures, pressures and flows within an open or closed-loop system.
Heat exchangers are a common type of closed-loop control application where both pneumatically and electrically actuated control valves can be used to regulate water, steam and condensate. A partial graphical representation of a typical heat exchanger using a control valve is shown in Figure 1.
In order for a pneumatically actuated valve to interface with an electrical control signal generated by a controlling device, there are three methods of controlling valve position: a current/voltage-to-pneumatic converter (I/P or E/P); a converter and pneumatic positioner combination; or the electro-pneumatic valve positioner. Choosing a valve positioner for control offers several advantages over using a converter only.
As modern heat exchange system design demands higher efficiencies, so has the need for improving control valve performance. The accurate modulation and control of valves that offer fast response and accuracies within 2% of set point, add up to less waste steam and condensate fall-out and removal. Some of the advantages of using valve positioners include:
Faster Speed of Response
A heat exchanger that does not respond quickly enough to temperature changes brought on by process load changes/upsets to the steam system can be caused by something as simple as an incorrect hot water or steam valve or valve actuator being used. Sometimes, changing from an electrically actuated valve to a pneumatically actuated valve that utilizes a positioner can make the difference, provided the heat exchanger can still meet facility capacity requirements.
Control Higher or Varying Differential Pressures Across Valve
If higher differential pressures or varying differential pressures across the valve plug cause the valve position to change, a positioner automatically adjusts the air pressure to the actuator to “re-position” the plug according to the control signal. When a positioner is used, the valve has its own closed-loop control based on the input signal and stem position feedback. This is accomplished through the stem position feedback linkage, pneumatic amplification relay and adjustment of the output pressure to the actuator according to the input control signal. This closed-loop control circuit is integral to the positioner and therefore maintains consistent closing and opening forces (thrust) dictated by the differential and valve position.
Control of Large Diaphragm / Piston Actuators
Larger diaphragm or piston actuated control valves can require increased air volumes, increased forward and reverse flows, and increased air pressures that are not generated by traditional converters (I/P or E/P). For example, a larger pneumatic diaphragm style actuator may have an internal volume of 0.30 ft3 (cubic feet) that must be filled in order to move a valve fully to the open or closed position. If a positioner is used that has a forward and reverse flow rating of 0.07 scfs (standard cubic feet per second), that same valve will open or close within 4.3 seconds. Without the use of a positioner, the open or closing time is much reduced. Most I/P and E/P converters are only rated for 0.02 scfs forward flow volumes and have reverse flow volumes of even less. That means a valve may open within the required system response time but close at an unacceptable rate.
Positioners also supply air pressures to the actuator at or near their supply pressures to move the valve into the desired position. They become the “powering force” regardless of how much air (psi) is required to the actuator in order to move the valve where it needs to be to achieve force-balance equilibrium. An I/P or E/P converter cannot and does not operate in this manner. The converter supplies an output pressure based on the linear electrical control input signal only. Usually, their pressure spans are within 30 psi with normal operating ranges of 3-15, 3-27 or 6-30. Those air delivery pressures can only be changed by 2-3 psi. While the electro-pneumatic positioner may receive the same type of linear electrical control input signal, it is not bound by the calibrated operating range restrictions. It will deliver pressures to the actuator up to its regulated supply setting to the actuator.
Being able to deliver these higher pressures is very helpful especially in overcoming the negative effects of hysteresis and deadband that are always present within larger actuator and valve combinations. Friction is the most common contributor to those negative effects and is always in opposition to the air pressure generated from the control signal at dynamic conditions. The positioner is always comparing desired set point with the stem position and adjusting the air pressure to the actuator so the friction effects are minimized.
Control Action Change
The concept of control action is more easily understood by considering a pneumatically actuated control valve that is air-to-open with an electro-pneumatic positioner installed. With direct control action, as the electrical input signal to the positioner increases so does the pneumatic output from the positioner to the actuator. The valve opens and modulates proportionally while flow through the valve increases or decreases accordingly.
With reverse control action, as the electrical input signal to the positioner increases, the pneumatic output from the positioner to the actuator decreases. Sometimes system design changes due to piping re-routing or control logic changes within the controller, computer, or DDC require a control valve to operate in this manner. While the use of a positioner cannot change the valve’s function, it can switch its control action from direct to reverse and vise-versa.
Change of Control Valve Flow Characteristic
Installed inherent flow characteristics (design characteristics) of a control valve are determined by the type of plug, seat, cage, etc. that are used. Sometimes the linearization of a non-linear flow characteristic is necessary so that the process valve gain is constant, regardless of the controller output. Changing or adjusting the flow profile is necessary to tighten control accuracy and improve performance. Depending on the design and type of positioner, linearization can be accomplished by switching a mechanical cam or by digitally / electronically reprogramming a new performance curve into the positioner. Utilizing a positioner to change the inherent flow characteristic can be a more cost effective solution than installing a new valve or changing out the valve trim.
Limited Travel with Reduced Lift
Reducing the amount of lift or limiting a valve’s travel is sometimes required in a control system for performance or safety reasons. Valves are sometimes mistakenly oversized resulting in inefficient operation at the lower 5-10% of their travel. Similar to split-ranging the input signal to match the full stroke of the valve as described in the previous section, the positioner’s pneumatic output signal can be compressed to match the full electrical signal input. This allows full input resolution of the control signal to be applied to the first 5-10% of the valve stroke. But there is a trade-off between a valve’s ability to respond to small control signal changes and having more input span resolution. Hysteresis and deadband effects are greatly amplified and can negatively affect control valve performance with such a high “turndown.” Operating a control valve in this manner should only be a temporary measure until an appropriately sized valve can be installed.
Sometimes a valve’s travel or lift is limited for safety reasons or process protection. If the positioner’s set-up and calibration is changed for these reasons, the installation of a mechanical travel stop should immediately follow. Mechanically limiting the travel assures the valve will not open beyond this point in the event of a positioner failure and does not cause an unsafe condition.
Digital Communications / Diagnostics Capability
Microprocessor-based electro-pneumatic valve positioner designs offer the ability to embed significant intelligence and communications into the electronics. This can be advantageous when interfacing the positioner to control systems that utilize digital communications such as HART, BACnet, Modbus or LonWorks protocols.
Sensors within the positioner provide information associated with valve stroke and thrust, output pressures to the actuator, enclosure temperature, valve seat/plug wear and valve performance. On-board diagnostics and alerts are commonplace within digital valve positioners along with historian data logging of abnormal valve performance events.
In order to take advantage of the additional information, sometimes operations, engineering and maintenance personnel must become familiar with many more control parameters and process variables. Software utility applications that allow valve positioner interfacing to a central computerized control system or PC are the most common method of monitoring and changing those parameters. Using a software utility can ease the complexity through its display of graphical views of valve parameters. The software helps the end-user interactively set-up, tune and maintain the control valve / positioner and save the configuration into a software file for later reference or download.
A valve positioner supplies the pneumatic power to an actuator regardless of how much air is required to move the valve and will deliver air pressures to the actuator up to the regulated supply pressure. This helps overcome the effects of hysteresis and deadband caused by friction. Adding a valve positioner also increases system accuracy through closed-loop control action integral to the valve. Upon correctly tuning the loop, system performance can be greatly improved by reducing waste associated with excess steam, water, and condensate generation.
Editors’ Note: This article is an excerpt from a whitepaper originally published by Spirax Sarco.