Calibration issues and solutions for control valves equipped with intelligent valve positioners

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Calibration issues and solutions for control valves equipped with intelligent valve positioners. Intelligence is one of the directions for the development of industrial control and automation today and in the future; it has become a trend and a prominent feature in the development of various new technologies, methods, and products in the fields of industry and automation. Since intelligent valve positioners outperform conventional ones in terms of control accuracy, environmental adaptability, commissioning, maintenance, and operating costs, their practical use is becoming increasingly widespread. As a result, more problems are arising during on-site calibration. How to address these issues in order to enable better use of intelligent valve positioners is a problem that our on-site installation and commissioning personnel must confront. 1 Working principle of intelligent valve positioners Intelligent valve positioners generally adopt a modular design, featuring an internal modular base that allows for easy replacement of sub-modules on-site without the need to disconnect wires or conduits. Some sub-modules can be installed in this module base: I/P converters, electronic and microprocessor unit components, pneumatic actuators, valve position sensors, pressure gauges, etc. Some of these sub-modules, such as pressure gauges, are optional. Its internal structure is shown in Figure 1: Figure 1 Internal structure of the intelligent valve positioner. The intelligent valve positioner is equipped with a highly integrated microcontroller; it uses the principle of electrical balance (digital balance) instead of the traditional principle of force balance. The input signals from the control system are converted into pneumatic positioning signals through a microprocessor and an I/P converter, thereby enabling control of the valve position. Digital signals for opening, stopping, and closing are used to drive the action of the pneumatic actuator. The valve position feedback signals are converted into digital signals by high-precision position sensors before being fed to the microprocessor for feedback comparison, thus achieving both electrical/pneumatic conversion and precise valve positioning. The pressure sensor integrated in the intelligent valve positioner is used to capture the air pressure signal sent by the valve positioner to the valve actuator, thereby further improving the positioning accuracy of the valve and providing a basis for the self-diagnosis of the intelligent positioner. The principle of electrical balance for intelligent valve positioners is shown in Figure 2: Figure 2 Principle of electrical balance. 2 Main features of intelligent valve positioners: (1) High output force and fast response speed ； (2) High precision in adjustment (minimum travel resolution of up to ±0.05%), enabling accurate positioning ； (3) Simple installation, highly automated calibration ； (4) Nearly maintenance-free operation, which means time savings and ease of use ； (5) Manual and automatic calibration functions with zero position and travel range ； (6) Features optional or programmable output characteristics ； (7) It has a strong self-diagnosis function ； (8) The gas consumption is much lower compared to traditional valve positioners ； (9) The set values and the limit values of control variables can be selected and set ； (10) It is possible to set the dead zone of the control valve ； (11) The online adaptive program ensures high-quality control even under harsh environmental conditions ； (12) The implicit parameters embedded within the intelligent valve positioner can provide many functions ； (13) The positioner can be configured in a flexible and simple manner, for example regarding valve characteristics, travel limits, or multi-stage operation ； (14) Changes in temperature and compressed air pressure have minimal, almost negligible, impact on intelligent valve positioners. 3 Problems encountered during debugging and solutions During the years of calibration and use of valves equipped with intelligent valve positioners, we have faced various problems and faults; the following are some methods for dealing with common issues: 3.1 The intelligent valve positioner fails to perform automatic calibration. Usually, when calibrating an intelligent valve positioner, we can use the automatic calibration function built into the controller to carry out the calibration of the control valve. However, sometimes automatic calibration does not succeed; in such cases, the manual calibration function on the controller can be used. Once manual calibration is successful, automatic calibration usually works fine thereafter. 3.2 The intelligent valve positioner cannot be calibrated automatically or manually. When neither automatic nor manual calibration of the intelligent positioner is successful, the following steps should be followed for inspection and troubleshooting: (1) Verify that there are no issues with the air supply, and that the air lines of the valve positioner are not blocked. (2) Check whether the valve sticks during operation due to installation issues or other problems ； (3) Check whether the parameters of the valve set in the intelligent locator are correct; parameters such as the valve’s model and size should match the actual values. It is particularly important to ensure that the valve model corresponds to the one actually in use, as the valve model includes many default settings provided by the manufacturer. If the wrong valve model is selected in the operator’s settings, it can lead to unforeseen errors ； (4) An inappropriate installation position of the valve positioner feedback rod can also prevent the intelligent positioner from completing automatic and manual calibration. Usually, due to transportation or other reasons, the locking nut on the feedback rod of the intelligent positioner loosens, causing the feedback rod of the valve positioner to deviate from its original installation position; in such cases, it is necessary to readjust the installation position of the feedback rod. Typical intelligent control valves are equipped with dedicated feedback rod positioning holes; by shutting off the air supply to the control valve, it is possible to determine the position of the feedback rod directly through these positioning holes. Due to their high adaptability, smart valve positioners generally do not malfunction if the installation position of the feedback rod is slightly offset; however, if the offset is too large, the valve positioner will be unable to carry out automatic or manual calibration ； 3.3 The control valve has high leakage; the problem persists even after adjustment. Control valves undergo rigorous calibration and testing by the manufacturer before leaving the factory. After installation on-site, and once it is confirmed that the process piping is clean and that the valve components are not stuck or damaged, an automatic calibration using an intelligent controller is carried out, which generally prevents significant leakage. If the problem persists after automatic calibration of the valve, the following methods can be employed: (1) For air-operated valves, a 20mA signal can be applied first to bring the valve into a fully closed state; thereafter, the two locking nuts on the valve stem can be used to adjust the stem until it can no longer be moved downward. Then, apply a signal of around 12 mA, and use the two locking nuts on the valve stem to adjust it so that it moves downward by a quarter turn. (2) For air-operated valves, a 4mA signal can be applied first to keep the valve in a fully closed state; then, the two locking nuts on the valve stem are used to adjust the stem until it can no longer be moved downward. Then, apply a signal of around 12 mA, and use the two locking nuts on the valve stem to adjust it so that it moves downward by a quarter turn. The above methods have been successfully applied on multiple occasions, resolving the issue of the control loop being unable to operate due to high leakage in control valves; moreover, this tuning method can also be used with control valves equipped with non-intelligent valve positioners. 3.4 Valve positioner oscillation 3.4.1 Issues with the installation of control valves (1) Problems in valve installation cause sticking of the valve stem. The control valve was not installed vertically, which caused the valve stem to stick, and this may lead to oscillation of the control valve. (2) The gas line connection is incorrect. In large-diameter intelligent control valves, there is a gas amplifier before the valve actuator’s air supply. Due to the lack of installation diagrams, the air supply input for the intelligent actuator was mistakenly connected to the exhaust port of the gas amplifier. As the exhaust port of the gas amplifier provided insufficient air supply during the valve’s movement, this caused oscillations in the valve’s operation during adjustment. 3.4.2 Process pipeline issues: Check whether there are devices on the process pipelines such as volume amplifiers that can cause pulsating flow of the fluid. If such devices are present, they should be adjusted first before calibrating the control valve; otherwise, it may also lead to oscillations in the control valve. 3.4.3 Adjusting the sensitivity of the intelligent positioner: Finally, the issue of oscillation in the valve positioner can be resolved by using the operator to adjust the settings for the valve’s response sensitivity, valve dead zone, and valve gain. Of course, in many cases, even when there are issues with the installation and piping, we can address valve oscillation problems by directly modifying the parameters related to the valve’s response sensitivity, dead zone, and gain in the valve positioner. While this approach allows the valve to continue to be used, it will inevitably cause significant damage to the valve over time, affecting its service life. Additionally, a blockage in the air circuit within the valve positioner can also prevent the positioner from completing calibration or cause oscillation in the valve positioner; this issue must be resolved first before dealing with other faults. 4 Conclusion The intelligent valve positioner is a new generation of valve positioner that integrates a microprocessor and utilizes the principle of electrical balance. As time goes by and technology advances, its applications will become increasingly widespread. Due to its strong self-diagnosis function, it usually provides indications of which component in the intelligent locator is faulty when a problem occurs; however, sometimes these indications can also mislead us. When dealing with issues related to smart valve positioners, it should be kept in mind that the failure rate of these devices is quite low; generally, the problems stem from other components, but they manifest themselves in the valve positioner. This requires us to approach the issue from a holistic system perspective. In many cases, the standard methods used for fixing faults in conventional valve positioners are also applicable.