In the United States and around the world, government officials, regulators, and even everyday citizens are growing more and more conscious of the impact industry has on the health and safety of our population and the environment. At the same time, from a business perspective, there is a heightened push toward efficiency and productivity in industrial operations as companies worldwide are increasingly focused on their bottom-line margins.
Driven by these trends, industrial process control operations are proactively looking for better ways to monitor and measure changes in their application environments. As such, asset monitoring, asset management, and predictive maintenance solutions are gaining popularity.
Evolution of Technology
Past iterations of process control utilized pneumatic signals to control the position of valves. The 3-15 PSI control signal was the ubiquitous standard for all instruments, as well as for positioning control valves using spring diaphragm pneumatic actuators. One of the problems with this method of control was that it was capital intensive, requiring miles of copper tubing to transmit pressure control signals.
The advent of the 4-20 mA control signal represented an evolution of technology. It improved communication between the control room and the final element controller, making control much easier and cheaper. Further developments in digital communication enabled process control plants to quickly and inexpensively exchange significant amounts of data between the control room and final control elements. This means users can now transmit information from valves and sensors back to a central location to monitor trends and anomalies so appropriate action can be taken to diagnose and fix problems.
Focus On Control Valves
Control valves are a key part of most process applications, and, as a result, the monitoring and management of these assets is critical to the safe and efficient operation of a process plant.
Although the control systems in modern process plants are extremely sophisticated, the power medium utilized is often compressed air, a paradoxically old technology power medium that doesn’t easily fit into today’s rapidly advancing digital world. In fact, compressed air typically hinders the gathering of accurate diagnostic data. So, even though the latest pneumatic process control actuators and positioners utilize sophisticated communication elements, the technology at the heart of most control-valve actuators (i.e., compressed air) is rather antiquated. As such, a mix of pressure sensors and position sensors is often required to monitor valve thrust demand and position.
Specifically, a typical control valve utilizing a spring diaphragm actuator and smart positioner takes a 4-20 mA signal or a digital positioning signal via Hart, Foundation Fieldbus, Profibus, or similar communication protocol to position the valve. The electronic signal is converted inside the positioner into a pneumatic signal. Then, a derivative of the decades-old flapper and nozzle technology is used to increase or decrease the applied air pressure to the spring diaphragm actuator.
Within the positioner, there is a position sensor driven by a linkage attached to the stem of the control valve. This mix of new and old technologies often results in less-than-precise control, which over a period of time can negatively impact the process itself and the efficiency of the entire plant.
A Complex String of Changing Calculations
Figure 1. Chevron Bakersfield uses HART communications to control and monitor CVA-actuated control valves.
Several derivations are required to monitor today’s typical pneumatically powered control valve. For example, in order to monitor valve thrust demand, the air pressure acting over the diaphragm of the actuator needs to be measured. The air pressure is then multiplied by the active area of the diaphragm to calculate the total force applied pneumatically.
However, most diaphragm actuators have an opposing spring, and that spring force must be subtracted from the applied pneumatic force to estimate the net thrust demand of the valve. In addition, the process is further complicated because the spring force is dependent on the preload applied during assembly as well as the position of the valve – the spring force changes with every valve position. So the position of the valve needs to be carefully measured, and then the spring force calculated and subtracted from the overall applied pneumatic force.
The computation takes place within the smart positioner. The resulting data on position and valve thrust demand can then be used to monitor changes in thrust demand. Also monitored is the pressure of the air supply at various positions in the assembly. This is done mainly to ensure the functionality of the compressed-air supply and the positioner assembly rather than the valve itself.
There are many manufacturers utilizing this methodology to gather data from the control valve assemblies and feed it into an asset management system to produce predictive maintenance data.
An Improved Approach
Figure 2. Dwell time data displayed on CVA data logger viewer shows time spent by control valve in various positions.
Today the advent of new technology in electric control valve actuators eliminates the need for the often-clumsy combination of old and new technologies. Such technologies employ electric power and electronic means for valve monitoring, providing a useful, cost-efficient, and more precise way to monitor the response time and positioning accuracy of the valve. Sensors in the valve positioner are capable of accurate and speedy measurement, data can be transmitted in a timely manner back to the control room, and the valve position can be directly related to the process variable, providing significant benefits to process control engineers. Also, detrimental effects such as friction and position overshoot can be monitored and eliminated with the advanced motion control of the electric actuator.
Direct Measurement for Improved Monitoring and Diagnostics
The latest generation of totally electric control valve actuators incorporates several features and characteristics that make them ideal for a wide range of flow control and process applications.
For example, the CVA electric control valve actuator (Figure 1), which was introduced to the marketplace about two years ago, can provide thrust sensing directly from the valve stem without the interpolation of the interposing spring force and without the inaccuracies of pressure measurement, such as those accrued with the indirect pneumatic method of thrust measurement.
Extremely accurate position sensing, as well as direct measurement of valve stem thrust, are clear advantages over traditional methods. The data is gathered faster and more accurately and can be transmitted over a digital communications link via Hart, Foundation Fieldbus, Profibus, or other protocols. Because the information is directly measured and not interpolated, it is a good fit for asset management, predictive maintenance, and efficiency records and planning.
Furthermore, the electronic control of an electric actuator eliminates the electric-to-pneumatic interface and therefore provides many other significant advantages to control valve operation. One such advantage is the elimination of any leakage or bleeding of the air supply, which can have a cumulative negative efficiency effect on plant operations.
The latest electric control valve actuators can also perform their own self-diagnostic tests. These tests are similar to a partial-stroke test; however, they can be initiated at null points in the process operation.
Figure 3. Data collected from a 2 percent partial-stroke test on the CVA actuator.
Specifically, a +/-2.0 percent partial stroke test will allow monitoring of the speed of reaction, friction, and full functioning of the control valve and actuator assembly. Data from these step change tests can be recorded within the actuator or transmitted via a digital communications link back to the main controller. This means that regular, identical tests can be initiated for any given actuator-valve combination, and the results can be progressively monitored to ensure no deterioration of performance or valve condition.
The latest control valve actuators can also monitor the total accumulated travel of both sliding stem and rotary valves. Total travel gives a measure of wear on the stem packing or seats of the valve. It’s a useful guide for predicting when the valve packing should be replaced.
This data need not be dependent on a sophisticated digital communication system to be retrieved. The latest control valve actuators have an on-board data logger and non-intrusive human machine interface (HMI) via wireless connectivity, which allows a PC or PDA to access the information without removing actuator covers or even physically touching the actuator. This offers the advantage of allowing personnel to easily monitor remotely located valves or awkwardly placed valves from catwalks or grade level.
The non-intrusive HMI capability and an intrinsically safe PDA often can be used in hazardous areas to gather data, check on the configuration of the actuator, and even reconfigure the actuator and control valve assembly without removing covers or the need for a hot work permit.
Failure Mode When Power Is Lost
One of the key features of traditional spring-diaphragm actuators is that the applied air pressure is balanced against the spring of the actuator to give the positioning capability. For that reason, most process control valve and actuator assemblies utilize a spring to provide a positive failure mode on loss of air power.
Figure 4. A technician using a PDA to collect data logger information from the control valve assembly.
Electric actuators do not have this inherent capability, and, in the past, this has been an impediment to the extensive use of electric actuators in process control. Today, however, major advances in super-capacitor technology and dependability overcome this obstacle. Super capacitors are now available that can reliably store large amounts of power in a small volume, allowing electric actuators to drive, at full load, to a pre-programmed position – including the fully closed position that a spring diaphragm unit is often built to deliver – if electric power is lost. Therefore, actuators that incorporate this super-capacitor capability can provide reliable failure-mode position shutdown when electric power is lost.
Monitoring & Diagnostic Advantages
Today’s electric control valve actuators are a good fit for many process and flow control applications and offer an advanced way to monitor and collect important diagnostic and operational data. They provide direct measurements of important valve-related data, without the need for interpolation or complex calculations, which can result in more accurate input for making diagnostic decisions as well as for asset management programs, predictive maintenance, and efficiency record-keeping and planning. They link easily and cost-effectively to most control systems and offer personnel a convenient way to nonintrusively monitor and retrieve needed data in the field.
Chris Warnett has over 32 years experience in the valve and actuator industry. A registered mechanical engineer, Mr. Warnett has worked in actuator design and application in both Europe and the United States. He has extensive experience working with valve diagnostics and is the author of a guidebook about using valve actuators as diagnostic instruments. He currently serves as International Sales Director for Rotork Process Controls and the Rotork CVA product line. Mr. Warnett can be reached at firstname.lastname@example.org or 585 247-2304, ext. 282.