The quest by process plants to optimize efficiency and production capacity is leading to a greater focus on preventive maintenance. This, in turn, calls for maintenance pre-planning and more active asset management.
Such efforts certainly should extend to control valves. After all, they play a critical role in most operations. A poorly performing control valve can cause significant losses because of shutdowns, lost production, reduced efficiency and unsafe events.
The positioners that maintain and control the valve setpoints can provide crucial insights. That’s why plants increasingly are turning to smart valve positioners. These devices not only position the valve actuator highly accurately but also monitor and record data, and offer robust diagnostic functions as well as other features. Devices with integrated digital communications can give plant operators greater visibility and control over critical production assets, resulting in improved plant performance and greater operational efficiencies.
Valve Positioner Basics
A valve positioner is a device mounted on the actuator that exerts or reduces air pressure as necessary to ensure the valve achieves the correct position. When there’s no positioner, the pneumatic (air) control signal goes directly to the actuator. Once installed, the positioner intercepts the analog/digital signal and outputs a pneumatic signal proportional to the bench range of the actuator.
A valve positioner acts as an interpreter between the control valve assembly and the control system. Its job is to translate output signals from the control system and adjust the air to the actuator that moves the valve to the desired position. The positioner also may take position feedback from the valve stem/shaft and send it back to the control system.
Generally speaking, control valve positioners come in three types:
• Pneumatic positioners. These receive a pneumatic signal from the controller and output a pneumatic signal to the actuator.
• Analog (or electro-pneumatic) positioners. Here, the input signal generally is 4–20 mA rather than pneumatic. These devices, which are mounted directly and axially to the actuator stem, suit quarter-turn rotary actuator applications.
• Digital (or smart) positioners. Such units also receive an analog signal or a communication protocol such as HART, Foundation Fieldbus or Profibus but their built-in microprocessors convert the analog signal to a digital signal for more precise control. Digital positioners also can offer diagnostic data, e.g., on valve position and friction as well as response history, that are invaluable for setting predictive maintenance schedules. Modern positioners not only collect data about the valve/actuator but also automatically let users know how the instrument and its assembly are performing and assist with troubleshooting and maintenance.
Smart control valve positioners allow tighter control over the process variable by increasing the speed and accuracy of the actuator response. The positioner, by ensuring the valve is in the right place, helps in overcoming factors such as friction that affect performance as well as problems like non-linearity and deadband. In addition, positioners can amplify or reverse an input signal as needed.
Control valve technology doesn’t change as quickly as other areas of automation. Instead, valve products gradually evolve over time to keep pace with the changing user requirements and industry standards.
For example, while most valve positioners are side-mounted, new top-mounted designs use a linear connection that mitigates hysteresis between the positioner and actuator by feeding back the actual position of the actuator in a faster, more consistent manner. This approach provides significantly improved flow control.
Other recent developments enable smart digital valve positioners to deliver data regarding valve stroke and thrust, output pressure to the actuator, positioner temperature, valve seat/plug wear, high/low friction and valve performance.
Many users now are demanding that valve positioners provide predictive diagnostics — because it’s critical to predict failures in the valve, control signal and positioner before they impede operations. Improvements in diagnostics play an important role by enabling the move from traditional corrective maintenance to predictive maintenance. Diagnostics with valve positioners also offer the ability to test valves to determine if maintenance or replacement is required.
Valve equipment manufacturers have developed positioner technology that provides real-time performance statistics. Their objective is to support methods for evaluation of valve health, operation and configuration. These capabilities, in turn, allow for predictive maintenance, intelligent alarm management, self-surveillance in accordance with NAMUR NE107 recommendations, and service management.
Some digital positioners come with valve diagnostic software as a Device Type Manager (DTM) for integration into control systems based on Field Device Tool technology. The positioner DTM enables the user to edit a complete “health” report on the valve with all data of configuration and diagnostics. In addition, it can control a partial stroke test that offers operators a tool to check the trouble-free function of emergency shutdown valves.
Plants strive to minimize their fugitive emissions to keep employees and communities safe, lessen environmental impact, comply with stringent air-quality regulations, optimize energy consumption and maximize operating efficiency. Early detection of fugitive emissions via leak monitoring enables facilities to promptly schedule maintenance to curb potential air pollution and avoid fines.
There’s no clear indication of just how much control valves contribute to overall gas and vapor leaks. Nevertheless, monitoring valves for fugitive emissions certainly is prudent. Fortunately, some digital positioners for pneumatically actuated valves feature comprehensive diagnostics tools that continuously check for fugitive emissions to help process plants meet the ANSI/FCI 91-1 standard and comply with regulators’ emissions guidelines. Equipped with integrated network communication and utilizing specialized valve packing and pressure switch gauges, these devices — in the event of a leak — will trigger an immediate alarm, alerting system controls through a standard industrial communications protocol such as Profibus PA, Foundation Fieldbus or HART through ordinary discrete outputs; such positioners can be configured to close the valve without any external communication.
Control valves can play a huge role in helping a process plant improve productivity and efficiency. In most cases, ensuring an efficient and successful operation such as batching (Figure 1) requires a precise combination of valves, meters, sensors and other equipment.
You must consider several essential aspects to choose the correct valve positioner:
1. Understand the technology. The smart valve positioner has become the standard at process plants. However, too many end users aren’t taking full advantage of its functionality. Doing so offers important operational benefits that can help a facility do more on a tight budget.
Using a smart positioner on control valves with rotary or linear actuators enables a plant to:
• implement precise control over operations;
• increase the speed of response to changes in the process;
• minimize valve stem packing friction and the resulting hysteresis;
• negate flow-induced reactions to higher pressure drops; and
• compensate for internal force imbalances.
The first step in leveraging the capabilities of digital valve positioners is to understand how to most effectively implement them in a process operation. It’s essential to invest in tools such as plant asset management software and valve self-diagnostics applications, which can capture, consolidate and present information in an actionable format.
The latest generation of positioners allows for collection of the control valve assembly’s performance while the process plant is running, and also stores information in the positioner memory for analysis. For instance, through continuous self-diagnostics such a positioner can check every aspect of its operation — as well as the health of the valve itself. This could include an indication that plugs or seats have worn out, supply air pressure is high or low, or high or low friction exists. Operators can view the alarm list to see a description and categorization of all failures, determine when the failures occurred and review instructions for addressing the problems.
In addition, equipment designers are eliminating the need for handheld devices for valve configuration by enabling users to set up the device using push button controls at the positioner itself. A consistent auto-start procedure maintains trims, gains, etc., at the optimal settings for control and speed; keeps the valve from over-shoot; and avoids the common problem of “chatter mode.”
2. Compare the alternatives. When thinking about making the move from analog to smart digital valve positioners, keep in mind that smart positioners actually can be less expensive than their older analog counterparts. Indeed, due to all the analog accessories built into smart devices, sizeable cost savings are possible.
In addition, newer smart digital positioners are more reliable and have more installation options than analog positioners. They are microprocessor-based and can provide valuable fieldbus communications and diagnostic information.
Digital positioners also have a significant edge because they have fewer moving parts than analog positioners and are designed to automatically self-calibrate. They monitor critical variables such as valve travel and thrust, air pressures, temperature and valve friction, and provide real-time data so users can identify and fix any performance problems before they escalate into emergencies.
Finally, it’s much easier to set up and install digital valve positioners than analog ones because digital devices can be calibrated and configured electronically (Figure 2). This not only means that control loops can get up and running faster during the commissioning process but also greatly decreases the risk of human error.
3. Consult with the experts. Few operating companies today have specialists who are knowledgeable and up-to-date on the latest designs and features in valve technology. So, to make the best technical and economical choice, seek outside help, particularly from experts at vendors. They know the factors most important for the selection of positioners and can offer guidance for picking the right positioner for a specific application and environment. In addition, most valve positioner manufacturers offer training to help end users learn how to mount and set up a digital positioner and to perform commissioning and troubleshooting.
4. Review all considerations. Every technology, including the current breed of smart control valve positioners, has overlooked advantages as well as potential limitations.
Most process plants under-utilize several capabilities of digital positioners. For example, opting for the same type of smart device for both linear and rotary valves can significantly reduce the time, effort and cost for training instrumentation technicians and other personnel. The advent of advanced diagnostics also provides a real-time evaluation tool for valve health and operating parameters — helping to manage both the overall wellbeing of crucial equipment assets and the plant itself.
Modern digital positioners do have a few noteworthy drawbacks and aren’t always the optimum solution for a particular application or environment. This is the case for applications lacking a digital control signal or power. Also, avoid digital positioners when there’s no compressed air or gas source and in environments with temperatures that are excessively high (above 212°F) or low (below -40°F).
In addition, users with simpler applications may save money by opting for pure pneumatics (3–15 psi) or a current-to-pressure (I/P) converter. Hysteresis also is possible with some linkage kits.
Some of the under-appreciated issues that could complicate a digital positioner’s successful implementation include incorrect mounting kits, loose and worn linkages, and insufficient pressure from a compressed air source. Excessive vibration on integral-mounted positioners could require the use of a remote-mounted positioner.
In addition, plant personnel deploying smart positioners must become familiar with a digital environment and, if necessary, acquire additional software and computer skills. The set-up and commissioning of a digital positioner differs markedly from that of a familiar analog or hybrid device because the user encounters many more parameters and choices.
It’s important to understand that digital positioning technology can’t resolve all the issues associated with traditional types of positioners. So, perform due diligence to verify the chosen option really offers the stated advantages and, especially, to ensure it has the correct diagnostic capabilities to alert operators of potential problems.
A Compelling Case
By upgrading control valve positioners from analog to digital, process plants can maximize productivity and improve their competitiveness in today’s marketplace.
The specific benefits of this approach include:
• minimizing environmental, asset and worker safety risks;
• improving compliance with fugitive emissions regulations;
• maintaining operational efficiency and reliability;
• enhancing product quality;
• increasing plant productivity;
• shortening maintenance time; and
• reducing capital costs.
Adding smart positioners with digital communication and diagnostic capabilities also assists plants in contending with the loss of experienced personnel and optimizing staff productivity. The latest generation of smart positioners helps operators do more diagnostics locally and better understand overall valve performance at a lower cost. Moreover, they enable a single operator to monitor the performance of hundreds of devices. Control room personnel can access and analyze data supplied by smart instruments to determine what valves are malfunctioning and identify the root causes of problems.
JIM BRAXTON is Pampa, Texas-based regional sales manager, flow instrumentation, for Badger Meter, Milwaukee. Email him at email@example.com.