Control valve users are turning more frequently to smart valve positioners. Smart valves, however, are not that new. One company introduced the first smart valves more than ten years ago, with the introduction of their integrated control loop. Since then, smart valves and valve, positioners invaded the process market.
This invasion has been pushed forward with the expectation of much greater performance from these new products. To gain acceptance the marketers have been extolling the performance virtues of these products and attempting to make inroads with additional diagnostic features. Digital positioners instrumented to deliver peak performance, as a side benefit, provide valuable diagnostic information for predictive maintenance strategies.
Therefore, the question is how one makes use of the diagnostic information to improve the maintenance function. As part of the answer, this article classifies, into four distinct groupings, the diagnostic capabilities and explains the predictive maintenance capability of most control valve products on the market.
Plant maintenance product is equipment reliability
Before we get started, it is important to understand the true need for these products in the modern process plant. With business becoming more global there has been an ever-increasing demand to provide products to the market at competitive prices. To meet these competitive demands and remain viable, it is more important than ever to provide products at the lowest possible cost. Hence, business looks at its cost structure and identifies plant maintenance expenditures as the single largest controllable expense item. Money saved in the expense category goes directly to the bottom line. Robert Boggs, in his article Plant Maintenance: Changing From A Necessary Evil to A Profit Center, states, "The actual product that plant maintenance sells is equipment reliability. Plant maintenance becomes a profit center when it can economically and successfully provide process uptime." And further, "Plant maintenance is one of the remaining major opportunities to improve company profits. Surveys of maintenance in industry reveal some alarming statistics. Maintenance costs are rising between 10 to 17.5 percent annually."
Much of the rising cost is associated with the preventive maintenance strategy of the 1980s. Many plants discovered the high cost of preventive maintenance and now look for better alternatives. Predictive maintenance strategies that, in the past, appeared to be costly are now being seen as opportunities to leverage plant assets to attain higher levels of production with greater asset reliability. Plant shutdowns involving removal of a full bank of control valves from the line for servicing is now a dead issue. Many control valve users look for ways to determine which valves need service and, more importantly, which do not.
Predictive maintenance is based on the belief that control valves operate trouble-free for long periods, but eventually begin to wear. To identify a change in performance early enough, control valves need to be monitored in real-time or at periodic intervals depending on the criticality of the process area. This approach helps the maintenance professional avoid unplanned downtime and the associated expense of lost production. Using the diagnostic technology the new smart valves offer helps you deploy a predictive maintenance strategy in your plant and turn plant maintenance into a profit center.
Just what are you paying for?
Many manufacturers would have you believe the new digital valve controllers exceed the performance of their analog predecessors and provide the additional benefit of on-board diagnostics. Simply stated, "it's digital, it has to be better." As is true with any purchase, buyers had better beware. Some digital valve controllers actually have lower levels of out-and-out performance when compared to their analog predecessors. However, each provides some level of smart self-diagnostics that aid the maintenance professional in keeping plant equipment operating at optimum levels. So let's look at the various smart valve diagnostic capabilities and outline how you can exploit them.
Smart control valve diagnostic capabilities cover a considerable range. It is important to understand the product capability differences and the advantages of each. The solution for periodic diagnostic testing of dumb valves comes in the form of a portable strap-on diagnostic tool. With this tool, the user captures diagnostic data periodically on even the dumbest valves in the plant for later trend analysis. The entry-level products in the smart valve world are the HART-based valve positioners--sometimes referred to as digital valve controllers. Next up, the high-performance digital controllers have the advantage of a greater power budget, and therefore, additional sensors to perform diagnostic testing equaled only by the strap-on tools. In addition, these high-performance controllers have a built-in capability to check themselves and determine if they are operating within a predefined performance range.
Portable diagnostic testing tools
If you do not want to pay the price for smart control valves, the portable diagnostic tool provides periodic testing of valves driven by analog or pneumatic signals. Modern portable diagnostic tools developed for the process industry only work on air-operated valves. Specialty diagnostic tools, first developed for the nuclear power industry, provide unique diagnostic testing capability and work on either air or motor-operated valves.
The portable test equipment usually consists of an industrialized computer; a signal marshalling box and a series of diagnostic sensors including transducers for stem position, supply pressure, signal pressure, and actuator pressure(s). The sensors and test equipment connect to the valve while in line, but not in service. Diagnostic software running on a portable computer supplies the analog or pneumatic signal to stroke the valve. The diagnostic sensors measure the critical parameters while the valve strokes. The computer drive captures the sensor data for analysis. A basic baseline test takes less than 10 minutes. For most valves this consists of a ramp test to stroke the valve from open to close and back thereby capturing a valve signature.
A signature is a collection of data lines from a series of sensors that graphically depict the performance of the control valve as it ramps open or closed. The typical valve signature consists of the valve stem position and the packing friction over the full stroke of the valve. One can generate a separate signature for each of the valves major subsystems, including the actuator, the positioner, and the I/P transducer when these components exist. These tests should be run periodically. The data, usually presented in graphical form, can be telling when an experienced reliability engineer examines it.
An effective predictive maintenance strategy would trend this data to spot degraded performance. Therefore, it is important to have a baseline test of the device while it is in good operating condition. This is best done when it is new or in new condition. The better valve manufacturers routinely perform this baseline test before they ship valves from the factory and provide the baseline data for your trend analysis. Contact your valve manufacturer for further information.
More advanced tests run using the portable tool require up to thirty minutes. These test capture data relative to hysteresis, dead band, step response, step sensitivity, step resolution and frequency response.
Most major valve manufacturers offer portable diagnostic systems. Prices start in the low $40,000 range (1998 prices) with the basic diagnostic test configuration and range upward to just below $100,000 for systems with all the bells and whistles. The investment in this diagnostic testing equipment seems ominous. However, this investment, when used properly, generally provides complete payback in a single plant turn-around.
HART-based valve controllers
Many of the smart valve positioners on the market offer a travel log level of diagnostics. Indicators telling how many cycles or how much total travel the plug has experienced are input signals for this level of diagnostics. This is meaningful if the user knows precisely how many cycles or travel a given valve takes before the valve packing needs to be replaced or repairs need to be performed. However, experience shows that a given style of valve has widely varying wear characteristics based on the application or service conditions. Therefore, this travel log allows you to use predictive maintenance techniques and it usually results in a periodic changing of wear components similar to the costly method of preventive maintenance.
Most HART-based valve positioners also offer valve signature testing capability similar to the basic test performed with the strap-on tool. A reliability workstation or portable computer running the manufacturer's software conducts this intrusive test when the positioner is out-of service and connected via HART to the computer. A HART connection to your computer or plant network requires the use of a HART modem.
Multiple digital positioners in your plant require a separate maintenance and reliability network, since the HART signal will not pass into most control system networks. A HART Analog I/O card integrates the HART signal into the control network; however, this solution is limited and often costly.
The user can configure the signature test to ramp from any point in the stroke to any other point but is generally ramped over the full stroke and back. Experienced reliability engineers can best interpret the signature graphs to isolate mechanical wear and its effect on valve performance. The advantage of having a smart valve for this test is the time saved in setting up the portable equipment.
HART-based positioners operate on the power available from the 4-20 mA command signal line, which is less than one-tenth of one watt. This power budget is the main limiting factor for performance and diagnostic features on these positioners.
Available for around $1,500, the advanced diagnostic versions of these positioners is a good value when using the full diagnostic features. You also need the software and a HART modem to use the diagnostics. HART hand-held devices only allow access to device configurations and alarms. The user should be aware that some manufacturers have several versions of the HART positioner but only the advanced versions have all the sensors necessary to do meaningful valve signatures. The diagnostic sensors in the advanced versions include stem position, supply pressure, actuator pressure(s), and most also have a sensor for ambient temperature.
High-performance digital controllers
High performance, digital controllers designed to operate with slightly more than seven watts of power provide a marked difference in positioner performance. In addition, there is ample power to supply the needs of the on-board diagnostic systems and analog output channels. The low-end systems consist of a valve mounted PID controller and integral digital positioner. The diagnostic sensors include a high-resolution stem position sensor, supply pressure, actuator pressure(s), an ambient temperature sensor, and power supply sensing. High-end systems include these features plus additional sensors to measure critical process metrics like line pressure upstream and downstream of the valve seat and process temperature. These high-end systems constitute a fully integrated process flow loop that offers diagnostics on the valve mechanical systems and on the process.
Diagnostic tests are intrusive testing routines similar to the strap-on tools mentioned earlier. The diagnostic metrics can be collected at up to forty data lines per second whereas the HART-based positioners are limited to around three data lines per second. Predictive maintenance strategies use this data to trend the mechanical subsystem performance as well as the overall mechanical health of the valve. The additional process signature lets the experienced reliability engineer identify process components that affect the optimal operation of the local plant systems. These digital controllers start at about twice the entry price of the HART positioners and currently use the MODBUS communications protocol. Diagnostic software is usually included with these controllers at no additional charge.
Real time diagnostic monitoring
Designed to constantly monitor equipment health, fully automated diagnostics provide the reliability engineer with alerts when the diagnostic tool detects deviation from normal operation. Baseline tests for these automated tools consist of data taken over a baseline period with the device installed and operating in normal process conditions. Baseline data collection occurs during the initial setup phase. A specialized program uses the data to generate a model that uniquely identifies the normal performance for the valve and process. An applet running in the valve controller compares actual performance to the modeled performance and sends an alert to the reliability engineer when deviation occurs.
Unlike the smart HART-based positioners and high-performance controllers, this level of smart diagnostic features built-in expertise from the factory experts. Real-time valve and process performance data are continuously monitored with the valve in operation in the process. Upon detecting a deviation, the controller captures a data set that encapsulates the alert and sends the data set to the diagnostic tool. The diagnostic tool interprets the data for the user and indicates where the problem might be, or suggests additional tests if needed to diagnose the exact fault condition. A neural network automatically examines charts, graphs, and other data to identify symptoms of problems sometimes too subtle for even the well-trained service engineer to distinguish. If, for example, the air pressure degrades because of a leaky connection, supply pressure sensors detect the decline; the software interprets the information and relays it to the maintenance professional via a reliability workstation.
These systems obviously demand a price premium above the price for the smart valve positioners and high-performance controllers. Used for a single valve or large groupings of valves, health maintenance systems are scalable to your needs. In addition, equipment health management systems also perform the same real-time automated symptom fault recognition on other plant assets including rotating equipment and instrumented fixed equipment like heat exchangers.
Todd Roeller is Product Manager EHM Solutions at Flowserve, Flow Control Division in Springville, Utah. Contact him at [email protected] and (801) 489-2602.