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.