Process Instrumentation is generally thought of as measurement device that receives a physical input and produces an electronic output. Pressure, temperature, level and flow transmitters fall into this category. However, there is instrumentation that does just the opposite: it receives an electronic input and produces a physical output. A valve positioner function in this way and are equally critical to maintaining a reliable process control system. Many of the same strategies that have been presented here can be applied to a valve positioner. Product tiering, advanced diagnostics and best practices should be considered.
Product tiering is the application of produce with a different level of performance for different applications. Manufacturers of control valves offer several different valve positioners specifically for this reason. Applications that are critical in terms of quality, safety or operation should use a premium tier product. These will have better specifications and include advanced control features. Control applications should use a core tier product which balances performance and cost. Finally, general applications like utility steam, can use a conventional tier product. This strategy ensures the best product for a given application. It also ensures the best valve positioner investment.
Advanced diagnostics are also available on a valve positioner. Rather than providing diagnostic data about the process, it will provide diagnostics data about the valve. The partial stroke test (PST) is a function that has gained wide acceptance. From a reliability standpoint it ensures that the valve is functioning properly. Hysteresis, packing friction and response time are all monitored during this process. This data can be compared to baseline data to determine valve performance. Rather than having to fully stroke a valve, the PST can be done online, maintaining the availability of the process.
There are a number of best practices that can be associated with a valve positioner. These will be developed fully in subsequent posts. Included in these are the application of a single- versus double-acting valve positioner, using the positioner for valve characterization as well as operational limits and elimination of mechanical linkage for stem position feedback. I would be remiss if I didn’t mention a valve positioner is also available as a wireless device. In short best practices are the application of the product tiering and advanced diagnostic strategies that are critical to process reliability.
While they perform a function that is different than measurement instrumentation, the selection of a valve positioner is critical to an overall process reliability strategy. There are many considerations that should be made including the strategies that should be presented here. These all lead to a process that is available and profitable. And that is most critical to the operation of a plant.
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