Plants tend to run longer in between turnarounds, or the period of time in which a plant is shut down for maintenance. These turnaround time intervals can be three, four, five, or even longer depending on the process and the industry.
Emerson’s Riyaz Ali, whom you may recall from many process safety and valve-related posts, recently wrote a Valve magazine article, Optimizing Plant Performance Using Predictive Diagnostics via a Digital Valve Controller on a Control Valve. In the article, he discusses the importance of diagnostics on these valves that must operate for longer periods between turnarounds.
Of all the automation-related equipment in the plant, the control valve may be the one with the greatest impact on the performance of the plant’s process. Riyaz notes:
…the valve (final control element) is truly the heart of the process control loop as it actually physically touches the process and controls it.
Valves can be located in difficult to reach and hazardous locations. They are often left to run until the next turnaround and sometimes have performance issues ignored. The introduction of digital valve controllers, also known as smart positioners, has enabled diagnostics to be added to provide early identification of performance problems. Riyaz shares:
The smart positioner receives feedback on the valve travel position as well as supply and actuator pneumatic pressures. This allows the smart positioner to diagnose not only its own workings, but also the valve and actuator to which it is mounted.
Often the valve with a digital valve controller will try to compensate for a poorly tuned control loop by continuously adjusting its position to try to maintain the loop setpoint. When the diagnostics from the valve are integrated back with the automation system, they can be used to flag the need to retune the loop. Riyaz writes:
…when total accumulated valve travel is relatively low, yet the number of travel reversals is very high, this is a possible indication the digital valve controller tuning may be too hot, and the valve may be oscillating, or the process itself may be varying significantly.
Riyaz enumerates some of the problems which these diagnostics can uncover, including valve packing, actuator pneumatic leakage, sticking valve, increased packing friction, process material buildup on the valve shaft, shaft seizure in bearing, valve stem or shaft fractures or bending, bearing corrosion, actuator spring break or permanent set, and slow air exhaust.
Some diagnostics are available only when the valve is bypassed or taken out of service. These diagnostics are useful during turnarounds to identify the valves that should be physically removed and sent to the shop for repair.
Intelligence from on-line or in-service diagnostics from microprocessor-based, digital valve controllers is used while the valves are in control of the process. These on-line, in-service diagnostics permit the valves to operate longer before requiring that they be taken out of service during a plant turnaround–increasing overall plant availability.
Riyaz closes the article with the importance of connecting the diagnostics with the automation system through asset management software. This path provides:
…24-hour-per-day monitoring of variables such as valve travel, actuator output pressures, travel setpoints and other factors in real time while also performing an analysis.
This connection also provides a path to the plant data historian where long-term trends can spot slowly degrading performance issues.
Given the importance of the control valve in the majority of plant control loops, having these diagnostics to spot problems can result in less variability and fewer unplanned shutdowns. For a more complete description of these diagnostics, give Riyaz’ article a read.