My good friend Riyaz Ali, whom you may recall from earlier process safety and digital valve controller-related posts, was kind enough to send me an article he’s recently written. The subject is performance diagnostics for control valves.
Since control valves are in the heart of the action controlling the flow of the process, they are usually located in difficult places such as hazardous areas, outside, cramped spaces, etc. These control valves are also expected to operate for longer periods without shutdown, as I highlighted in a recent plant turnaround post.
In the article, Riyaz shares how digital valve controllers receive positioning feedback on valve travel in addition to supply and actuator pneumatic pressures. Combining this information allows the on-board diagnostics to diagnose the digital valve controller itself, the valve, and the actuator.
Riyaz highlights five on-line (in service) diagnostics including supply pressure, relay adjustment, travel deviation, current-to-pressure (I/P) & relay integrity, and air mass flow rate performed by these smart positioners.
The supply pressure diagnostic examines the travel setpoint, travel, and supply pressure to look for any supply abnormalities in and to detect any supply droop during large travel excursions. This can help spot supply line restrictions that may have cropped up.
The relay adjustment diagnostic measures supply pressure, port A pressure, and port B pressure. This diagnostic monitors crossover pressure on double-acting actuators. The diagnostic also helps in setting the crossover pressure on large-volume actuators.
The travel deviation diagnostic looks at travel setpoint, travel, port A pressure, and port B pressure. It monitors actuator pressure and travel deviation from setpoint. This help plant personnel find stuck control valves, active interlocks, low supply pressure, or travel calibration shifts.
The I/P & relay integrity diagnostic looks at travel setpoint, travel, and I/P drive signal. This diagnostic checks for conditions such as plugging of the I/P primary, I/P nozzle, failures in the instrument diaphragms and O-rings, and I/P calibration shifts.
The final diagnostic that Riyaz highlights is air mass flow. It looks at supply pressure, relay position, port A pressure, and port B pressure. It estimates airflow through the relay and helps to detect leaks in the actuator or tubing to the actuator. It can also detect leaks downstream of the relay.
Riyaz shared this story of the air mass flow diagnostic with me. A process manufacturer has a double-acting actuator on a critical application. They discovered that a change in load did not move the valve, even though the automation system was correctly driving the signal to the digital valve controller.
Through the air mass flow rate diagnostic, they were able to detect that air was leaking and were able to make up the leaking air to move the valve to specified position. This helped tide them over until the next planned shutdown to inspect the actuator. When they opened the actuator, they discovered a missing o-ring that separates the top and bottom port of the actuator.
These air mass flow rate diagnostics help discover the problem to allow them to perform a workaround to run the valve until they could plan appropriate action to inspect the internals of actuator.
Riyaz notes that you can initiate all of these diagnostics together in a one-button sweep to help quickly troubleshoot issues around an improperly performing valve. In the article, Riyaz offers more detailed descriptions of the diagnostics and pictures of what the diagnostics look like from the AMS Valvelink software. I’ll update the post and link to the article once it’s posted and available on the web.