Following a brief “swoop-down” to quickly remedy some process equipment issues, a plant experienced erratic behavior of a critical valve—critical because it provided surge control for a large compressor. No one was comfortable proceeding with the startup with this valve behaving erratically. Manual control was out of the question, and the plant manager put the startup on hold.
The controls technician saw no obvious cause, but he remembered a tip from witnessing a factory technician performing a rebuild during a recent shutdown. The factory techs routinely replaced a pneumatic relay—a small component that’s effectively an amplifier for the positioner’s current-to-pneumatic (I/P) converter. He removed the existing relay and found the culprit: water droplets could be seen on the gasket surface of the relay. After blowing out the air supply piping and replacing the positioner’s supply regulator (airset) with one that included a filter, he installed a new relay and reassembled the instrument. The erratic behavior was gone, and the startup proceeded.
This positioner was a recent-vintage, fieldbus digital valve controller (DVC). But like all positioners used on air-actuated valves, it has at its heart a flapper-nozzle I/P converter. Positioners are designed and manufactured to function in all manner of harsh and hazardous environments—extreme heat and cold, explosive atmospheres, even in driving rain or hose down—but most aren’t designed to work underwater, and all function best with clean, dry instrument air.
At a recent conference, Fisher Controls DVC specialist Sean Raymond told us an easy majority of the troubleshooting calls he gets are revealed to have wet instrument air as their root cause. “There’s a nine-thousandths orifice in the I/P converter—it doesn’t take much water to screw it up,” Raymond said. The orifice is tiny because instrument air isn’t free, and lower demand can matter in a total-cost-of-ownership comparison. It’s also a concern where natural gas has to be used in lieu of air and low emissions are a priority.
The “power budget” of the positioner is also important, especially for those who rely on a 4-20 mA signal—any digital diagnostics, algorithms or memory has to function on low current (less than 4 mA). So, the I/P converter is designed to be energy-efficient as well. A larger orifice in the flapper-nozzle means more power is consumed in analog circuitry and less is available for device (“thing”) intelligence and memory.
Silicon for processors and memory has made great strides in the 30-plus years since the first products were introduced. But wherever the microprocessor world interacts with the physical world, we have transducers, like the I/P converter with its 0.009-in. orifice. No matter how magical our silicon gets, it still needs mechanical, electro-pneumatic converters and actuators to move a valve and affect the process.
If, like us, you have a population of smart positioners from around 1999, how much more capable are the positioners you would specify today? These days, we’re pondering what things we might want to connect to the Internet or the cloud. Like panning for grains of gold among the silt and effluvium coming from our many generations of microprocessor-based devices, we must be choosy about what things we monitor. We want to detect the beginnings of malfunctions that delay the plant startup, or cause instability and shutdowns. For the facility’s most critical valves, should you merely add a HART multiplexor or a wireless adapter to a vintage positioner? When over 50% of issues can arise from moisture in the instrument air, it might make more sense to replace the entire device with one that can routinely detect impending degradation in the mechanical “circuitry.”
For the plant whose startup was delayed, the cost of lost production would have paid for a dozen of the jazziest, feature-laden positioners, and the software to monitor them. Operations and maintenance organizations shouldn’t assume that they can merely connect our “things” from the last century and reap the benefits—sometimes a device upgrade, perhaps even a protocol upgrade, is required, and is easily justified.