These applications include chemical reactor feed shutdown, pressure swing absorption, mineral pressure oxidation isolation and steam generator shutdown.
Isolation valves are typically outfitted with open and closed limit switches that provide valve position feedback. Unfortunately, the on/off nature of the switches are said to limit the ability to diagnose valve performance, so a degrading valve will go unnoticed until it fails outright. Such a failure can have serious process implications, and it can pose safety risks as well.
Hart-enabled Fieldvue 4400 valve position transmitters are claimed to offer a cost-effective solution to address this and other issues. The transmitter incorporates a linkage-less design. As compared to limit switches, it is expected to provide much higher reliability, along with valve diagnostics and Safety Integrity Level (SIL) 2 capabilities—the latter often required in critical isolation valve applications. The position transmitter also includes limit switch outputs, so it can be retrofitted into an existing on/off switch circuit, while also taking advantage of the valve position information.
According to Emerson, the position transmitter offers an inherent advantage over limit switches since it provides a continuous position indication throughout the valve stroke to a host, such as a distributed control or an asset management system, via HART. If a valve is failing to reach end-of-stroke or hanging/sticking during transition, this is immediately apparent from the position signal. Also, a failure of the transmitter itself is instantly detected as the continuous signal is lost, so it can be flagged for repair.