Control valves are the workhorses of process automation. They touch the process and execute the control strategy coming from the automation system. Any valve seat leakage can impact the level of control and lead to safety issues, unplanned downtime, quality issues, increased maintenance costs, and more.
To avoid seat leakage, control valves typically have been placed on a routine maintenance schedule, which take the valves out of service. This can result in unnecessary work being performed if the valves are OK, or the leakage may go undetected if a valve is not scheduled for maintenance or the schedule is delayed for the next scheduled downtime period.
I caught up with Emerson’s Damon Meadows, a member on the Instrument and Valve Services (IVS) team, who shared with me a new whitepaper that has just been published, Using Acoustic Emissions to Determine In-Service Valve Seat Leakage.
As the title indicates, the IVS team has applied acoustical measurement technology to determine if there is control valve seat leakage—by the sound it makes. The principle is based on the fact that fluid loss is accompanied by acoustic energy loss. This Acoustic Emission (AE) can be measured to determine the actual leakage rate. The great part is that this testing is performed while the valve is in service.
The IVS team provides a service to perform these measurements. They use AE equipment capable of “hearing” signals in the range from 75 KHz to 1 MHz. When process fluid leaks past the valve sealing surface, the sound waves travel through the fluid, pipe, and valve walls. The signal is filtered by the AE equipment to remove noise not associated with the valve leakage.
Process conditions can vary widely which requires skilled personnel to operate the AE equipment. For example, in high temperature applications above 125°C (257°F), a waveguide must be used since the sensor cannot survive these temperatures. Insulated piping is another consideration in some applications.
Emerson’s Fisher Valve team has developed acoustic correlations for the valve parts they manufacture. These correlations help provide the level of accuracy to the acoustical measurements in the field to determine if there is leakage.