As Published by COMPRESSORtech2 Magazine October 2016 issue
by Karl Hoffower – Condition Monitoring and Reliability Expert for Failure Prevention Associates
Combining ultrasound and vibration sensing adds precision to recip valve analyses
Over the past decade, ultrasonic condition monitoring of reciprocal compressor valves has become more widely known. However, it does not seem to be widely used.
Ultrasonic testing measures high-frequency sound waves, well above the range of human hearing. These ultrasound devices record the high-frequency signals for analysis later. Trending valve cap temperatures is the most common condition monitoring technique for monitoring
compressor valve health.
Ultrasonic testing of compressor valves and vibration monitoring of rotating components are an informative, preventative-maintenance practice. Compressor valve deficiencies with opening, closing or leaking may be diagnosed using the ultrasound recording functions.
Steven Schultheis, a Shell Oil Co. engineer, addressed the issue in a paper presented at the 36th Turbomachinery Symposium in Houston in 2007.
“Trending valve temperatures have proven to be valuable in identifying individual valve problems, but are most effective if the measurement is made in a thermowell in the valve cover.” Schultheis wrote. “Ultrasound has proven to be the preferred approach to analysis of valve condition.”
Failure Prevention Associates completed an experiment with a major midstream gas transmission company to see if this type of condition monitoring tool can effectively find fault conditions well before other technology used.
Ultrasound meters (such as the SDT270 from SDT Ultrasound Solutions) have digital readouts that indicate the level of ultrasound detected. These devices have been used for decades to “hear” air, gas and vacuum leaks. The intensity or amplitude of the signal is expressed in decibels — microvolts. (dB[A] μV). The dB(A) is a common intensity unit for sound intensity; μV designates the engineering reference unit being used with a piezoelectric sensor.
Converting an airborne ultrasound detector with a contact sensor allows a technician to monitor what is happening inside a machine, whether it is a bearing, steam trap, or valve.
Ultrasound detectors are designed operate in a specific and narrow frequency band. Then through “heterodyning” step high frequency sounds down into an audible format that the technician can hear though headphones. During the heterodyning process the quality and characteristic of the original ultrasound signal is preserved.
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