The Industrial Vibration Quarterly - presented by The Modal Shop - is a newsletter with one mission: helping industrial plants use vibration to enhance safety and reduce downtime by teaching readers how to make sure their systems are functioning properly. Welcome to our debut issue.
Proximity Probe Cable Length Mistakes
What Gap Voltage Is Not Telling You
Make just a half-meter mistake by connecting the wrong extension cable and the proximity probe vibration alert on a critical gas turbine will occur at five mils instead of four. Or a critical compressor may shut down at seven mils vibration instead of eight. The "green zone" for gap voltage - the gap voltage tolerance - is too wide to alert technicians of the error. There are two ways to make sure proximity probes are working correctly:
Perform a DC probe curve test measuring DC voltage output at known gap distances
Simulate vibration with an oscillating target, confirming operation from probe tip to alarm & display
In The Modal Shop's test of a 5 m proximity probe system, an incorrect extension cable caused output to drop 25%. Worse yet, the gap voltage was still within the acceptable range and by adjusting the probe position could be made to look like the perfect DC gap voltage. Fixing the gap voltage without fixing the cable error caused the system's output to drop even more...
In the book "Engineering Asset Management: An Insurance Perspective," written by Ian Barnard and available on various sites including the Reliability Web bookstore, annual calibration of instrumentation and controls is noted as a "recommended best practice" for steam turbines (see page 76). The book encourages readers to share the positive steps they take to ensure safety and reliability with both the plant's corporate and insurance Risk Engineers. If calibration of vibration instrumentation is part of your department's job description it could be beneficial from both a safety and insurance perspective to keep those records on file and make the insurance auditor or Risk Engineer aware that routine confirmation of the vibration monitoring system takes place.
Question of the Quarter
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