Airborne and Structure Borne Ultrasound

Airborne Ultrasound

Compressed air escaping a flange, an electrical contact arcing inside a sealed enclosure and a rolling element beginning to fail all produce sound at frequencies well above the audible range. Ultrasound instrumentation isolates that band and listens only to it, which allows each of these faults to be detected over plant noise that conceals it from every conventional technique.

The practical advantage of airborne measurement is reach. A leak is detected from several metres away, through steel grating, without shutting down the process and without touching the equipment. Survey areas are scanned systematically with a detector and a suitable horn, a background reading is established with the source quiet so that leak sound can be compared against normal plant noise, and each hit is confirmed before it is recorded with an estimated leak size and the running cost it represents.

Airborne ultrasound detects:

• Compressed air, steam, process gas and nitrogen leaks, including valve stem and flange sealing
• Electrical arcing, corona discharge and tracking, including inside enclosures that cannot be seen
• Steam traps that have failed open or passed while still appearing closed
• Mechanical seal and packing leakage on pumps and agitators
• Insulation breakdown on high-voltage equipment where the fault is not visible

Structure Borne Ultrasound

Structure borne measurement is made with the sensor fixed directly to the machine casing. The high-frequency signal of a bearing or gear defect travels through the structure, and mounting the sensor on a prepared flat surface at a defined location lets the instrument separate that signal from everything else running nearby. It is the same instrument and the same principle as the airborne survey, with the sensor mounted rather than swept.

Structure borne ultrasound is applied to:

• Early-stage rolling-element bearing and gear mesh defects in gearboxes, pumps, fans and motors
• Gearbox lubrication condition, including oil degradation and the absence of the bearing and mesh signals it should carry
• Pumps, compressors and other assets where enclosure noise masks conventional vibration analysis

Every reading is recorded as an ultrasonic image with its measured sound level and its precise location, and confirmation readings are taken after repair to verify that the leak has stopped, the arcing has cleared or the bearing signal has returned to normal.