Understanding Ultrasound Analysis

1. Definition: What is Ultrasound Analysis?

Ultrasound Analysis (or Airborne/Structure-Borne Ultrasound) is a condition monitoring technology that involves listening for high-frequency sounds that are well outside the range of human hearing. Humans can typically hear sounds up to about 20 kilohertz (kHz). Ultrasound instruments are designed to detect sounds in the range of 20 kHz to 100 kHz.

These high-frequency sounds are generated by friction, turbulence, and electrical arcing. An ultrasound instrument detects this high-frequency sound, translates it into an audible signal that can be heard through headphones, and measures its intensity (amplitude), which is displayed as a decibel (dB) level. This allows inspectors to “hear” problems that would otherwise be completely silent.

2. How It Works: Heterodyning

The core technology inside an ultrasound instrument is called heterodyning. This is an electronic process that accurately converts the very high-frequency, inaudible ultrasonic signal into a lower-frequency signal within the audible range, without changing the sound’s original characteristics. This means that the “hissing” sound of a compressed air leak or the “crackling” sound of an electrical arc will sound like a hiss or a crackle in the headphones, making the diagnosis highly intuitive.

3. Key Applications in Maintenance

Ultrasound analysis is a versatile technology with several high-value applications:

a) Leak Detection

This is the most common and financially beneficial application. The turbulent flow of a gas (like compressed air, steam, or nitrogen) escaping from a pressurized pipe or vessel creates a significant amount of broadband ultrasound.

• Procedure: An inspector uses a handheld ultrasound device with an airborne sensor to scan an area. The instrument is highly directional, and as it gets closer to a leak, the audible signal in the headphones will get louder and the dB reading on the meter will increase.
• Benefits: Finding and fixing compressed air leaks can save a plant tens or even hundreds of thousands of dollars per year in wasted energy costs.

b) Electrical Inspection

Electrical faults like arcing, tracking, and corona in medium and high-voltage electrical equipment produce ultrasound.

• Procedure: An inspector can safely scan enclosed electrical cabinets from the outside. The ultrasound generated by a fault will escape through air gaps in the cabinet seals.
• Benefits: This provides an excellent, non-contact method for detecting serious electrical faults before they lead to an arc flash event, enhancing plant safety. It is also a great screening tool to use before opening a panel to perform thermography.

c) Mechanical Inspection (Condition-Based Lubrication)

Ultrasound is also highly effective for assessing the condition of rolling-element bearings and guiding lubrication practices.

• Procedure: A contact ultrasound sensor is placed on a bearing housing.
• Interpretation:
  • A healthy, well-lubricated bearing will produce a low, steady “hissing” sound.
  • A bearing that needs lubrication will have a higher dB reading. A technician can then apply grease slowly, stopping as soon as the dB level begins to drop, thus preventing over-lubrication.
  • A bearing with a defect (like a spall) will produce a repetitive “crackling” or “popping” sound as the rolling elements strike the flaw. This provides very early warning of bearing failure.

4. Ultrasound vs. Vibration Analysis

For bearing analysis, ultrasound and vibration are complementary. Ultrasound is often better at detecting very early stage failures (Stage 1) and lubrication issues. Vibration analysis is better at diagnosing the exact nature of a later-stage fault (e.g., distinguishing an outer race fault from an inner race fault) once it becomes visible in the vibration spectrum.

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