Ultrasound Inspection: A Beginner's Guide

What is ultrasound inspection

Ultrasound inspection is a condition monitoring method based on the high-frequency sound produced by friction, impacts and turbulence. People cannot hear it; an ultrasonic detector picks it up and shifts it into the audible range. It is used to find compressed air and gas leaks, diagnose bearings, control lubrication, inspect electrical equipment, and check steam traps and valves.

How it works

1. Friction, impacts and escaping gas generate ultrasound.
2. The detector's sensor picks up the signal: an airborne sensor at a distance, a contact sensor on the machine housing.
3. The instrument shifts the signal into the audible range (heterodyning) and displays the level in dBµV.
4. The operator listens on headphones and compares the level with a reference or previous readings.

Ultrasound fades quickly with distance, so the source is easy to locate even in a noisy plant: the signal rises sharply as you get closer. SDT detectors work up to 100 kHz.

Eight application areas

SDT groups ultrasound applications into eight areas. Each has its own page on this site.

1. Mechanical

Bearings, gearboxes, pumps, fans. Place the contact sensor on the bearing housing at the same point every time and compare the level with a reference or trend. A rising level and impacts in the sound point to wear or lack of lubrication. More: mechanical inspection.

2. Bearing lubrication

Over-greasing is as harmful as under-greasing. Ultrasound shows when friction in the bearing has dropped to its minimum and greasing should stop. This is done manually from the signal level or with the LUBExpert assistant. More: lubrication.

3. Compressed air and gas leaks

Scan pipework, fittings, valves, hoses and seals with an airborne sensor, then follow the sound to the source. Tag each leak and log it with a loss estimate, for example in LEAKReporter. More: leak detection.

4. Electrical equipment

Defect	How it sounds
Corona	steady continuous buzzing
Tracking (surface discharge)	irregular crackling
Arcing	loud pops, sharp bursts

Inspect from a safe distance, through viewing windows or cabinet vents, with a parabolic or airborne sensor. More: electrical inspection.

5. Steam traps

Condition	Ultrasound	Temperature
Working (cycling)	clear open and close cycles	hot inlet
Failed open	continuous steam noise with no pauses	cannot be told from a working trap by temperature alone
Failed closed	silence or a very weak signal	cold or cooled inlet

Temperature only confirms the ultrasound result. The method is covered in detail in the article on testing steam traps at a power plant. Area page: steam and condensate.

6. Hydraulics

Internal leakage in valves and cylinders, cavitation in pumps. A contact sensor compares the signal on the housing in different operating states. More: hydraulics.

7. Valves

Control, relief and check valves, solenoids. Compare the signal upstream and downstream: flow noise with the valve closed means internal leakage. More: valves.

8. Tightness

Ship hatch covers, tanks, clean rooms, isolators. An ultrasonic transmitter goes inside, and the operator looks for points outside where the signal escapes. For ship hatches SDT offers the SHERLOG system with reports in SHERLOGReporter. More: tightness testing.

Choosing an instrument

Instrument	Capabilities	Best for
SDT200	up to 100 kHz, built-in airborne sensor and IR pyrometer, memory for 4,000 readings, 10 s signal recording	leak surveys, bearing checks, panel inspection, a first instrument for training
SDT270	2 channels, IR thermometer, laser tachometer, ICP accelerometer input, signal recording up to 80 s for analysis in UAS3, ATEX version available	regular route-based data collection, trending, hazardous areas
SDT340	ultrasound and vibration channels, IR thermometer and tachometer, colour display, focUS mode	a comprehensive diagnostics programme, signal analysis

All three work with Ultranalysis Suite 3, which stores routes, readings and trends.

Inspection routine

Before the round. Plan the route, charge the instrument, check the sensor and settings, review the history for each point.

During the round. Follow the route, take readings at the same points, record the signal wherever the sound differs from normal, tag defects.

After the round. Download the data to UAS3, compare with previous readings, prepare the report and repair requests.

Learning to hear defects

Sound	Meaning
smooth noise	normal operation
clear periodic clicks	a working cycling steam trap
continuous hissing	a leak
buzzing	corona
crackling	tracking
grinding, impacts	wear, bearing defect
rattling	loose fixing

Start with obvious leaks, build a library of recorded reference sounds, and compare similar machines with each other. Personnel competence in ultrasound testing is certified under ISO 18436-8; ISO 17359 sets general requirements for condition monitoring.

Building a programme

1. Assessment. Walk the site, identify critical equipment and the costliest problems: leaks, bearing failures, steam.
2. Pilot. Pick an area, train two or three specialists, start regular rounds and record results.
3. Expansion. Extend routes to other areas, link work requests to the maintenance system, standardise procedures.
4. Development. Add permanent monitoring on critical assets and signal analysis.

Cost and benefit

Compressed air leak surveys and steam trap testing pay back fastest: their losses translate directly into electricity and steam. Condition-based greasing reduces grease consumption and bearing failures. Cost depends on the configuration: sensors, software, training. KEG TRK quotes SDT instruments on request and helps calculate the payback for your site.