Ultrasonic leak detection uses high-frequency sound waves (typically 38–40 kHz) to identify leaks, bearing faults, and electrical anomalies that are inaudible at human hearing frequencies, enabling maintenance teams to locate and quantify problems without shutting down equipment. In Singapore's energy-conscious industrial environment, where compressed air alone typically accounts for 20–30% of a manufacturing facility's electricity bill, systematic ultrasonic leak detection is one of the highest-return maintenance investments available. The technique also extends to steam system integrity, bearing lubrication condition monitoring, and high-voltage electrical partial discharge detection, making it a genuinely multi-purpose PdM tool. This guide covers the physics of ultrasonic detection, the primary applications in a predictive maintenance programme, instrument selection, inspection procedures, and how to quantify the energy savings achieved.
How Ultrasonic Detection Works
When a fluid passes through a small orifice under pressure differential, as occurs in a compressed air leak, a failing valve seat, or a cracked pipe fitting, the turbulent flow generates broadband noise extending into the ultrasonic range above 20 kHz. Ultrasonic detectors use a directional sensor to convert this high-frequency signal into an audible tone through heterodyning, allowing the operator to scan an area and home in on the leak source by following the increasing signal strength. Because ultrasonic signals are highly directional and attenuate rapidly with distance, the technique is effective even in noisy industrial environments where background machine noise would mask lower-frequency leak sounds, letting an operator locate a small leak in a noisy compressor room that would be completely undetectable by ear. Coltraco Ultrasonics, whose instruments are distributed in Singapore by Unitest Instruments, specialises in ultrasonic condition monitoring across compressed gas, maritime, and building services applications.
Compressed Air, Gas and Steam System Applications
Compressed air leaks are among the most significant sources of energy waste in manufacturing facilities: a 3mm diameter leak at 7 bar loses approximately 36 litres of free air per minute, translating to several hundred dollars of electricity cost annually per leak, and facilities with ageing distribution systems commonly have leak rates of 20–40% of total production. A systematic ultrasonic audit walks the entire compressed air distribution system (headers, drop lines, flexible hoses, quick-connect fittings, regulators, filters, pneumatic tools) scanning each section, tags and photographs each leak location, estimates flow rate from signal level and system pressure, and prioritises repairs by estimated energy cost. For Singapore facilities under EMA's Energy Efficiency Fund or ISO 50001 requirements, documented leak surveys with before/after energy measurements are a recognised improvement measure; PUB also references compressed air efficiency in its industrial water efficiency guidelines, since compressor cooling systems are often significant water consumers. A follow-up survey after repairs confirms leaks have been sealed and quantifies the savings achieved, data valuable for reporting under the Energy Conservation Act for prescribed large energy users.
Failed steam traps are a major source of energy loss: a failed-open trap continuously passes live steam to the condensate return, wasting energy and treated boiler water, while a failed-closed trap causes condensate backup, water hammer, and reduced heat transfer. A working trap produces a characteristic pulsing or cycling ultrasonic signal as it periodically opens and closes; a failed-open trap produces a continuous high-level signal with no cycling, and a failed-closed trap produces little or no signal even under normal upstream steam pressure. Steam trap inspection should be combined with infrared thermography, since thermography shows temperature distribution across the trap and pipework while ultrasonics confirms whether it is cycling normally, together giving a definitive diagnosis without any system isolation or disassembly.
Bearing Lubrication Monitoring and Partial Discharge Detection
Rolling element bearings in good condition with adequate lubrication produce a characteristic low-level ultrasonic signal; as lubrication degrades, the oil film breaks down, grease dries out, or contamination enters, metal-to-metal contact increases and the signal level rises, detectable before any change in vibration or temperature. Ultrasonic bearing monitoring is used to optimise re-lubrication intervals: greasing a bearing that does not yet need it wastes lubricant, can cause over-greasing pressure damage, and may introduce contamination, so by monitoring signal level during re-lubrication the operator knows exactly when the bearing has received sufficient grease and stops at that point. This grease-by-condition approach reduces both lubricant consumption and premature bearing failure from over-greasing, particularly valuable for motors with vertical shafts and sealed bearing housings.
Partial discharge (PD) in high-voltage electrical equipment (switchgear, transformers, cables, insulators) produces ultrasonic emissions as well as radio-frequency signals and light; ultrasonic PD detection can be performed externally on switchgear enclosures using a contact probe or airborne sensor, without physical access to energised components. PD is a symptom of insulation degradation that, if allowed to progress, leads to tracking, arcing, and ultimately catastrophic failure; early detection allows the affected equipment to be scheduled for maintenance during a planned outage rather than failing in service. For Singapore substations and high-voltage distribution equipment operated under EMA licence conditions, documented PD inspection records demonstrate due diligence and are increasingly expected by insurance underwriters and asset owners.
Instrument Selection for Ultrasonic PdM
Key specifications to consider when selecting an ultrasonic instrument for a PdM programme:
| Specification | What to Look For |
|---|---|
| Operating frequency | 38–40 kHz is standard for compressed air and bearing work; some instruments offer adjustable frequency |
| Sensitivity | Sufficient to detect small leaks at a working distance of at least 5 metres for safe stand-off from high-voltage equipment |
| Directional sensor | A parabolic focusing dish or a flexible tube extension for confined spaces improves source location accuracy |
| Contact probe | For bearing and steam trap inspection where surface-borne ultrasound is the signal of interest |
| Signal display | A numerical dB level display enables quantitative trending; an analogue bar graph is adequate for basic scanning |
| Data logging | Route-based logging enables trending over time. Essential for bearing lubrication monitoring |
Calibration and Instrument Verification
Ultrasonic leak detection instruments used for quantitative purposes (estimating leak flow rate or trending bearing signal levels), must be verified for consistent sensitivity and calibrated at appropriate intervals. Unitest Instruments provides SAC-SINGLAS accredited calibration across multiple measurement disciplines and can advise on calibration requirements for specific ultrasonic instruments.
For instruments used purely for qualitative scanning (find the leak, not measure it), formal calibration may be less critical, but a sensitivity check using a reference ultrasonic source should still be performed at the start of each inspection campaign to confirm the instrument is functioning correctly. See our article on compressed air leak detection and energy savings for a detailed discussion of the economics of leak management.
Building an Ultrasonic Inspection Programme
An effective ultrasonic inspection programme follows a structured cycle:
- Baseline survey: Walk all compressed air, gas, and steam distribution lines; tag and document all leaks found; estimate flow rates; photograph each location.
- Repair campaign: Prioritise repairs by estimated energy cost of each leak. Most repairs are straightforward (tighten a fitting, replace a quick-connect, reseal a thread) and can be completed by in-house maintenance staff.
- Verification survey: Confirm all repairs have eliminated the leak; document the before/after comparison.
- Periodic repeat: Re-survey at intervals of 6–12 months, as new leaks develop continuously in active compressed air systems.
For bearing monitoring, establish baseline signal levels for each bearing measurement point and incorporate ultrasonic readings into the standard vibration route. For steam trap surveys, conduct a full trap audit semi-annually for facilities with large steam systems.
Contact Unitest Instruments to discuss ultrasonic instrument options from Coltraco and other brands suited to your specific PdM applications in Singapore.