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 from a leak detection programme.
How Ultrasonic Detection Works
When a fluid (gas or liquid) 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 that extends 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 (frequency down-conversion), allowing the operator to scan an area and home in on the source of the leak 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. The operator can 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.
Application 1 — Compressed Air and Gas Leak Detection
Compressed air leaks are among the most significant sources of energy waste in manufacturing facilities. A 3mm diameter leak in a compressed air line at 7 bar (gauge) loses approximately 36 litres of free air per minute — translating to several hundred dollars of electricity cost annually per leak. Facilities with ageing compressed air distribution systems commonly have leak rates of 20–40% of total compressed air production.
A systematic ultrasonic compressed air audit involves:
- Walking the entire compressed air distribution system — headers, drop lines, flexible hoses, quick-connect fittings, regulators, filters, and pneumatic tools — with the ultrasonic instrument scanning each section
- Marking each leak location with a numbered tag
- Estimating leak flow rate from the instrument signal level and the known system pressure
- Photographing each leak location for the inspection report
- Prioritising repairs by estimated flow rate (and therefore energy cost)
For Singapore facilities under EMA's Energy Efficiency Fund (E2F) or ISO 50001 energy management requirements, documented compressed air leak surveys with before/after energy measurements are a recognised energy improvement measure. PUB also references compressed air efficiency in its industrial water efficiency guidelines, as air compressor cooling systems are often significant water consumers.
After repairs are completed, a follow-up ultrasonic survey confirms that leaks have been sealed and quantifies the energy saving achieved. This before/after data is valuable for reporting under the Energy Conservation Act for prescribed large energy users.
Application 2 — Steam Trap and Steam System Inspection
Failed steam traps are a major source of energy loss in steam-using facilities. A failed-open trap continuously passes live steam to the condensate return or drain, wasting both energy and treated boiler water. A failed-closed trap causes condensate backup, water hammer, and reduced process heat transfer. Both failure modes are detectable with ultrasonic instruments.
A working steam trap produces a characteristic pulsing or cycling ultrasonic signal as it periodically opens to discharge condensate and closes when steam approaches. A failed-open trap produces a continuous high-level signal with no cycling. A failed-closed trap produces little or no signal even when the upstream system is under normal steam pressure.
Steam trap inspection should be combined with infrared thermography — the two techniques complement each other. Thermography shows temperature distribution across the trap and adjacent pipework; ultrasonics confirms whether the trap is cycling normally. Together, they provide a definitive diagnosis of trap condition without any system isolation or disassembly.
Application 3 — Bearing Lubrication Condition Monitoring
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 the bearing — the metal-to-metal contact increases and the ultrasonic signal level rises. This change is detectable before any change in vibration or temperature, making ultrasonics a sensitive early-warning tool for bearing lubrication condition.
Ultrasonic bearing condition monitoring is used to optimise re-lubrication intervals — greasing a bearing that does not yet need grease wastes lubricant, can cause over-greasing pressure damage, and may actually introduce contamination. By monitoring the ultrasonic signal level before and during re-lubrication, the operator knows exactly when the bearing has received sufficient grease (the signal level returns to its baseline) and stops adding grease at that point.
This approach — grease-by-condition rather than grease-by-calendar — is an advanced practice that reduces both lubricant consumption and premature bearing failure from over-greasing. It is particularly valuable for motors with vertical shafts and sealed bearing housings where over-greasing is a common cause of premature failure.
Application 4 — Electrical Partial Discharge Detection
Partial discharge (PD) in high-voltage electrical equipment — switchgear, transformers, cables, and insulators — produces ultrasonic emissions as well as radio-frequency signals and light. Ultrasonic detection of PD can be performed externally on switchgear enclosures using a contact probe or airborne sensor, without any physical access to the energised components.
PD is a symptom of insulation degradation in high-voltage equipment and, if allowed to progress, leads to tracking, arcing, and ultimately catastrophic insulation failure. Early detection through ultrasonic scanning allows the affected equipment to be scheduled for maintenance or replacement 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 in asset condition monitoring 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.