What Is Ultrasonic Testing?
Ultrasonic testing for predictive maintenance is a listening technology. It detects the high-frequency sound energy that equipment and systems produce when they are leaking, discharging electrically, experiencing friction, or operating abnormally. Unlike ultrasonic thickness gauging, which transmits sound pulses through material to measure wall dimensions, predictive maintenance ultrasonic testing is a passive detection method — our instruments receive the ultrasonic emissions that faults and losses naturally generate, and we interpret those signals to identify, locate, and quantify the condition.
The technology works because of a useful acoustic property of industrial environments. Plant noise from compressors, pumps, fans, conveyors, and general operations is concentrated below 20 kHz — the upper boundary of human hearing. Above that threshold, the ambient noise floor drops sharply. The ultrasonic emissions from compressed gas leaks, steam trap blow-through, electrical discharge, and bearing friction exist in this quiet zone, making them clearly detectable against a low background even in noisy facilities where audible methods are useless.
Airborne and Structure-Borne Detection
Ultrasonic instruments operate in two distinct modes, each suited to a different class of fault condition. Airborne detection uses an open-air sensor — typically a piezoelectric element behind a resonant cone or parabolic reflector — to capture ultrasonic energy propagating through the atmosphere. This is the mode for compressed air leak surveys, vacuum leak detection, steam trap assessment, and electrical discharge surveys. Modern instruments with focused sensor arrays detect significant leaks at distances of 15 to 30 meters in typical plant conditions, though systematic survey routes work at closer range to ensure smaller leaks are not missed.
Structure-borne detection uses a contact probe pressed against the equipment surface to sense high-frequency stress waves traveling through solid material. When rolling element bearings operate with inadequate lubrication, microscopic metal-to-metal contact between rolling elements and race surfaces generates stress waves in the 24 to 50 kHz range that propagate through the bearing housing. When a closed valve is passing fluid internally, the turbulent flow across the seat gap transmits ultrasonic energy through the valve body. Heat exchanger tube leaks produce detectable acoustic emissions through the tube sheet. In each case, the contact sensor accesses these signals directly through the equipment structure.
Heterodyne Frequency Translation
Ultrasonic signals exist above the range of human hearing, so the instruments use a process called heterodyning to translate detected ultrasonic frequencies down into the audible range. The heterodyne receiver mixes the incoming ultrasonic signal with an internally generated reference frequency, producing an output signal at the difference frequency — typically shifted down into the 1 to 5 kHz audible range. This translation preserves the acoustic character of the original signal, which is critical for diagnosis. A compressed air leak produces a steady, broadband rushing sound. A corona discharge produces a steady buzzing. Tracking produces an intermittent crackling. Arcing produces a harsh, erratic snapping. A bearing with lubrication starvation produces a dry, gritty friction sound. The trained inspector hears these qualitative differences through headphones while simultaneously reading the quantitative dB intensity on the instrument display, combining both channels of information for accurate classification.
What Are the Signs Your Facility Needs Ultrasonic Testing?
Ultrasonic testing addresses failure modes and energy losses that other predictive maintenance technologies detect poorly or not at all. The following conditions indicate that your facility would benefit from professional ultrasonic testing services.
- Compressors run loaded at or near capacity for most of the operating day and no systematic leak survey has been performed — a significant fraction of generated compressed air may be escaping through leaks that are individually small but collectively substantial
- Compressed air energy costs represent a material operating expense and you need near-term reduction opportunities that require no capital investment
- Steam traps have not been surveyed in the past 12 months, or the last survey relied on temperature-only methods that miss traps failing partially open or blowing through intermittently
- You have experienced electrical faults in medium- or high-voltage systems and want detection capability that complements thermographic inspection — ultrasonic instruments find corona, tracking, and arcing conditions that may not yet produce thermal signatures visible to infrared cameras
- Isolation valves must seal for safety, quality, or energy reasons and you lack a non-intrusive method to verify seat integrity without pulling valves from service
- Your vibration program monitors rotating equipment effectively but does not address lubrication adequacy between collection intervals — ultrasonic screening fills this gap with rapid route-based checks
- Bearing lubrication follows calendar schedules rather than actual condition, and you suspect some bearings are being over-greased while others receive insufficient lubricant
- Heat exchangers are process-critical and you have experienced tube leaks or bypass conditions that reduced efficiency or caused cross-stream contamination
- Vacuum systems suffer from leak-in that causes quality problems, contamination, or efficiency losses and you need a way to pinpoint ingress locations from outside the system
Our Ultrasonic Testing Approach
Our ultrasonic testing services are structured to deliver quantified findings with clear economic significance. Every leak is estimated in cost impact. Every bearing condition is classified against its baseline. Every electrical discharge finding is categorized by type and severity. We provide prioritized, actionable information — not a list of dB readings.
Survey Methodology and dB Threshold Classification
Our instruments measure signal intensity in decibels, and we apply application-specific dB threshold systems to convert raw readings into maintenance decisions. For bearing monitoring, we establish an individual baseline dB level for each bearing in good, properly lubricated condition. An increase of 8 dB above baseline indicates changed lubrication conditions — often correctable with proper grease application. A 12 dB increase suggests a developing mechanical condition warranting investigation. A 16 dB or greater increase indicates an advanced condition approaching failure. These thresholds are equipment-specific; the value lies in consistent measurement technique and comparison to each asset’s own baseline.
An 8 dB increase above baseline indicates changed lubrication conditions; 12 dB suggests a developing mechanical condition; 16 dB or greater indicates an advanced condition approaching failure.
For compressed air leak surveys, we record the dB level at a standardized 12 to 15 inch measurement distance from each leak source. Instrument-specific calibration charts correlate these readings with estimated leak flow rates in CFM, which we convert to annual energy costs using the facility’s compressor specific power (kW per 100 CFM) and blended electricity rate. This economic quantification allows the repair list to be sorted by cost impact so maintenance captures the largest savings first.
Tagging, Mapping, and Severity Ranking
At each detected condition, we record the location with photographs and descriptions detailed enough for a maintenance technician to find and repair the issue without the ultrasonic instrument present. Compressed air leaks are tagged with numbered markers at the detection point and cataloged on a facility map. Steam traps are identified by tag number and classified into four categories: operating normally, failed open (blowing through), failed closed (blocked), or suspect/indeterminate requiring follow-up. Electrical findings are classified as corona, tracking, or arcing with severity ratings that drive response timelines.
The survey report organizes all findings by both location and severity, providing a prioritized work list. We also identify systemic patterns — recurring leak types such as worn quick-connect fittings or deteriorated hose connections, over-pressurized zones, or trap populations with high failure rates — that point to corrective actions beyond individual repairs, including hardware standardization, pressure optimization, or zone isolation strategies.
Integration with Other PdM Technologies
Ultrasonic testing does not operate in isolation within our programs. Electrical ultrasonic surveys are correlated with thermographic inspection data — a connection showing both ultrasonic discharge activity and an infrared thermal anomaly has higher diagnostic confidence than either finding alone. Bearing ultrasonic readings are compared against vibration analysis trends to cross-validate condition assessments. Steam trap ultrasonic results are combined with temperature data from infrared surveys for confirmation of failure mode classification. This multi-technology correlation reduces false positives, increases diagnostic certainty, and builds a more complete picture of asset health than any single technology provides.
Applications We Cover
Compressed Air Leak Surveys
We survey the complete distribution system from compressor discharge through aftercoolers, dryers, receivers, main headers, branch lines, drops, hose connections, regulators, filters, lubricators, quick-connect fittings, and point-of-use equipment. In most facilities, the distribution network includes thousands of individual joints and connections, and leak accumulation is ongoing. We recommend comprehensive initial surveys followed by semi-annual or annual re-surveys, with targeted surveys after system modifications or maintenance activities that disturb connections.
Steam Trap Assessment
Using both airborne and structure-borne methods, we classify every trap in the system. Properly operating traps produce characteristic ultrasonic signatures — intermittent cycling for thermostatic and thermodynamic types, steady low-level flow for float and inverted bucket types. A failed-open trap generates high-intensity continuous ultrasonic energy as live steam passes through the orifice at velocity. A failed-closed trap produces no flow signature. For each failed-open trap, we estimate steam loss rate based on orifice size and system pressure, then calculate annual energy cost at the facility’s steam generation rate — translating technical findings into dollar figures that drive repair prioritization.
Steam trap failure identification rates of 12-25% of the installed population on initial survey — consistent with the 15-30% industry benchmark for unsurveyed systems.
Source: DOE and ESA data
Bearing Condition Monitoring
Structure-borne ultrasonic monitoring detects lubrication inadequacy — the earliest stage of bearing distress — with higher sensitivity than vibration analysis in many applications. We use it as both a screening tool (route-based readings that flag bearings running above baseline for detailed vibration investigation) and a lubrication management tool (guiding grease application in real time, with the technician applying lubricant while monitoring the dB level and stopping when the reading drops to baseline). This approach eliminates both under-lubrication and the over-lubrication that accounts for a significant fraction of premature bearing failures in calendar-based grease programs. The technology is especially effective on equipment below 300 RPM where conventional vibration analysis loses sensitivity.
Valve Leak-Through Detection
Structure-borne testing detects internal leakage by sensing turbulent flow when fluid passes through a valve that should be fully sealed. The contact sensor is placed on the valve body downstream of the seat, providing immediate indication of whether the valve is holding. This capability is valuable for confirming isolation valve integrity before permit-required maintenance, verifying check valve function in condensate and process systems, and testing safety valve seat tightness without removing the valve for bench testing.
Electrical Discharge Classification
Medium-voltage and high-voltage electrical systems can develop corona, tracking, and arcing conditions that produce distinctive ultrasonic emissions. Corona — ionization of air molecules around conductors or connection points — generates a steady buzzing signature and produces ozone and nitric acid that degrade insulation over time. Tracking — conductive path development along insulating surfaces from contamination and electric field stress — produces an intermittent crackling and represents active surface degradation. Arcing — high-energy discharge across a gap or damaged insulation barrier — produces a harsh, erratic signature indicating active insulation failure requiring prompt correction. Our surveys classify emissions into these categories, assign severity ratings, and correlate with thermographic data where available. Metal-enclosed switchgear is surveyed through acoustic ports or ventilation openings without door removal, avoiding arc flash exposure.
Heat Exchanger Tube Leak Detection
Contact testing on tube sheets and exchanger shells detects acoustic emissions from active tube leaks while the exchanger remains in service. The method identifies that a leak exists and localizes it to a region of the tube bundle before the unit is opened, allowing maintenance teams to stage materials, allocate resources, and plan tube plugging or replacement work in advance. This pre-outage diagnostic capability directly reduces outage duration and improves repair quality for shell-and-tube, plate, and air-cooled exchangers.
What Results Do Companies Typically See?
Ultrasonic testing delivers results that are quantifiable in energy saved, failures prevented, and maintenance efficiency gained. The outcomes are often the most immediately visible of any predictive maintenance technology because the energy loss categories it addresses produce ongoing, measurable cost that stops as soon as the identified condition is corrected.
Compressed air energy cost reduction of 20-35% following the initial leak survey and repair campaign.
- Compressed air energy cost reduction of 20-35% following the initial leak survey and repair campaign, with the specific percentage depending on system age, prior maintenance rigor, and the facility’s commitment to repairing identified leaks promptly
- Steam trap failure identification rates of 12-25% of the installed population on initial survey — consistent with the 15-30% industry benchmark for unsurveyed systems per DOE and ESA data, confirming that systems without regular survey programs carry substantial recoverable energy losses
- Electrical discharge detection in 3-8% of medium-voltage components during initial surveys, with findings ranging from minor corona requiring monitoring to active tracking requiring scheduled repair to incipient arcing requiring prompt intervention — the most critical findings preventing potential insulation failures that carry six-figure repair costs and extended outage durations
- Bearing lubrication-related failure reduction of 25-40% at facilities that transition from calendar-based greasing to ultrasonic-guided application, primarily through elimination of over-lubrication damage that destroys seals and elevates operating temperatures
- Valve leak-through identification that eliminates energy waste from passing isolation and control valves, improves process control accuracy, and confirms isolation integrity before permit-required maintenance — a safety benefit that is difficult to quantify but operationally critical
- Compressed air system capacity recovery — facilities frequently discover that leak repair eliminates the need for planned compressor additions or allows existing trim compressors to unload, deferring or eliminating capital expenditures for additional compression capacity
The return on investment for ultrasonic testing is typically the fastest of any predictive maintenance technology. Compressed air leak surveys and steam trap assessments produce findings with immediate, calculable economic impact that routinely exceeds the annual program cost within the first survey cycle. This makes ultrasonic testing an ideal entry point for facilities building a predictive maintenance program that need early wins to demonstrate value and build organizational support for broader technology deployment. Our team delivers the survey execution, quantified reporting, and follow-up verification that turn ultrasonic findings into sustained energy and reliability improvements for your operation.