How to Detect a Leaking Tube Using a Simple, Inexpensive Ultrasonic Leak Detector?

A minuscule, weeping pinhole in a PTFE tube, deep inside a heat exchanger bundle, is a quiet thief of product and performance. A water test or dye test to locate it is messy, takes a long time, and is typically inconclusive on a very little leak. A new breed of cheap, portable ultrasonic leak detectors can 'hear' the invisible. The turbulence of a high pressure gas jetting out even the smallest pinhole creates a distinct, high frequency ultrasonic hiss, much beyond the range of human hearing. The sensitive microphone and circuitry of the detector convert the ultrasonic scream into an audible tone and visible meter reading. The technician merely scans the air around the suspected region and can locate the exact source of the leak.

The Challenge: A Small Tube Leak in a PTFE Exchanger
PTFE shell-and-tube heat exchangers are commonly utilised in corrosive applications, including acid pickling lines, chemical processing and semiconductor fabrication. One leaking tube can lead to cross-contamination between the shell-side and tube-side fluids, resulting in product deterioration, corrosion of downstream equipment, or environmental issues. It's a classic field service task to find the specific leaking tube out of hundreds.

The main disadvantages of traditional approaches are:

Hydrostatic test (water fill): The shell side is filled with water and pressurised. Water drops mean leaks. However, PTFE is slightly porous and can take in water, leading to false signs. Then the bundle has to be dried, a process that takes hours.

Dye penetrant test: A fluorescent dye is applied and the bundle is viewed under UV light. This is dirty, needs a good clean and PTFE can soak up the dye.

Soap bubble test: Spray the tubesheet with soap solution. Bubbles indicate a leak. This works but is slow on heavy loads and leaves soap residue.

There is a clean, fast, non-invasive alternative: an ultrasonic leak detector.

How an Ultrasonic Leak Detector Functions
The ultrasonic leak detection PTFE exchanger tube method is based on a basic principle of fluid dynamics, namely the turbulent flow of a pressurised gas through a very small opening (pinhole or fracture). This turbulence produces a wide range of sound, including a large amount in the ultrasonic frequency range, generally 20-100 kHz, which is above the maximum limit of human hearing (around 20 kHz).

An ultrasonic leak detector includes:

An ultrasonic frequency sensitive directional microphone (often a parabolic reflector or focussed waveguide).

The signal processor removes lower frequency background noise (motors, pumps, conversations, etc.) and turns the ultrasonic signal into an audible tone (heterodyning).

Headphones to listen to the transformed tone.

Visual display (LED bar graph or LCD) of the signal strength.

Point the microphone at a gas leak, and the detector makes a hissing or rushing sound in the headphones, and the meter climbs. The sound stops when it is pointed away. The microphone is directional, letting the user " home in " on the specific source .

The detector is a bat-like electronic ear that can hear the silent, supersonic scream of a tiny, high-pressure leak.

Leak Detection Procedure Step by Step
The following method is performed on a PTFE shell and tube exchanger that has been taken out of operation, drained and separated.

Step 1: Configure the exchanger
Drain shell side entirely. Any liquid left inside the shell will weaken the ultrasonic signal and may preclude detection.

Isolate the tube side from attached pipework. The tube side is open to atmosphere (pressure equal to atmospheric pressure).

If possible, dry the bunch. For important applications, a short blast of warm, dry air through the shell side will remove moisture.

Step 2: Gas Pressure on the Shell Side
The shell side is linked to clean, dry inert gas (compressed air, nitrogen or argon). The gas pressure is increased to a value which will produce a noticeable jet through any pinhole, but not over the safe operating pressure of the exchanger. Typical test pressure is 10-30 psi (0.7-2 bar). Higher pressures give a stronger ultrasonic signal but may risk damage to PTFE tubes or seals.

The pressure must be stable and maintained during the test. If the leak is on the shell side the pressure will drop. If the leak is tiny it may take a steady feed to keep the pressure up.

Step 3: Configure Ultrasonic Leak Detector
Install new batteries and test the detector using a known leak such as a small hole in a piece of tubing to determine if it is working.

Put the earphones in and adjust the volume to a comfortable level.

If possible, adapt the frequency filter to the common range for gas leaks (often 35-45 kHz). Some detectors feature a fixed filter .

Zero the meter or set the sensitivity so that background ultrasonic noise (e.g. from fluorescent lights or pressurised air leaks elsewhere) gives a low, continuous reading.

Step 4: Inspect the Tubesheet Side
The technician is standing at the tubesheet face, the end of the bundle where the tubes can be seen. The detector's microphone wand is carefully moved across the face, passing each tube opening. Scanning is performed at a distance of 5–15 mm from the tube ends. The wand is placed right into each tube.

For a tight, non-leaking tube: No change in headphone tone or meter reading on detector. There is no gas jet therefore no ultrasonic signal.

If a tube is leaking: A noticeable hissing or rushing noise is heard through the headphones. The meter registers a higher reading." The sound lasts for as long as gas is leaving. Move the wand around until the sound is loudest, and you've found the perfect tube. The faulty tube can be marked with a marking (e.g. "Row 4, Tube 17")

Step 5: Confirm and record
Re-scan the suspect tube at a slightly different angle to confirm the signal.

Compare with adjacent tubes to avoid a false positive from a nearby leak.

Log the location and the meter reading (as a relative measure of leak size). A big leak makes a powerful loud signal . A very small pinhole makes a weaker intermittent signal .

4. Depressurise the shell side before doing any repair work.

The approach is clean, very fast, non-invasive and does not use any chemicals. Scanning a full bundle of 500 tubes takes 10-15 minutes.

Technical Correctness: Parameters and Limitations
The differential pressure is key
Ultrasonic detection is most effective when there is a large pressure differential across the leak. In case of a tube leak the shell side is under pressure (say 2 bar) and the tube side is at air pressure (0 bar). The differential of 2 bar forms a sonic or subsonic jet which produces ultrasound. If the differential is too low (e.g. 0.1 bar) the gas velocity may not be high enough to give a noticeable signal. We propose a minimum differential of 0.5 bar (7 psi), with 1-2 bar being best.

Case of Gas Medium
The method involves a leaking medium of gas. Liquids (water, oil) flowing through a small hole do not produce substantial ultrasonic (but can make some noise from flashing if the pressure is high enough). Hence the shell side needs to be drained and pressurised with a gas, usually dry air or nitrogen. Never try this test with a liquid filled shell side.

Filtering and Background Noise
Industrial plants are noisy places. However, the background noise i.e. motors, pumps, fans, vibrating equipment, is concentrated in the audible frequency range (below 20 kHz). The ultrasonic leak detector's electronic filter filters out these lower frequencies and only allows signals over a predetermined threshold (e.g. 35 kHz) to pass. This gives the detector high immunity to plant noise . However, the following sources may interfere:

Other pressurised gas leaks nearby (e.g., compressed air lines, pneumatic valves). These should be temporarily turned off or the detector employed when there is low activity.

Ultrasonic welders or cleaners (in the same building). These generate loud ultrasonic and can be noticed from a long way off. The test should be scheduled while such equipment is not in use.

Fluorescent light ballasts (earlier magnetic types) ultrasonic noise. The scanning can be performed with the lights off or the detector protected, the detector can be sensitive to them

Leak Size and Sensitivity
Most handheld ultrasonic leak detectors can detect gas leaks as little as 0.1 standard cubic foot per hour (approximately 0.0028 m3/h) under optimum conditions. This is about the leakage rate of a PTFE exchanger tube with a 0.1 mm hole under a pressure of 2 bar. Smaller leaks (e.g. micro-cracks) may not produce enough ultrasonography to be detected. In such instances a more sensitive equipment (e.g. acoustic emission sensor) or a different approach (e.g. pressure decay) may be necessary. However, for most field service applications the ultrasonic approach detects the bulk of the leaking tubes which influence process performance.

Safety Tips
Hearing protection. The detector is provided with headphones that allow you to shut off the loud plant noise. However, if the detector is used without headphones, the audible tone can be loud enough to cause hearing damage if used at high intensity for extended periods of time. Follow the manufacturer's safety instructions.

Pressurised gas: Pressurised shell side. Make sure all connections are tight and the pressure does not exceed the rating of the exchanger. Purchase a pressure regulator. Do not stand directly in front of a tube if pressure could cause debris to be discharged.

Gas Selection: If you are utilising nitrogen or another inert gas, make sure you have good ventilation to avoid displacing the oxygen. Do not use inflammable gases.

Benefits over other leak detection methods
Method Clenliness Speed Equipment CostPost-Test Cleanup False Positives
Ultrasonic Leak Detection DeviceVery clean (no chemicals) .Very quick (minutes/bundle)Low-moderate (300-300-1,500)Low (with sufficient filtration)Nobody
Hydrostatic water fillWet, needs to drySlow (hours, plus drying time)Pump, hose, lowDrying required Moderate (permeation)
Dye penetrant Dirty Slow Low (dye, UV lamp)Heavy cleaning Low
Soap bubble Clean, but leaves some residueSlow for big bundlesVery low (soap solution)Low Soap removal
Pressure decay gauge Clean Indirect (only locates, does not pinpoint) Moderate (pressure gauge) N/A (no location)None
The ultrasonic detector is ideally suited for a field service setting when time is restricted and cleanliness is critical (pharmaceutical, semiconductor or food grade exchangers).

Practical Example: Locating a Pinhole in a Pickling Line Exchanger
A PTFE shell-and-tube exchanger suspected of leaking in a steel pickling line. The shell side contains 15% HCl at 80°C; the tube side contains cooling water. The water outflow exhibits a small pH drop, showing acid breakout. The bundle is taken off, drained and put on supports. The shell side is pressurised with compressed air to 15 psi (1 bar). A $600 ultrasonic leak detector is switched on. The technician scans the tubesheet and hears a distinct hiss from tube #83. Meter reads 80% of scale. The tube is labelled. Subsequent removal verifies a 0.2 mm pin hole from mechanical abrasion. The tube is plugged and the exchanger is returned to service. Diagnosis complete in 20 minutes.

Conclusion: Listen to the Invisible for Fast and Reliable Leak Detection
The simple ultrasonic leak detector is a powerful, modern and highly efficient tool for locating a leaking tube in a PTFE bundle. It converts a time-consuming chemical test into a rapid, electronic scan. By pressurising the shell side with gas and scanning the tubesheet face with a portable detector, the high-frequency hiss of even a very small pinhole is quickly transduced to an audible tone and a visual reading. The approach is clean, non-invasive, does not require chemicals or drying time and works in noisy plant conditions thanks to electronic filtration. In the field, the ultrasonic leak detector is the service specialist's device of choice for quickly and reliably locating tube leaks in PTFE heat exchangers.

The best diagnostic tools are those that can listen to the invisible. You can't see a tiny hissing leak in a dense bundle of PTFE sheets, but you can hear it-with a bat-like electronic ear that turns a silent problem into a clear, undeniable signal.