How to find a gas leak at an industrial site: methods and instruments

Gas leaks at an industrial site are found with two kinds of instrument. An ultrasonic leak detector or an acoustic camera locates a leak of any pressurised gas but cannot tell which gas it is. Gas-specific instruments show what is leaking and at what concentration: a PID for VOCs, an SF₆ leak detector for SF₆, a thermal conductivity detector for helium and hydrogen, an analyser for methane.

Two questions: where is the leak and what is leaking

Before choosing an instrument, decide which question you need answered.

• Where is the leak? Pressurised gas escaping through a gap creates turbulence, and turbulence produces ultrasound. SDT uses ultrasonic instruments to find compressed air and gas leaks: the instrument hears the flow, and the chemical composition does not matter to it. An acoustic camera works the same way but shows the sound source on an image.
• What is leaking and how much? This needs a sensor that responds to a specific substance. No single instrument sees every gas: each detection principle has its blind spots.

In practice the methods are combined: ultrasound quickly covers the line and finds the point, and a gas instrument confirms the gas and measures the concentration.

Table: gas, method, instrument

Gas or task	Method	What it gives	Example instrument
Compressed air, nitrogen, any pressurised gas	Ultrasound	Leak location	SDT340, acoustic camera SonaVu+
VOCs, benzene, solvent vapours	Photoionisation detector (PID)	VOC concentration from ppb	Ion Science Tiger XT
Methane, natural gas	Ultrasound for location; infrared (NDIR) analyser for concentration	Location and CH₄ concentration	SDT340; Rapidox 1100 CH₄
SF₆, C4-FN	Negative Ion Capture leak detector	Leak location from SF₆ in air, sensitivity down to 1×10⁻⁸ cc/s	Ion Science SF6 LeakCheck P1 XTL
SF₆ in GIS room air	Fixed detectors	Continuous 0–2000 ppm monitoring with alarms	Rapidox SF6 Fixed Detection System
Helium, hydrogen, refrigerants	Micro thermal conductivity detector (MTCD)	Leak location	Ion Science Panther PRO
Oxygen deficiency after an inert gas leak	O₂ analyser or gas detector	O₂ content	see Oxygen gas analyser: how to choose

Instrument data are taken from manufacturers' pages and KEG TRK catalogue cards.

Ultrasound: location of any pressurised gas leak

An ultrasonic leak detector suits the first walk-down: pipelines, flanges, valves, compressed air connections. The procedure and five search methods are covered in How to find a compressed air leak, and the physics of the method in Ultrasonic leak detector: how it works.

Limitations of the method:

• the gas must escape under pressure, otherwise there is no turbulence and no ultrasound;
• the instrument does not distinguish gases: a nitrogen leak and a methane leak sound the same;
• ultrasound does not measure gas concentration.

Hazardous areas need an explosion-protected instrument. In the SDT range this is the SDT270 in ATEX execution, and the SonaVu+ acoustic camera has an ATEX version.

PID: volatile organic compounds

A photoionisation detector measures VOCs, benzene, solvent and fuel vapours, including in LDAR programmes. According to the catalogue, the Ion Science Tiger XT covers 1 ppb to 20,000 ppm with a 10.6 eV lamp and has a library of more than 750 compounds.

An important limitation: according to Ion Science, a PID does not respond to methane and ethane, to SF₆ and other fluorinated compounds, to hydrogen or CO. More in Gas and VOC leak detection: Ion Science PID detectors and Methane leaks: what to use and why a PID will not work.

SF₆: a dedicated leak detector

SF₆ needs an instrument sensitive specifically to SF₆. The Ion Science SF6 LeakCheck P1 XTL uses Negative Ion Capture technology, needs no warm-up and responds in under 0.5 s. Standard sensitivity is 1×10⁻⁷ cc/s, the HIGHsense version 1×10⁻⁸ cc/s. The new version also detects C4-FN. According to the manufacturer, after exposure to high gas concentrations the instrument recovers without losing sensitivity.

A leak detector answers where the gas is escaping. Gas quality inside the compartment is measured by an analyser; the difference is covered in SF6 leak detector or gas analyser. An SF₆ leak in an enclosed room is dangerous because it displaces oxygen; see Oxygen displacement in enclosed GIS rooms.

Helium, hydrogen, refrigerants: thermal conductivity

The Ion Science Panther PRO uses a micro thermal conductivity detector (MTCD) and a built-in piezo pump that keeps sample flow stable in draughts. According to the manufacturer, sensitivity is 5×10⁻⁶ cc/s for helium and 3.8×10⁻⁶ cc/s for hydrogen, with a T90 of about 1 s; the main applications are helium, hydrogen, ammonia and refrigerants. The manufacturer notes that the instrument does not sample remotely: the probe is brought to the leak point.

Site survey procedure

1. List the gases on site and, for each, define the task: find the location, measure concentration or monitor continuously.
2. Check that the instruments are approved for the work zone (ATEX execution for hazardous areas).
3. Walk the pressurised lines with an ultrasonic leak detector or scan the area with an acoustic camera, and mark the leak points.
4. At the points found, confirm the gas with an instrument for that gas and measure concentration if needed.
5. In enclosed rooms with inert gases and SF₆, check oxygen content before entry.
6. Record locations and readings, schedule repairs and re-check afterwards.

Which instrument to choose

• Walking the lines and locating a leak of any pressurised gas: SDT340 or the SonaVu+ acoustic camera.
• SF₆ leaks at a substation: Ion Science SF6 LeakCheck P1 XTL.

KEG TRK supplies SDT ultrasonic instruments, Ion Science detectors and Rapidox gas analysers in Kazakhstan. Price on request.

Sources

• SDT Ultrasound Solutions. Compressed Air & Gas Leak Detection.
• Ion Science. Technical/Application Article 01: What is a PID?
• Ion Science. SF6 LeakCheck P1 XTL Leak Detector.
• Ion Science. Introducing SF6 LeakCheck P1 XTL.
• Ion Science. Panther PRO Gas Leak Detector.
• KEG TRK catalogue cards: Tiger XT, Rapidox 1100 CH₄, Rapidox SF6 Fixed Detection System.