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LEOS - Leak Detection Technical Description

Diffusion method

The physical principle of operation is based on the air-tight so-called "LEOS sensor tube" that is permeable to gases and vapors within a certain diffusion period. The sensor tube is laid alongside the monitored pipeline and is purged with clean air in periodic intervals. Between these measuring cycles the tube remains at normal pressure. If there is a leak, the medium carried by the pipeline gets in contact with the wall of the sensor tube, diffuses (=penetrates) through the tube wall after a certain time, and forms a locally high vapor concentration inside the tube at the point of contact. This works regardless of whether the leakage medium gets in contact with the sensor tube as liquid or gas (=vapor) – and it even works when the leakage medium is dissolved in water (for example if sensor tube and pipeline are buried below groundwater level or in the seabed/riverbed). Since the described
diffusion process occurs with most chemical substances, there forms an expanded long air sample inside of the sensor tube that represents the atmosphere outside the tube.

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Automatic integral self-test of the system

The correct operation of all LEOS components is integrally checked with every measurement by means of the test gas. Amplitude and arrival time of the test peak normally stay within a small variation range (so-called "test peak window"). In case of deviations a "system alarm" is automatically triggered.

Special applications

The analysis capabilities of LEOS can be extended in a flexible way by implementation of additional
gas analyzers in the air flow. This allows selective and sensitive detection of special toxic gases such as chlorine or carbon monoxide.

A detailed chemical analysis (such as gas chromatography, etc.) of the air sample from the leak location is also possible, since the LEOS system directly transports the leakage substance as vapor phase to the measuring station.

Periodic measuring cycles

In adjustable time periods, this air column is purged through out of the tube and passed through several gas sensors. The signals of these gas sensors are recorded as a function of the purge time (see figure below). At the same time during this purge process, filtered air flows into the tube line in order to fill it with pure air for the next diffusion period.

Leak localization

When the vapors from the leakage pass the measuring station, its gas sensors show a clear signal peak (leakage peak) that will trigger a "leak alarm" when exceeding an alarm threshold. Before each purge process, a small amount of test gas (hydrogen, butane or propane test gas, etc.) is also injected at the end of the tube and is transported along with the air flow through the entire sensor tube line. The resulting so-called "test peak" will also be detected by the gas sensors and its travel time marks the total length of the tube line. The location of the leak along the tube line (and thus also on the pipeline) can be determined accurately from the travel times of test peak and leakage peak, as well as the measured air flow velocity.