Common Detection Methods

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Detection Method Details

Biological:

Experienced personnel will walk along a pipeline, looking for unusual patterns nearby, smelling substances that could be released from the pipeline or listening to noises generated by product escaping from a pipeline hole. A metal rod can be placed against a pipe and to an ear, listening for escaping gas. Trained dogs are also used to smell substances released from a leak.

Temperature Change:

Some leaks can be detected by temperature changes in the soil. Temperature sensors such as an optical time domain reflectometer, are used to detect changes of temperature in the immediate surroundings of a leak.

Ground Penetrating Radar (GPR):

GPR can accurately pinpoint buried pipeline leaks without digging. The leaking substances can be ‘seen' at the source by the radar via the changes in the surrounding soil's electrical parameters. A handful of papers has recently been written reporting results of GPR's effectiveness as a tool for detecting leaks in utility pipes. Over the last 20 years several important tests have been conducted mapping controlled releases of fluids in test pits using the GPR. These confirm GPR's sensitivity to subtle changes in soil moisture. Many other experiments have been reported under more realistic conditions where a pipe is actually leaking and the leak is in need of detection. Some have noted the difficulty of GPR in detecting these changes in wet, clay soils.

Acoustic devices:

Noise is generated as the gas escapes from the pipeline. Due to the limitation of the detection range, it is usually necessary to install many acoustic sensors along the line.

Sampling devices:

If the product inside a pipeline is highly volatile, a vapor monitoring system can be used to detect the level of hydrocarbon vapor in the pipeline surroundings. This is usually done through gas sampling. The sampling can be done by carrying the device along a pipeline or using a sensor tube buried in parallel to the pipeline. The response time of the detection system is usually from several hours to days.

Negative pressure:

When a leak occurs, a rarefaction wave is produced in the pipeline contents, which propagates both upstream and downstream. Pressure transducers can be used to measure pressure gradient with respect to time. Usually two sensor are used for each pipeline segment.

Flow or pressure change

If the flow or pressure rate of change at the inlet or outlet is higher than a predefined figure within a specific time period, then a leak alarm is generated.

Mass or volume balance

If the difference between an upstream and downstream flow measurement changes by more than an established tolerance, a leak alarm will be generated. This method allows the detection of a leak that does not necessarily generate a high rate of change in pressure or flow.

Dynamic model based system:

This technique attempts to mathematically model the fluid flow within a pipeline. The method requires flow, pressure, temperature measurements at the inlet and outlet of a pipeline, ideally also pressure/ temperature measurements at several points along the pipeline.

Pressure Point Analysis (PPA):

Based on the assumption that the pressure in the line drops due to a leak. An appropriate decrease in the mean value of the pressure measurement generates a leak alarm.

References:
http://www.netl.doe.gov/scng/publications/t&d/tsa/pipe_leak.pdf