Natural Gas Leak Detection in Pipelines

Natural gas is an important commodity traded frequently in the energy market. Because natural gas is a low-density gas at ambient pressure and temperature, safe and economical handling during transportation, storage, and utilization requires taking special measures and sophisticated technologies. The pipeline is one of the most widely applied transportation methods for chemicals. However, the accidents on pipeline leak have generated great safety concerns. A natural gas pipeline differs from a water or crude oil pipeline because of the change in the gas thermal phase.

Leak detection systems range from simple visual line walking and checking to complex arrangements of hardware and software. Unfortunately, none of the methods is universally applicable. It’s operating requirements that dictate which method is the most cost effective.

Leak detection in natural gas pipelines mainly involves direct and indirect methods.

Direct Methods

Direct methods represent the direct detection of leakage substances, which mainly include the artificial line patrol method, acoustic method, chemical method, stress method, magnetic leakage method and robot method, etc. Their main feature is to directly identify pipeline leak with the help of human vision or various special sensing devices. Such methods sometimes are also known as the hardware detection method.

Flammable gas detection method: when gas leakage occurs outside the pipe, it can be detected by manually inspecting the ground surface, cable channels and cellar wells nearby to recognize the presence of combustible gas.

Odorization method: Humans are particularly sensitive to certain gases, such as ether (EM). In this method, trace amounts of leakage recognition gases are added to some natural gas. If you smell something very smelly, you can judge that a leak has occurred in the nearby pipeline. Then, gas companies can use the instrument to detect the nearby pipelines. This method has been widely used in urban natural gas.

Determine and check the displacement of the drain: The gas pipeline is supposed to be drained on schedule. If the water quantity suddenly increases, the infiltration of groundwater into the drain may be considered. In this case, the gas pipeline may leak due to damage. Besides, there are the observation plant method, airborne infrared method, optical fiber sensing technology and other methods. The direct detection methods are simple in principles and convenient in operation, but they require a high cost of manpower and the pipelines cannot be monitored online in real time.

Indirect Methods

In indirect methods, pipeline leak is detected by calculating and analyzing pressure, flow, temperature and other data provided by the data collection system. Because the detection results are obtained by calculation instead of direct detection, the indirect methods are also called the indirect inference method or software based detection methods, such as the mass balance method, pressure wave analysis method, real-time model method and statistical leak detection method.

Pressure method: Pressurize the test tube and close the both ends of the valve. If the pressure drops, there is a leak in this part. In this method, the pipeline must be shut down.

Method of listening: For pressurized pipes with shallow buried soil, we can listen to the sound on the ground surface, by which the leakage point can be found. This approach depends largely on the experience of the listener.

Correlation method: The two sensors of the correlator are placed at the both ends of the pipeline under investigation. The position of the leakage point can be detected by the microcomputer processing of the correlator. This method is demanding for operators and expensive equipment may be required.

Flow method: Compare the difference in fluid flow at the inlet and outlet of the pipeline. When the flow difference exceeds a certain value, leakage ought to be considered. Greatly affected by the variation in parameters such as fluid temperature and pressure, the measurement is not accurate enough. In addition, this method cannot locate the leakage point.

Pipeline modeling detection: In this method, system parameters are estimated online by establishing a real-time dynamic model of the pipeline, and leakage faults are diagnosed by comparing estimated values with measured values. According to different modeling, there are two methods: state estimation and system identification.

Method based on signal processing: In this method, neither modeling nor the measurement of the flow signal is required, so it has lower requirements of field conditions. The method includes the pressure gradient method, negative pressure wave method, pressure point analysis method and statistical analysis method.

Wall parameter change detection method: determine whether there is leakage by detecting the pipe wall defects. Through this method, detailed information about the quality of the pipeline can be obtained, which constitutes the basis for the need of overhaul. Its disadvantage is: the use of expensive equipment, a high cost of detection, inability to achieve real-time monitoring. At present, there are several commonly used methods: leakage flux test, ultrasonic test, eddy current test and so on.

References

• Liu, Y., Chen, B., Chen, Y., Zhang, S., Guo, W., & Cai, Y., et al. (2015). ‘Methane storage in a hydrated form as promoted by leucines for possible application to natural gas transportation and storage’. Energy Technology, 3(8), 815-819.

• Pan, X., & Karim, M. N. (2015). ‘Modelling and monitoring of natural gas pipelines: new method for leak detection and localization estimation.’ Computer Aided Chemical Engineering, 37, 1787-1792.

• Murvay, P. S., & Silea, I. (2012). ‘A survey on gas leak detection and localization techniques.’ Journal of Loss Prevention in the Process Industries, 25(6), 966-973.

• Viswanathan, B. (2017). Chapter 3 – natural gas. Energy Sources, 59-79.

• (2014). Conversion factors - pipeline rules of thumb handbook (eighth edition) - 22. Pipeline Rules of Thumb Handbook, 743–782.

Alfa Chemistry is a professional analytical testing company, and has long been committed to promoting the long-term development of the analytical testing industry. We sincerely welcome colleagues from all walks of life to come to negotiate and cooperate.