Leak-acoustics Generation And Propagation Characteristics For Natural Gas Pipelines

With the rapid development of the natural gas industry,the pipelines are paved increasingly and researches on the leak detection and location method are getting more and more important.Among the methods,acoustic method is the hotspot because it has been proven to be the most effective,true real-time and on-line leak detection method,which provides quick leak detection,high sensitivity,accurate leak location,and low false alarm rate.All along,studies on leak detection and location based on acoustic method have been focused on the signal processing of acoustic waves and acoustic sensors,but the leak-acoustics generation characteristics and propagation model for natural gas pipelines are not considered thoroughly,which is a key point influencing the signal recognition.To study on the generation mechanism,the leak-acoustic amplitude calculation equation has not been concluded.When the propagation characteristics are researched,the influences of gas flow on the attenuation model are without consideration,and the waveforms of the acoustic waves are not considered also.And the amplitude characteristics extracted from the weak acoustics lack systematic researches.The leak location method focuses on the calculations of velocity and time difference.The leak-acoustics generation and propagation characteristics are researched based on the fundamentals of fluid dynamics,gas dynamics and aeroacoustics,with the help of computational fluid simulation and the experimental fitting.Then the leak-acoustics calculation equation and the attenuation model are obtained,which are applied for leak detection and location.The main work and conclusions can be seen clearly as follows:(1)The leak-acoustic generation model is established and the semiempirical and half-theory formula is concluded,which are studied by the theory,the computational simulation and experiments.The acoustic source characteristic is that the gas compressibility and unsteady flow lead to turbulent fluctuations which result in the source fluctuations.The rarefaction wave characteristic is that the acoustic waveform will pervade itself with the longer propagation distance.The gas attack wave characteristic is that the acoustic amplitude can be calculated by the direct gas attack pressure,which is used to conclude empirical formula of acoustic amplitude.And the formula can be verified by the experiments.(2)The wavelet transform method combined with blind source separation method is firstly proposed,which is applied to fitting the waveforms of the original signals and the signals in the propagation process.The experimental results measured in 10 mm and 42 mm pipelines can be processed by the proposed method to judge the leakage time and to make up the amplitude of the weak signals.And two coefficients including lasting time and time coefficient are defined to describe the acoustic waves.Through the two coefficients,the acoustic wave is elongated in the propagation process.(3)The propagation equation is deduced in the gas pipelines.The amplitude attenuation models are established in theory,in simulation and in experiments.Firstly,the theoretical attenuation model is established in the viscous flow with uniform velocity considering gas flow,turbulent effect and viscothermal effect.Then the computational fluid dynamic simulation model of the acoustic waves is researched by the three-dimensional time domain pulse to obtain the propagation characteristics.The time coefficient changing law obeys exponential curve when the acoustic wave propagates in the gas.When the gas flow and the acoustic wave are in the same direction,it is bad for the waveform keeping as the Reynolds number gets larger.While in upstream,it is good for the waveform keeping as the Reynolds number gets larger.The attenuation factor gets smaller as the propagation distance gets longer.The equivalent length of the reducing diameters is obtained.The attenuation factors are calculated in experiments and in theory in 10 mm gas pipelines.When the original signals are processed,the upstream errors calculated in theory are less than 6.0%.When the wavelet transform(WT)signals are processed,the upstream errors calculated in theory are less than 7.0% while the downstream ones are less than 12.0%.The attenuation factors are calculated in experiments and in theory in 42 mm gas pipelines with the WT signals.The upstream errors calculated in theory are locating-0.634%~-1.940% while the downstream ones are locating-1.842%~-2.046%.And the relationship between the attenuation factors and the pipe diameters is clarified.(4)The new location method based on the attenuation model.When the original signals are used to leak location,the errors are locating-0.821%~0.119% with the traditional location method while the errors are locating-2.297%~2.423% with the new one.When the WT signals are used to leak location,the errors are locating-0.768%~0.705% with the traditional location method while the errors are locating-1.55%~7.646% with the new one with the theoretical attenuation factors and the errors are locating-2.603%~2.653% with the new one with the experimental attenuation factors.Then the location errors are obtained in 42 mm pipelines with the WT signals.The errors are locating at the order of 0.1% with the traditional method,of which the largest one is-0.780%.The ones are locating at the order of 0.01% with the experimental factors,of which the largest one is 0.054% while the ones are locating at the order of 0.1% and 1% with the theoretical factors,of which the largest one is 1.211%.And the location method based on the same side sensors is proposed because the attenuation factors are not fixed.