Researches On Low Flow Rate Calculation And Reference Wave Of Time Difference Method And Cross-Correlation Ultrasonic Gas Flowmeter

With the popularity of natural gas in China,ultrasonic flow-meters have been widely used in gas pipeline transportation and trade measurement.Compared with the traditional gas flow meters,ultrasonic gas flow meter bas the advantages of bi-directional measurement,high precision and repeatability,no moving parts inside and almost no pressure loss inside the pipeline.It becomes the best choice for gas transportation and trade.However the measurement accuracy of ultrasonic gas flow-meter is seriously affected by fluid flow profile,especially under high flow rate.To reduce the measurement error,two over-diameter paths are adopted as the object paths in multipath fusion of six-path ultrasonic gas flow-meter under low flow rate condition.The.experiment results showed that this method can reduce the relative error of flow measurement under the low flow rate condition.A method of using waveform at the junction between the first and second envelopes of received ultrasonic signal as the reference waveform is proposed to solve the problem of 5?s layered value of transition time(Ultrasonic sensors are operated at 200kHz),And experiments were carried out to verify its effectiveness.The main research contents and results of this paper are listed as follows:1)The multi-path fusion algorithm at low flow rate was studied.The average velocity of the chord line in the low flow velocity state is very small.Limited by the sampling frequency,the measurement resolution of the transit time is far from satisfaction,which results in the low accuracy and repeatability using the conventional multi-path fusion method.Two-path fusion algorithm using over-diameter paths is proposed to improve measurement accuracy under low flow rate condition.2)A new reference waveform selection method is proposed,using the waveform at the junction between the first envelope and the second envelope as the reference waveform.The attenuation and distortion of the received ultrasonic signal are very serious when the ultrasonic wave propagates in the gas pipeline,especially when the flow rate is high.The gas velocity in the pipeline and the installation direction of the ultrasonic transducer will further affect the ultrasonic signal propagation and cause the fluctuation of the received ultrasonic signal.There exist 5 ?s layered problem in the transit time calculated by the cross-correlation algorithm when ultrasonic sensors are operated at the frequency of 200 kHz.The reason for this problem is that the maximum peak value and the second largest peak value of the cross correlation result are very close.Under the effect of noise,the maximum peak point will fluctuate between the two adjacent peaks.The method of using the waveform at the junction of the first envelope and the second envelope as a reference waveform can effectively solve the problem of 5?s layered value of transition time in the medium-high flow rate area that the conventional reference waveform is powerless.3)The above research content was experimentally verified on the six-path ultrasonic 'gas flow-meter experiment platform.Experiment results verify the effectiveness of two-path fusion algorithm using over-diameter paths at low flow rates and the new reference waveform.The experiment was divided into low flow rate area and medium-high flow rate area.The experiment results showed that in the low flow rate area,the abnormal flow rate point was eliminated,the relative error of the flowmeter was reduced from 10%to 3%,and the repeatability was improved from 7%to 2%.In the medium-high flow rate area,the relative error of the flowmeter was reduced from 3%to 1.5%and the repeatability was improved from 4%to 1%.