With the advantages of high precision,low pressure loss and wide range ratio,gas ultrasonic flowmeter has been more and more widely used in natural gas trade measurement occasions such as West-to-East Gas Pipeline,China-Myanmar Gas Pipeline and Gas Storage and Distribution Station.Ultrasound flowmeter calculates the flow value according to the time difference between the forward and backward propagation of ultrasound in the unif-orm velocity distribution flow field.In fact,the velocity distribution in the pipeline is not uniform due to the existence of disturbing elements and rough pipe wall upstream and downstream.For DN500 and DN1000 pipelines,considering the existence of single elbow or double elbow in upstream pipelines,according to the given five flow boundary conditions,the velocity distribution of the flow field in the pipeline is analyzed by CFD simulation;the actual flow through the pipeline is calculated according to the flow field analysis;at the same time,the time difference of the downstream and countercurrent time of the ultrasonic wave propagating through the sound channel in the flow field is calculated.The flow rate is calculated by time according to the criterion;then the errors of the two flow rates are analyzed by comparing the flow rate with the flow rate under boundary conditions;and the errors caused by the changes of pipe diameter,channel length and channel angle are also discussed.Finally,based on the above research,the error of ultrasonic flowmeter is evaluated and the improvement measures are given.Specific research contents are as follows:(1)For DN500 and DN1000 pipelines respectively,considering the existence of single elbow or double elbow in upstream pipeline according to the code,the velocity distribution of the flow field downstream of the spoiler at different length of straight pipe section is studied according to the given five flow boundary conditions.The results show that the velocity distribution downstream of the two kinds of spoilers is not uniform,but with the increase of the length of the straight pipe section,the velocity distribution of the flow field gradually returns to uniformity.Through the analysis of the length index of straight pipe section needed to restore symmetrical and fully develop turbulent velocity distribution,the uneven velocity distribution caused by double elbows on different surfaces is more serious than that caused by single elbow,and the larger the diameter and flow rate of the pipe,the more serious the flow distortion caused by the two kinds of turbulent elements.(2)Based on the velocity distribution of each flow field in the pipeline,the flow rate in the pipeline is calculated with the idea of integral,and the time of forward and backward flow of ultrasonic wave through the sound channel in each flow field is calculated,and the flow rate is calculated according to the standard of the ultrasonic flowmeter.The flow calculated by the two methods is compared with the given boundary flow,and the error magnitude and law of the two flow rates are analyzed.The results show that the flow errors based on flow field and the calculation errors based on standard are larger than those of single elbow;with the increase of length of straight pipe section,both errors decrease gradually;the larger the diameter and flow rate of pipe,the larger the flow errors and calculation errors caused by flow state distortion;and the velocity distribution of multi-channel ultrasonic flowmeter is uneven compared with that of single channel flowmeter.The flow field has better measurement accuracy.(3)Quantitative analysis of the errors caused by the changes of the diameter,length and angle of the vocal tract.Comparing the errors caused by the variation of pipe diameter,channel length and channel angle with the non-uniform velocity distribution,it can be seen that the most important factor affecting the measurement accuracy of the ultrasonic flowmeter is the velocity distribution of natural gas in the pipe section.The maximum calculation error of the ultrasonic flowmeter caused by the errors can reach-3.52%.(4)According to the measurement error requirement of ultrasonic flowmeter,the errors caused by pipe diameter,channel length,channel angle variation and non-uniform velocity distribution of flow field are evaluated,and the improvement measures are put forward:the installation length of straight pipe section of single elbow and double elbow downstream ultrasonic flowmeter is specified;the improvement method of measuring pipe section based on flow regulator is put forward." The improved triangular channel arrangement method improves the flow field adaptability of mono-channel ultrasonic flowmeter.Based on the least square method,the correction coefficient of velocity distribution is corrected by using three parameters of straight pipe length L,pipe diameter D and flow rate,and the calculation error of mono-channel ultrasonic flowmeter downstream of single elbow and double elbow is controlled within the allowable range.Finally,it is suggested that the real-flow calibration be adopted.Accurate method eliminates the errors caused by the changes of pipe diameter,channel length and channel angle. |