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Main Factors Affecting The Accuracy Of Ultrasonic Flow Measurement

Mar 11, 2019

  Ultrasound flowmeter work is not only related to the main body of the ultrasonic flowmeter,Xi'an Huaheng Instrument Manufacturing Co.,Ltd.is also directly related to the reliability of pressure,temperature measuring instruments,chromatographic analyzer and flow computer equipment.In addition,the accuracy of ultrasonic flow measurement will be affected to a certain extent in the aspects of staff operation,environmental impact,design selection,equipment installation,etc.Its influencing factors are complex and diverse.The relationship between the influencing factors of the measurement accuracy of the ultrasonic measurement system is shown in Fig.2.


  In practical production applications,noise,impurities in natural gas,gas composition analysis,pressure and temperature measurement are the four main factors that have great influence on the accuracy of ultrasonic flow measurement.


  1.noise


  Because of the high speed of air flow and the existence of elbows,valves and other kinds of obstruction pipe,noise will be produced in the natural gas transmission station,which has been fully considered in the design and installation of the measuring device.However,due to the change of working conditions,such as flow rate,pressure and temperature,noise and vibration under different power of compressor,all kinds of unpredictable noise still make the ultrasonic flowmeter be affected by noise in the process of field use.Ultrasound flowmeter is particularly sensitive to noise generated by pressure-reducing elements,and even some low-noise valves have greater impact on the ultrasonic flowmeter than the pressure-regulating valves.This is because the valve with low-noise technology adjusts the noise frequency to a high frequency range(20 kHz)which is not sensitive to human ear hearing.The frequency range coincides well with the ultrasonic frequency of the ultrasonic flowmeter and is more sensitive to ultrasound.Flowmeters have a great impact.


  2.Natural gas impurities


  Natural gas flowing through the ultrasonic flow timing,water,iron sulfide powder and other impurities gradually accumulated in the surface of the flowmeter pipeline or on the ultrasonic probe,will affect the accuracy of the ultrasonic flowmeter,the impurities attached to the surface of the ultrasonic probe shorten the transmission time of ultrasound in the pipeline,affecting the sensitivity of the ultrasonic probe,at the same time,due to the attachment of impurities,metering pry up.The change of surface roughness or blockage of the rectifier plate in the running straight pipe section results in the change of gas flow pattern,which affects the accuracy and stability of the flowmeter.The experimental results show that:


  Dirt can cause a deviation of 0.3%or more for the flow output of some flowmeters.


  3.Gas Composition


  According to the conversion formula of working condition flow rate and working condition flow rate of ultrasonic flow measurement system,it is necessary to use compression factor to convert the working condition flow rate of natural gas into standard condition flow rate in natural gas trade measurement,while the compression factor under working condition needs to be calculated by using gas analysis equipment,such as chromatograph analyzer.Whether the gas composition detection is accurate and timely directly affects the measurement accuracy of the ultrasonic measurement system.In actual production,gas analysis equipment of some stations is not connected to flow computer,which loses the main function of gas analysis equipment.At the same time,there is a phenomenon of gas composition update in time in the stations without gas analysis equipment.The influence is more obvious in the metering stations with multiple gas sources.The experimental results show that the maximum volume flow deviation of the ultrasonic flowmeter can reach 0.6%under the standard reference condition due to the component change of the gas transmission trunk.


  4.Measurement of Pressure and Temperature


  According to the working principle of ultrasonic flowmeter,the accuracy of pressure and temperature measurement is directly related to the accuracy of standard flow calculation.In practical applications,although the pressure transmitter can work normally within the operating temperature range indicated by its nameplate,the difference between the measured values of the pressure transmitter in outdoor exposed environment and the measured values of the pressure transmitter with sunscreen protection can be as high as 18 to 20 kPa.In the field application of temperature transmitter,there are metal impurities and sand in the protective sleeve,which make the temperature measuring element unable to reach the thermal balance with natural gas in time,resulting in low measured temperature value and large measurement results.Therefore,to ensure the accuracy and reliability of pressure and temperature measuring instruments is very important to ensure the accuracy of the measurement system of ultrasonic flowmeter.