Research On The Measurement Of Stack Gas Flowrate By The Pitot Tube And Quantity Traceability

With the prevention and control of air pollution in China and further development of international carbon trading,it is more and more important to accurately measure the emissions of pollution gas and greenhouse gas.Stack gas emissions are the main sources of air pollutants and carbon dioxide emissions in China,few standard facility were used to calibrate large diameter gas flowmeter in China and lack of a complete system of quantity traceability,the stack flow is compiex and high dielectric turbulence,cause the accuracy of the measurement results is low.S-type pitot tube is the most widely used stack gas flowmeter,it's measurement performance is directly related to the accuracy of measurement results.In order to meet the demand for the quantity traceability of stack gas flow and slove the problem that the calibration coefficient of S-type pitot tube is affected by the transverse flow in the stack gas flow measurement.This paper mainly evaluated the new stack gas flow standard facility of National Institute of Metrology,and through the stack field test analysised the metrological characteristics of S-type and spherical pitot tube in stack flowrate measurement,researchde on the change of calibration coefficient of S-type pitot tube with transverse flow the standard facility,designed an improved cone type pitot tube which calibration coefficient is not sensitive to transverse flow.The main research of the paper are as follows:(1)Have a performance evaluation to the National Institute of Metrology's stack gas flow standard facility.By the test,the standard facility has a measurement capacity of flow range of 908~104840m~3/h and flow velocity range of 0.5~60m/s,the expanded uncertainty of the facility is 0.61%(k=2),it can calibrate the flowmeter with a largest diameter of 1 meter,improved the quantity traceability system of stack gas flowrate.(2)Studied the metrological characteristics of S-type pitot tube and ball type pitot tube in stack flowrate measurement by stack field test.Research found that compared with the standard flowrate of the field ultrasonic flowmeter,the measurement indication error of S-type pitot is 33.8%,the measurement indication error of ball type pitot's directional measurement is-6.2%,the measurement indication error of ball type pitot's non-directional measurement is-6.5%.directional measurement and non-directional measurement's accuracy of the results is similar.But,the ball type pitot tube has small aperture and is easy to block,so it can not be measured for a long time,S-type pitot tube can avoid this problem,but it needs to be improved it's structure to improve the metering performance.(3)The transverse flow of stack gas can cause the change of pitot tube's calibration coefficient,the influence of the structural parameters of the S-type pitot tube head on the calibration coefficient was analyzed under different yaw angels and pitch angles.It was found that witch yaw angle changes,the calibration coefficient of S-type pitot tube is approximately symmetrical distribution to 0°.With pitch angle changes,the calibration coefficient varies greatly and is unsymmetric.The variation of the static pressure hole's differential pressure is the main factor for the variation of the calibration coefficient of S-type pitot tube.When the radius of curvature of the tube and the distance between the total and the static pressure holes has increased,or the tube's proportion of internal diameter and external diameter has declined,the change of calibration coefficient of S-type pitot tube tends to be stable.(4)Designed an improved cone type pitot tube which calibration coefficient is not sensitive to transverse flow,and researched the influence of the structural parameters to the calibration coefficient.It was found that the total pressure hole's differential pressure is the main factor for the variation of the calibration coefficient of cone type pitot tube.Under the transverse flow,the total pressure hole's cone angle and the distance of connecting rod and pitot tube's head has increased,or static pressure hole's cone angle has declined,the change of calibration coefficient of cone type pitot tube tends to be stable.Finally,through the stack field test,the indication error of improved cone type pitot tube is 24.6%,compared with S-type pitot tube,the accuracy of measurement has been improved by nearly 9%.