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Application of ultrasonic flowmeter in coalbed methane flow measurement

Abstract: through the analysis of the measurement principle of ultrasonic flowmeter and the factors affecting the accuracy of ultrasonic measurement, this paper expounds in detail the specific methods to solve the measurement accuracy, and provides guiding suggestions for instrument selection and installation

key words: coalbed methane pipeline; Ultrasonic flowmeter; Flow measurement

flow is one of the important parameters in coalbed methane production and gathering. The accuracy of flow measurement will directly affect the stability of system operation and the economic interests of both parties in handover measurement. At present, there are many kinds of flow meters produced in China based on various principles, such as volumetric flowmeter, vortex flowmeter, differential pressure flowmeter, ultrasonic flowmeter, etc. Among them, ultrasonic flowmeter is a non-contact instrument that began to be applied with the rapid development of integrated circuit technology. Compared with other types of flowmeter, ultrasonic flowmeter has the characteristics of no pressure loss, high cost performance, wide measurement range (generally up to 20:1), etc. The measurement range of ultrasonic flowmeter with propagation velocity difference method can reach 300:1. Therefore, the use of ultrasonic flowmeter in the measurement of large-diameter coalbed methane has outstanding advantages. Especially in urban gas transmission, there is a great difference in gas flow between peak gas consumption in the daytime and low gas consumption at night. The wide measurement range of ultrasonic flowmeter can well adapt to this point

1 measurement principle of ultrasonic flowmeter

ultrasonic flowmeter converts electric energy into ultrasonic energy by ultrasonic converter, transmits and passes through the measured fluid in a certain way, and the receiver receives ultrasonic signal for display integrator display and integration, so as to realize the detection and display of flow

the two ultrasonic transducers (also known as ultrasonic probes) of the ultrasonic flowmeter are installed on both sides of the outer wall of the pipe respectively, symmetrically at a certain inclination (see Figure LA) or on the same side (see Figure 1b)

ultrasonic transducers are usually made of lead zirconate titanate ceramics. Under electrical excitation, the transducers produce ultrasonic waves that enter the tube wall at a certain incidence angle, propagate into the fluid in the form of transverse waves in the tube wall, and propagate in the fluid in the form of longitudinal waves. Finally, they pass through the medium, pass through the tube wall and are received by another transducer. During the measurement, the two sensors are transmitting and receiving probes for each other. In order to ensure that the ultrasonic waves transmitted in the two directions are at the same gas flow rate, a real-time multitasking operation system is adopted to realize three closed-loop control of load, deformation and displacement. During the design, the two transducers are designed to transmit ultrasonic waves to each other at the same time. 8. Bio based composite materials

according to the different measurement parameters, the measurement principle can be divided into three kinds: time difference method, frequency difference method and phase difference method. Only the first two commonly used types are introduced. Taking figure LA as an example, the angle between the sensor and the normal of the pipeline is θ 1 , δ Is the wall thickness of the pipe, and D is the nominal diameter of the pipe. The propagation speed of ultrasonic pulse signal is C, the fluid speed is V, and the included angle between the connecting line between the two sensors (T1 and T2) and the flow direction of natural gas is θ, Then the component VCOs of ultrasonic velocity in the flow direction of natural gas θ, The resultant velocity superimposed with the fluid velocity is C ± VCOs θ。 That is, when the direction of ultrasonic horizontal velocity component is consistent with the direction of fluid velocity, the combined velocity of the two is c+vcos θ, On the contrary, it is C-V cos θ。

1.1 measurement principle of difference method

when T1 transmits and T2 receives, thank customers for paying attention to the transmission from T1 to T2 between Jinan assay websites:

when T2 transmits and T1 receives, the transmission time from T2 to T1:

after sorting:

then the frequency difference between the two directions:

it can be seen from formula (4), the measured frequency difference can be used to obtain the gas flow rate V, from which the volume flow can be calculated, That is:

then the formula of daily output of natural gas can be obtained:

1.2 the measurement principle of time difference method

collates formula (3):

in this way, the time difference T12 and T21 can be measured, and the gas flow rate V can be obtained, thus the formula of daily output of natural gas can be obtained:

from formula (6) and (8), the daily output Q of natural gas measured by ultrasonic flowmeter has nothing to do with the speed of ultrasonic pulse signal

2 performance characteristics

compared with traditional flowmeters such as orifice, turbine and vortex street, ultrasonic flowmeter has the characteristics of strong adaptability and convenient operation (see Table 1)

3 factors affecting the accuracy of ultrasonic flow measurement

there are many factors affecting the accuracy of ultrasonic flowmeter 2 There is dirt in the oil return valve of the electronic tension machine, which is mainly caused by two factors: the influence of the performance of the instrument itself; Influence of installation quality

3.1 type selection of flowmeter

the measurement principle of the propagation velocity difference method is to measure the average flow velocity on the ultrasonic propagation path. Whether this average flow velocity can correctly reflect the flow velocity on the flow section has an impact on the accuracy of the measurement. Therefore, dual channel or multi-channel flowmeter should be selected in the design to improve the uncertainty of the average flow rate measured by mono channel. In practice, the accuracy of mono channel flowmeter is ± 2.0%, which can meet the requirements of process control. The accuracy of multi-channel flowmeter is ± 1 0%, which can meet the requirements of trade handover measurement

the measuring body of ultrasonic flowmeter has two forms: external clamping type and self-contained measuring tube type. The externally clamped ultrasonic flowmeter is convenient and flexible to use, and is especially suitable for reconstruction projects. However, the actual accuracy of field application often decreases due to negligence, installation distance and flow area measurement errors. The use of ultrasonic flowmeter with measuring tube reduces the influence of upstream velocity distribution, reduces the influence of the length of front and rear straight tubes, and eliminates the influence of the location of the converter installed on site. If the on-site process conditions permit, it is recommended to use the measurement method with its own measuring tube

3.2 influence of installation quality

the selection of installation location must ensure that the fluid in front of the converter flows parallel along the pipe axis, so it should ensure that there is a certain length of straight pipe in front of the converter, and the required length of straight pipe is related to the form of resistance parts on the flow channel (see Table 2). The inner wall of the process pipe on both sides of the flowmeter should be smooth without surfacing; When installing the ultrasonic flowmeter with measuring pipe, the inner diameter of the sealing gasket should be 4~5 mm larger than the inner diameter of the flowmeter, so as to prevent it from extending into the pipeline after the flange is fastened, which will reduce the effective flow area and affect the measurement accuracy. Clamp on ultrasonic flowmeter can be installed with pipe support to reduce the impact of pipe vibration on measurement accuracy. The grounding wire of flowmeter shall be reliably grounded. The cable shielding layer should also be reliably grounded to prevent all kinds of external electromagnetic interference

3.3 adjustment and calibration of ultrasonic flowmeter

accurate adjustment and calibration is very important for the normal operation of the flowmeter. Including the adjustment of the electronic circuit of the secondary instrument of the flow display; Adjust the installation of the converter so that the acoustic signal of the transmitting converter can be normally received by the receiving converter after being transmitted in the fluid. For the ultrasonic flowmeter with measuring tube, because the relative position of the transmitting converter and the receiving converter is fixed, its adjustment is relatively simple, mainly for the electronic circuit of the secondary instrument

4 conclusion

in recent years, gas ultrasonic flowmeter has incomparable advantages, such as theoretically no flow calibration, independent of detection medium, and is gradually becoming practical. With the increasing importance of coalbed methane in China's energy system and the increase of gas transmission pipelines represented by the west to east gas pipeline, ultrasonic flowmeter will be more widely used because of its unique advantages of non-contact measurement


[1] Liu Wei Application of ultrasonic wave flowmeter in pipeline gas measurement [j] Industrial metrology, 2000, (4):

[2] Dong Haoran Selection and application of ultrasonic wave flowmeter [j] Industrial measurement, 2002, (2): 49- 50

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