On-site pipeline flow measurement and analysis using portable ultrasonic flowmeter

The popular portable ultrasonic flowmeter is convenient and flexible to use. It is generally suitable for measuring the flow of large-diameter pipelines. However, the actual measurement accuracy of field applications is often reduced due to negligence, errors in the measurement of transducer installation distance and flow area. Improper installation may even cause the instrument to fail to work completely. Therefore, the installation and commissioning of the instrument on site is a very important link for measurement. This article analyzes the problems encountered in the field use of ultrasonic flowmeters and summarizes the method of accurate measurement of pipeline flow.

1 Principle of ultrasonic flowmeter

Ultrasonic flowmeter uses the characteristics of ultrasonic waves that can carry fluid flow rate information when propagating in a flowing fluid. By receiving and processing ultrasonic information passing through the fluid, the speed of the fluid can be detected and converted into flow. It has the following main features: ① It solves the measurement difficulties of large pipe diameters, large flows and various open channels and dark channels; ② There is almost no requirement for the medium; ③ The measurement accuracy is almost unaffected by the parameters such as temperature, pressure, density, viscosity, etc. of the measured fluid; ④ The measurement range is wide, generally up to 20:1.

The time difference method [1] consists of two pairs of ultrasonic generators and receivers forming two channels in the measured fluid, emitting ultrasonic pulses in the downstream and upstream directions respectively, and measuring the time difference △t between their propagation. The propagation time of

the ultrasonic pulse emitted in the downstream direction is

The propagation time of the ultrasonic pulse emitted in the countercurrent direction is

Where: L is the distance between the ultrasonic transmitter and the receiver, also known as the sound path; c is the ultrasonic propagation speed when the measured fluid is stationary; u is the speed of the measured fluid. The

propagation time difference of the ultrasonic wave in the downstream and upstream conditions is

Since the ultrasonic wave propagation velocity c is much greater than the fluid flow velocity u, it can be considered that c2-u2≈c2, so

Right now

It can be seen from the above formula that when the propagation speed of ultrasonic waves in a static fluid can be considered as a constant, the fluid flow rate is proportional to the time difference △t. By measuring △t, the flow rate can be obtained, and then the flow rate can be obtained.

2 Installation and commissioning of ultrasonic flowmeters

Generally, the installation of ultrasonic flowmeters should be considered from the following aspects: ① Understand the site conditions in detail; ② Determine the installation method; ③ Select the installation pipe section; ④ Calculate the installation distance and determine the probe position; ⑤ Pipeline surface treatment; ⑥ Probe installation and wiring; ⑦ Use an oscilloscope to observe the received waveform, fine-tune and fix the probe.

2.1 Installation of transducer [2]

(1) Correctly select the installation site

Generally, when the inner diameter of the pipeline is D, the length of the upstream straight pipe section should be greater than 10D, and the downstream should be greater than 5D. When there are resistance components such as pumps and valves upstream, the length of the straight pipe section should be at least 30 to 50D, and sometimes even higher, such as 70 to 100m from the pump room. When dual-channel or multi-channel measurement is used, the straight pipe section before the meter can be significantly shortened.

The length of straight pipe section that should be configured for different types of flow-blocking parts is listed in Table 1.

(2) Correctly select the installation method [3]

When the fluid flows parallel to the pipe axis, the penetration method (Z method) in Figure 1 can usually be used to obtain better accuracy. However, when the fluid flow direction is not parallel to the pipe axis and there is a velocity component in the radial direction, the reflection method (V method) in Figure 1 or the 2V method should be used. For configured fluid pipelines, especially when measuring the flow of large-diameter pipelines, where measurement errors are easily caused by interference from the upstream flow state, a more appropriate measure is to increase the measurement line, that is, to increase the propagation path of the ultrasonic wave, receive more flow velocity information in the propagation path, and average it to offset the measurement error caused by fluid disturbance. When increasing the measurement line, the transducer should be installed so that the ultrasonic wave propagation path is evenly placed on the flow cross section. It is generally believed that 4 measurement lines (i.e. 4 transducers) are sufficient.

(3) Determine the installation distance

The transducer should be installed according to the specific measurement method and the steps in the manual. The installation steps under various measurement methods are slightly different, but the principles are similar. The following uses the commonly used transducer installation of the penetration method (Z method) as an example to explain the method for determining the installation distance of a pair of transducers.

There is a basic principle for the installation of a pair of transducers, which is called the distance method [4]: ​​the transmitting transducer and the receiving transducer should be on the semicircle bisector of the pipe (for the V method, on the same semicircle baseline), and the axial distance h between the two transducers is determined based on the actual pipe inner diameter, wall thickness and the properties and parameters of the measured fluid to ensure the accuracy of the h value during installation. The calculation formula is:

h=2δ tanθ2+D tanθ3

Where: δ is the pipe wall thickness; D is the pipe inner diameter; θ2 is the angle at which the sound wave enters the pipe wall; θ3 is the angle at which the sound wave enters the fluid.

Current ultrasonic flowmeter display instruments generally have the function of calculating the installation distance. As long as the pipe material, pipe diameter, wall thickness and other pipe parameters are determined, the flowmeter can measure the installation distance h, and then use the above distance method to determine the installation position. The

above distance method can only be used in situations where the pipe conditions are relatively good. When the pipeline condition is not good, such as due to long use, serious scaling, and the actual thickness of the scaling is unknown, it is difficult to determine the installation distance h. In this case, the transit time method [5] can be used to install the transducer. [page]

Figure 1 Transducer installation

First, turn the sensitivity knob of the display instrument to the middle position, then fix one transducer on one side of the pipe as required, apply butter to the other transducer and move it up, down, left and right on the other side of the pipe, and observe the cycle display light or number of the display instrument. When the cycle display light is on or there is a number display, move the transducer slightly and observe the transit time. When the transit time is close to the given value, stop moving the transducer. At this time, if the first 4 digits of the transit time are stable, it means that the transducer has been installed. Fix the transducer and the instrument can work normally.

2.2 Installation of the display instrument

(1) Installation location of the display instrument

The installation location of the display instrument should be selected in a location with small vibration and impact, and pay attention to avoid the influence of electromagnetic fields. The instrument power supply should avoid causing voltage fluctuations. The operating ambient temperature should also be within the range specified in the manual. Except for special sealed instruments, it should be installed in a non-corrosive environment.

(2) Connection length

The connection between the meter and the transducer should be shielded. The connection length should be in accordance with the manual and generally not exceed 500m. However, in power plants, due to the large interference, the connection length is generally not more than 100m.

3 Actual cases and experience summary

In order to find out the factors that affect the accuracy of ultrasonic flowmeters, field tests were carried out when measuring the flow in the drainage system pipeline of Jiangsu Southeast Iron and Steel Plant. Several experiences are summarized.

(1) There should be a long enough straight pipe section before the meter (as shown in Table 1).

(2) The geometric dimensions of the pipeline should be accurate and the scale inside the pipe should not be too thick. From the relationship between flow rate and pipe diameter, it can be seen that for every 1% error in pipe diameter, there will be about 3% flow measurement error. Therefore, when using an ultrasonic flowmeter, the wall thickness must be measured accurately.

Pipe scaling not only causes pipe diameter errors, but also scatters sound waves, reducing the measurement accuracy of the ultrasonic flowmeter. When the scale is too thick, the meter may not even work. Table 2 is a calculation table for the impact of pipeline scaling on flow measurement.

For some pipes with serious scaling, you can take down the transducer first and knock the part where the transducer is installed with a hammer. The scale will fall off partially after the vibration. Then install the transducer on the pipe for test. If it still does not work, we can only suggest that the unit update the measuring section of the pipe to a stainless steel pipe.

(3) Install an exhaust valve on the upstream pipeline of the meter.

It is a common problem that there is air in the water pipeline, and the gas content in the source water pipeline is even higher. The gas in the pipeline occupies the effective cross-sectional area of ​​the pipeline, causing errors in the flow measurement. If there is too much gas in the pipeline, the ultrasonic wave will be scattered and the instrument will not work. Therefore, it is necessary to install an exhaust valve upstream of the instrument. The

exhaust valve should be installed more than 30D upstream of the instrument and should be exhausted once every certain period of time. Tests were conducted on the DN600 and DN900 pipelines of Southeast Iron and Steel Plant and found that when the exhaust valve was not installed, the flow meter over-measured the water volume by about 9% to 10%.

(4) The fluid in the pipeline does not fill the pipeline

. When the transducer is installed on the pipeline, the intensity signal received by the flow meter is still not displayed on the screen, and other pre-display programs are normal, indicating that the fluid in the pipeline does not fill the pipeline.

4 Conclusion

Through the understanding of basic principles and the summary of field test experience, the flow of the drainage system of Southeast Iron and Steel Plant was measured again. By comparing with the calibrated online flow meter, it was found that after considering the above interference accuracy factors and adopting the appropriate installation method, the measurement accuracy was improved from the first ±1.67% to ±0.81%, which met the test accuracy requirements.

References
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