What should be paid attention to when using portable ultrasonic gas flowmeter on site

Portable ultrasonic gas flow meter is a commonly used type of gas flow meter product, which has the advantages of high measurement accuracy, wide application range, stable performance, high sensitivity, easy operation and long service life. Today, I will mainly introduce what issues should be paid attention to when using portable ultrasonic gas flow meter on site, hoping to help everyone.

1. Some issues that should be paid attention to during on-site work

1.1 Selection of instrument frequency and probe frequency

In actual measurement, the corresponding measurement frequency should be selected according to the specific measurement pipeline, and the working frequency of the instrument should be matched with the probe. The measurement frequency of the general instrument is between 200KHZ and 1MHZ, and its measurement range is about from 4'' to 24''. There are generally three types of probe frequencies: 200KHZ, 500KHZ, and 1MHZ. The basic rule is that the lower the frequency, the larger the measurement pipe diameter. Therefore, in actual measurement, we should choose the correct probe according to the pipeline to match it.

1.2 Issues that should be noted when selecting points for measuring pipelines

The first is to select the length of the straight pipe to be measured. The ideal measuring point should be a straight pipe length of 20D upstream and 10D downstream, with no bends, valves, reducers, inserted thermocouples or other flow-disturbing elements in between. The second is the surface treatment of the probe installation point. The surface of the two probe installation points should be smooth and free of paint and rust. If the surface is rusty or has a paint layer, it should be polished. According to the probe installation spacing recommended by the instrument, mark the two probe installation points at the actual measurement point and polish the installation points.

1.3 Matters needing attention in measurement and installation preparation

Before measuring, you should first obtain the outer diameter and wall thickness of the pipe at the measuring point, as well as the temperature and pressure parameters for reference during measurement. Measure the outer diameter and wall thickness of the pipe at the actual measuring point, input the probe model, pipe diameter and wall thickness, gas type and other parameters into the instrument, and refer to the recommended sound path number in the external space of the measuring point and the minimum pressure table above, and input the sound path number (Travers) into the instrument (In actual measurement of external clamp-on gas measurement, except for the pipe diameter below 50mm, which has 3 or 5 sound paths, 1 sound path is generally selected in most other cases), and then obtain the installation spacing (Spacing) of the probe in the instrument. The work here is very important, and the above parameters must be obtained in actual measurement.

1.4 Matters needing attention during installation

(1) Laying of noise reduction materials

The installation of noise reduction materials is critical to clamp-on gas measurement and may directly affect the success of the measurement, so it should be taken seriously. Wipe the outer wall of the tube clean, peel off the inner protective paper of the noise reduction material, stick it on the outer wall of the tube, press and stick it tightly to avoid bubbles. After the installation of the upstream and downstream probes, please check whether the installation spacing of the probes is consistent with the Spacing in the instrument. If it is inconsistent and the difference is not large, please enter the actual spacing into the instrument instead of the recommended spacing. Otherwise, the probe needs to be reinstalled. The author found that this problem is often ignored in actual measurement.

(2) Use of signal amplifier

Install the signal amplifier. Generally, a signal amplifier should be installed between the probe and the connecting cable when measuring gas. If the gas pressure far exceeds the minimum measurement pressure requirement, you may consider not installing the amplifier. After installation, you can check the signal strength (the signal strength is divided into upstream signal strength SSUp and downstream signal strength SSDn, which are usually equal and range from 50 to 70) for reference. If the instrument reports a fault after installation: SignalStrengthtoohigh, you can remove the amplifier and try again.

1.5 Minimum pressure requirement at the measuring point

The most important condition for successful measurement is that the gas pressure at the measuring point meets the minimum measurement pressure requirement. Different pipe diameters, wall thicknesses, and different gas types have different minimum pressure requirements. The basic rule is that the larger the pipe diameter, the thicker the wall thickness, and the smaller the gas molecular weight, the higher the required minimum pressure. For specific minimum pressure requirements, please refer to the measurement pressure table in the relevant instrument manual (the general table lists three gases: air, natural gas, and steam. Other gases can be referenced and estimated according to molecular weight, such as N2 can be referenced to air).

2 Several possible faults in measurement

2.1 Low signal strength fault

Signal strength is the most important parameter in the measurement and diagnosis. If this parameter is abnormal, the instrument will usually report a fault and the measurement will fail. Generally speaking, the greater the signal strength, the more stable and reliable the measurement value, and the longer the reliable operation. When installing, try to adjust the position of the probe and check whether the coupling agent is sufficient to ensure the maximum signal strength.

The system can work properly when the signal strength in both directions is greater than 60.0. When the signal strength is too low, you should check the measurement point selection (pressure, straight pipe length), pipeline surface treatment, probe installation, coupling agent, probe spacing, and instrument settings.

2.2 Sound speed failure

Sound speed SNDSP (soundspeed) is another commonly used parameter in diagnostic parameters, which refers to the propagation speed of the ultrasonic signal actually measured by the instrument in the gas. When this value is compared with the theoretical sound speed of the gas in the instrument parameter setting, if the error exceeds a certain range (for example, the error exceeds ±20%), a sound speed fault will occur. At this time, the installation of the probe, the probe spacing, and the instrument settings should be checked. (The propagation speed of ultrasonic signals in common gases generally ranges from 300m/s to 1000m/s).

2.3 Signal quality failure

Signal quality refers to the quality of the received signal. Poor signal quality may be caused by large interference, poor probe installation, or the use of poor quality, non-dedicated signal cables. In general, the probe should be adjusted repeatedly and the coupling agent should be checked to see if it is sufficient until the signal quality is as high as possible.

2.4 “Time difference” indication fluctuates too much

"Total transmission time and time difference" can reflect whether the installation is appropriate, because the measurement calculation inside the flow meter is based on these two parameters. Therefore, when the "time difference" reading fluctuates too much, the displayed flow and flow velocity will also jump sharply. This situation indicates that the signal quality is too poor, which may be due to poor pipeline conditions, improper probe installation or incorrect parameter input.

2.5 Transmission time ratio failure

The transmission time ratio is used to confirm whether the probe installation spacing is correct. When installed correctly, the transmission ratio should be 100±3. When the transmission ratio exceeds the range of 100±3, check whether the parameters (pipe outer diameter, wall thickness, pipe material, lining, etc.) are input correctly, whether the installation distance of the probe is consistent with the data displayed in the instrument, whether the probe is installed on the same straight line of the pipeline axis, whether there is too thick scaling, whether the pipeline at the installation point is elliptical, etc.

3 Conclusion

Due to the complexity of the gas flow meter at its use site, it often cannot reflect its performance in the laboratory environment. Therefore, when using a portable clamp-on gas ultrasonic flow meter to conduct an online field comparison of the gas flow meter, the various parameter settings and preparations should be taken seriously, and the uncertainty of the measurement results caused by the complex conditions on site should be fully analyzed and evaluated to ensure the accuracy and reliability of the test results.