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Analysis of obstacles in the use of ultrasonic flowmeters
2012-3-12 10:28:29

在使用过程中会出现一些不正常的现象和故障,现将一些故障现象分析如下: There are some abnormal phenomena and failures in the ultrasonic flowmeter during use. Now we will analyze some failure phenomena as follows:

I. Failure phenomenon: Unstable reading changes drastically

Cause analysis: The pipeline where the ultrasonic flow sensor is installed has a large vibration or there is a device that changes the flow regime (such as a flow meter installed downstream of a regulating valve, a pump, or a reduction orifice). Solution: Modify the flow sensor away from the vibration source or move it to Upstream of changing flow regime

2. Failure phenomenon: Inaccurate reading and large error

Reason analysis: 1. Ultrasonic flowmeter sensor The sediments installed on the top and bottom of the horizontal pipe interfere with the ultrasonic signal. Solution: Install the sensors on both sides of the pipe. 2. The ultrasonic flowmeter sensor is installed on the pipe where the water flows downward, and the pipe is not full of fluid. Solution: Mount the sensor on a fluid-filled pipe section. 3. There are devices that make strong flow fluctuations, such as: venturi, orifice plate, vortex, turbine, or partially closed valve, which are within the range of the sensor transmitting and receiving, making the reading inaccurate. Solution: Install the sensor away from the above device, the sensor upstream from the device 30D, downstream from the device 10D or move upstream of the device. 4. The input pipe diameter of the ultrasonic flowmeter does not match the inner diameter of the pipe. Solution: Modify the pipe diameter to match.

Third, the failure phenomenon: the sensor is good, but the flow rate is low or no flow rate

Cause analysis: 1. The paint and rust outside the pipeline have not been removed. Solution: Clear the pipeline again and install the sensor. 2. The piping surface is uneven or the ultrasonic flowmeter is installed at the welding seam. Solution: Grind the pipe flat or away from the weld. 3. The roundness of the pipeline is not good, the inner surface is not smooth, and there is pipe-lining fouling. This may happen if the pipe is cast iron. Solution: Choose a place with smooth inner pipe material or lining, such as steel pipes. 4, the measured medium is pure or solid suspended matter is too low. Solution: Choose other types of instruments that are suitable. 5. The sensor is installed on the fiber glass pipe. Solution: Remove the glass fiber. 6, the sensor is installed on the casing, it will weaken the ultrasonic signal. Solution: Move the sensor to the unsleeved pipe section. 7, the sensor is not well coupled with the pipeline, the coupling surface has gaps or bubbles. Workaround: Reinstall the couplant.

Fourth, the failure phenomenon: When the control valve is partially closed or the flow is reduced, the reading will increase.

Reason analysis: The sensor is installed too close to the downstream of the control valve. When the valve is partially closed, the flowmeter actually measures the flow rate of the control valve to reduce the diameter and increase the flow rate. The flow rate increases due to the reduction of the diameter.

Solution: Move the sensor away from the control valve, and move the sensor 30D upstream of the control valve or move the sensor to the control valve 5D upstream of the control valve.

工作正常,突然超声波流量计不再测量流量了。 V. Failure phenomenon: The ultrasonic flowmeter works normally, and suddenly the ultrasonic flowmeter no longer measures the flow. Cause analysis: 1. The measured medium has changed. Solution: Change the measurement method. 2. The measured medium is gasified due to high temperature. Solution: cool down 3. The temperature of the measured medium exceeds the limit temperature of the sensor. Solution: Cool down 4. The coupling agent under the sensor is aged or consumed. Solution: Recoat the coupling agent 5. Due to high frequency interference, the meter exceeds its own filter value. Solution: Stay away from the interference source 6. Data loss in the computer. Solution: Re-enter the correct parameters. 7. The computer freezes. Workaround: restart the computer

Introduction to the principle of ultrasonic flowmeters: At present, two types of ultrasonic flowmeters are generally used, one is a Doppler ultrasonic flowmeter and the other is a time difference ultrasonic flowmeter. The Doppler type uses the phase difference method to measure the flow velocity, that is, a sound wave of a known frequency moves in the fluid. Because the liquid itself has a speed of movement, the frequency or phase of the ultrasonic wave between the two receivers (or transmitters) occurs. Relative change, the liquid velocity can be obtained by measuring this relative change; the time difference type uses the time difference method to measure the flow velocity, that is, a sound wave of a certain speed causes the propagation time between two receivers (or transmitters) due to the fluid flow. Change, and by measuring this relative change, the fluid flow rate can be obtained. Introduction to the installation method of ultrasonic flowmeter: At present, three installation methods are usually used: W type, V type, and Z type. The installation method is selected according to different pipe diameters and fluid characteristics. Generally, W type is suitable for small pipe diameters (25 to 75mm), V type is suitable for medium pipe diameters (25 to 250mm), and Z type is suitable for large pipe diameters (above 250mm). ) In short, in order to improve the accuracy and sensitivity of the measurement, choose a suitable installation method so that the measurement signal (that is, the difference) matches the secondary meter. In order to ensure the accuracy of the measurement of the instrument, a location that meets certain conditions should be selected: usually straight pipe sections above 10D upstream and downstream 5D above; no disturbance devices such as pumps and valves can be installed in the upstream 30D. The installation method of the ultrasonic flowmeter probe: The "coordinate installation" method is adopted, that is, the outer surface of the pipeline is cleaned, and a special coupling agent is applied. First, the position of one of the probes is fixed, and the circumference of the pipeline is measured with a paper tape to measure the circumference. Mark the fold marks and determine the position of the other probe track at 1/2 of the perimeter. Similarly, the track should be parallel to the axis of the pipeline. Then, according to the installation distance displayed by the instrument, determine the relative distance between the two probes on the track to ensure ultrasound. There is sufficient signal strength, usually the signal strength displayed on the panel is greater than 2%, until the reading shows stable, indicating that the installation and commissioning are completed, the meter can work normally.

的使用: Use of ultrasonic flowmeter :

(1) Zero flow inspection. When the pipeline liquid is still, and there is no strong magnetic field interference or strong vibration around the meter, the meter display will be zero. At this time, the zero point is automatically set to eliminate the zero point drift. Small signals must be removed during operation. , Usually the flow rate is less than 5% of the full range flow rate, automatic removal. The zero point can also be adjusted through the menu. (2) Instrument panel keyboard operation. Before starting the instrument, you must first set the parameters effectively, for example, using the unit system, installation method, pipe diameter, pipe wall thickness, pipe material, pipe roughness, fluid type, and distance between two probes. , Velocity unit, minimum speed, maximum speed, etc. Only when all the parameters are input correctly, the meter can correctly display the actual flow value. (3) Periodic calibration of the flow meter. In order to ensure the accuracy of the flow meter, regular calibration is required. Usually, a more accurate portable ultrasonic flow meter is used. Direct comparison, calculation using measured data: error = (measured value-standard value) / standard value, using the calculated relative error, correction coefficient, so that the measurement error meets the ± 2% error, which can meet the measurement requirements. The operation is simple and convenient, which can effectively improve the accuracy of measurement