1. Measurement principle of ultrasonic flow meter
Ultrasonic flow meter is a non-contact flow meter. The working principle is: when ultrasonic waves propagate in fluids, their propagation speed is affected by the fluid flow rate. By measuring the propagation speed of ultrasonic waves in fluids, the flow speed of the fluid can be detected and the flow rate can be calculated. Taking the most widely used transit-time ultrasonic flowmeter as an example, when ultrasonic waves propagate in a fluid, the propagation speed of ultrasonic waves in the downstream direction will increase and in the counter-flow direction, that is, the same propagation distance will have different propagation times. Then use The relationship between the propagation velocity difference and the measured fluid flow velocity is used to calculate the flow velocity and convert it into the flow rate. That is, when the ultrasonic beam propagates between the "upstream sensor" and the "downstream sensor" in the flow direction of the water medium in the pipeline, the flow of water will cause a slight change in the propagation time of the ultrasonic beam relative to the static propagation, and this propagation time The change is proportional to the flow rate of water. This is the measurement principle of the time-difference ultrasonic flowmeter. The theoretical expression of its relationship is as follows:
V=MD/sin2θ×△T/TupTdown
In the formula, M—is the number of straight-line propagations of the ultrasonic beam in water
θ—is the angle between the ultrasonic beam and the water flow direction
Tup—is the propagation time of the ultrasonic beam in the forward direction (the propagation time from the upstream sensor to the downstream sensor)
Tdown—is the propagation time of the ultrasonic beam in the reverse direction (the propagation time from the downstream sensor to the upstream sensor)
△T= Tup—Tdown
2. Ultrasonic wave Characteristics of flow meters
Ultrasonic flow meters are based on microprocessing technology and are mostly designed using integrated circuits and low-voltage wide pulse emission technology. In terms of measurement technology, in order to achieve higher resolution and larger measurement range, 0.1ns ultra-high resolution time measurement circuits are often used. It is specially used for liquid medium measurement, especially water measurement. Its notable features are: the accuracy level is ±1.0%, and it can be installed under pressure without stopping production. The host can be installed in the value control room and can also output standard signals such as current and pulse, and can use RS232 or RS485 interface communication for remote measurement data. Teleport. This flow meter has the advantages of high reliability, low power consumption, anti-interference, and easy installation and maintenance.
3. The basic structure and main installation methods of ultrasonic flowmeter
1. The structure of ultrasonic flowmeter
Ultrasonic flowmeters can generally be divided into field sensors (i.e. Probe), transmission cable and display host. The sensors are clamp-on, plug-in, and flange-type (i.e., pipe-section type). The display host is divided into fixed and portable. The portable host can be equipped with a clamp-on sensor to compare ultrasonic flowmeters that are fixed and online (on-site calibration). ) and installation is very easy.
2. Determination of the measuring point of the ultrasonic flowmeter
Ultrasonic flowmeter needs to select a suitable measuring point first, then input the water pipe parameters of the measuring point into the flowmeter, and finally connect the sensor to the flowmeter. (i.e. probe) is installed on the water pipe.
⑴General requirements for measuring points
The measuring points of ultrasonic flow meters are required to be on a straight pipe section of a certain length, that is, a pipe section with uniform water flow distribution is selected to reduce measurement errors.
⑵Principles for selecting measurement points
⑴The measurement point should be 10 times the length of the pipe diameter from the upstream (the direction of the water flow) and 5 times the length of the pipe diameter from the downstream (the direction of the water flow) A uniform straight pipe section (that is, the upstream and downstream valves are outside this length, or the inflection point of the water pipe is outside this length).
⑵ The material of the straight pipe section must be uniform and free of scars and cracks to facilitate ultrasonic wave transmission.
⑶The inner wall of the straight pipe section should be free of scale (if there is slight scale, it can be purged with steam or high-pressure water if conditions permit).
⑷The straight pipe section must be filled with water (whether vertical or horizontal pipe section).
3. Classification and main installation methods of ultrasonic flowmeter sensors
The installation quality of ultrasonic flowmeter sensors is directly related to the accuracy, credibility and operational reliability of water flow measurement.
⑴Classification of ultrasonic flowmeter sensors (probes)
There are three commonly used ultrasonic flowmeter sensors according to the installation methods:
Clamp-on sensor— During installation, the intended installation position on the outer wall of the pipe needs to be polished smooth, and then the sensor (probe) is attached to the outer wall of the pipe with coupling agent and then fixed with a special clamping device.
This method can easily measure water flow outside the pipe and is also suitable for portability. The disadvantage is that improper handling of the coupling agent can easily cause the signal receiving state to deteriorate and affect the stability of the measurement.
Plug-in sensor—during installation, use a drilling tool to insert the sensor (probe) into the pipeline without stopping production. The advantage is that it can achieve stable and reliable measurement even when the inner wall of the water pipe is scaled or there is air in the water.
Pipe segment sensor—When installing, you need to cut a selected straight pipe section and use a flange connection. The product has been specially calibrated at the factory. The advantage is that the sensor can be repaired without stopping production. It is characterized by high measurement accuracy.
⑵Installation of ultrasonic flowmeter sensors (probes)
The installation location of ultrasonic flowmeter sensors (probes) is generally to choose two sensors (probes) with the pipe axis in the water pipeline. The axis is in the horizontal direction or at an angle of 45 degrees with the horizontal plane of the pipe axis.
The installation methods of ultrasonic flowmeter sensors (probes) include Z, V, N, and W methods. Among them, N and W methods are suitable for water pipelines with a diameter of less than 50mm, and are rarely used due to difficulty in use and high cost performance. There are two commonly used methods:
a. "V" installation
"V" installation is the standard installation method, and the measurable pipe diameter range is 25mm-400mm. When installing sensors (probes), attention must be paid to the horizontal alignment of the upstream and downstream sensors (probes) so that their center line is consistent with the axis of the water pipeline.
b. "Z" method installation
"Z" type installation is generally suitable for thick water pipes or the water medium is not very clean or there is scale on the inner wall of the pipe, causing the "V" type installation. Install signal distortion conditions. Generally speaking, "Z" type installation is more suitable for water pipelines with diameters above 300mm. The measurable pipe diameter range of "Z" type installation is usually 100mm-600mm. When installing sensors (probes), it must be noted that the upstream and downstream sensors (probes) are in the same plane as the axis of the water pipeline, with the upstream sensor (probe) at a low position and the upstream sensor (probe) at a high position. (See the manual for the schematic diagram)
⑶Installation inspection of the ultrasonic flowmeter sensor probe
a. Mainly check whether the installation position of the sensor (i.e. probe) is suitable.
b. Whether the connection with the outer wall of the water pipe is smooth and tight.
c. Check the signal strength and signal quality through the host, and observe whether the sensor can receive the ultrasonic signal that allows the host to work normally.
4. Debugging of ultrasonic flowmeter
⑴ Enter the pipeline parameters according to the requirements of the flowmeter and record them.
⑵Adjust the installation position, spacing, and pipe joint degree of the upstream and downstream sensors (i.e., probes), and adjust the signal intensity received in both upstream and downstream directions to the strongest (the greater the signal intensity, the greater the measurement The more stable the value, the greater the credibility, and the more reliable it can run for a long time).
4. Remote transmission and sharing of ultrasonic flowmeter measurement data
Technicians usually implement ultrasonic flowmeter communication through PC or other communication equipment, using appropriate serial devices. The serial port cable connects the standard serial port of the ultrasonic flowmeter with the serial port of the host computer. Using software to issue preset commands on the host computer, the flowmeter can send out relevant response signals.
Technicians use the identification code of the ultrasonic flowmeter as the network address code, use the corresponding command set as the communication protocol, and use the current loop of the flowmeter and its OCT output to control the stepper (or analog) electromagnetic The opening of the valve and the relay output can control the power on and off of other equipment, which can realize both data collection and remote control (the data transmission hardware uses RS232 or RS485 interface communication when the distance is relatively short, and the current loop is used when the distance is longer. or wireless transmission), transmit the flow meter data to the corporate network through Ethernet to achieve data sharing among relevant secondary units within the company. And realize the real-time monitoring status of water metering.
5. Common faults and solutions in the use of ultrasonic flow meters
1. Fault phenomenon: The instantaneous flow meter fluctuates greatly.
⑴Cause of failure: Signal strength fluctuates greatly; the measured fluid itself fluctuates greatly.
⑵ Countermeasures: Adjust the probe position, increase the signal strength (keep it above 3%) to ensure stable signal strength. If the fluid itself fluctuates greatly, the position is not good. Re-select the point to ensure that the first 10D and the last 10D are 5D working conditions requirements.
2. Fault phenomenon: The signal of the external clip-on flow meter is low.
⑴Cause of failure: The pipe diameter is too large or the pipe is severely scaled or the installation method is incorrect.
⑵ Countermeasures: Use plug-in probes for pipes with too large diameter and severe scaling; reselect the installation method.
3. Fault phenomenon: The signal of the plug-in probe decreases after being used for a period of time.
⑴Cause of failure: The probe may be offset or the scale on the surface of the probe may be thick.
⑵ Countermeasures: Readjust the probe position and clean the probe emission surface.
4. Fault phenomenon: No display when starting up.
⑴Cause of failure: The power supply properties do not correspond to the meter ratings or the fuse is blown.
⑵ Countermeasures: Check whether the power supply properties correspond to the rated value of the instrument and whether the fuse is blown. If there are no above problems, please notify the manufacturer's professionals to deal with them.
5. Fault phenomenon: After turning on the instrument, there is only backlight without any character display.
⑴ Cause of failure: Generally, the program chip is lost. ⑵Countermeasures: Notify the manufacturer’s professionals to deal with it.
6. Fault phenomenon: The instrument cannot be used under strong interference on site.
⑴Cause of fault: The power supply has a large fluctuation range or there is interference from a frequency converter or strong magnetic field around it, or the grounding wire is incorrect.
⑵ Countermeasures: Provide stable power supply to the instrument; or install the instrument away from frequency converters and strong magnetic field interference; or set up grounding wires in a standardized manner.
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