Ultrasonic flowmeter is a non-contact flowmeter that is not commonly used in direct contact with the measured medium. It measures the flow velocity of the fluid by measuring the difference in the propagation time of the ultrasonic technology in the medium, and then calculates the flow of the fluid with high accuracy. For professional technicians, it is inevitable that they will encounter new instruments and instruments in their daily work. In addition to strictly following the requirements of the instrument, instrument manual, installation manual and other random data, it is more important to understand the instrumentation. The measurement principle and system composition, combining theoretical knowledge with field practice, can better serve the installation of field instruments. This paper summarizes a set of ultrasonic flowmeter fault diagnosis and elimination methods by analyzing the fault phenomenon and causes.

 

Ultrasonic flowmeter

Ultrasonic flowmeter is a meter that measures the volume flow by detecting the action of the fluid on the ultrasonic beam. The sensor of the ultrasonic flowmeter is an installation method that directly binds the sensor to the outer surface of the pipe to be measured, thereby realizing flow measurement. It solves the problem that the flowmeter of other principles must be broken and stopped during installation. It is the basic installation method of ultrasonic flowmeter. It has the characteristics of independent of pipe diameter, simple installation, no need to stop production, no pressure loss, etc. Ultrasonic flowmeter is commonly used. The measurement method is the propagation velocity difference method. The basic principle is to measure the difference between the velocity of the ultrasonic pulse along the water flow and the reverse water flow to reflect the flow velocity of the fluid, thereby measuring the flow rate. It uses the sound wave to propagate in the fluid due to the different flow directions of the fluid. The characteristics of different propagation speeds are measured by the difference between the downstream propagation time t1 and the countercurrent propagation time t2, thereby calculating the velocity and flow of the fluid flow.

 

First, the advantages and disadvantages of ultrasonic flowmeter

1. Ultrasonic flowmeter advantages

(1) Energy conservation. The instrument can be clamped in the appearance of the measuring pipe, does not touch the fluid, so it does not interfere with the flow field, and there is no pressure loss. Therefore, it is an ideal energy-saving instrument. Especially when the flow metering is large, the energy saving effect More significant.

 

(2) It is especially suitable for large-caliber flow measurement. Other flowmeters increase the cost more greatly with the increase of the diameter, and the cost of the ultrasonic flowmeter is basically independent of the diameter of the pipe to be tested. Therefore, the larger the diameter, the more advantages Significant.

 

(3) Solve the problem of flow measurement. Since the ultrasonic flowmeter is a non-contact meter, in addition to measuring general media such as water and petroleum, it can also flow the strong corrosive medium, non-conductive medium, explosive and radioactive medium. Measured, and is not affected by fluid pressure, temperature, viscosity density.

 

(4) It is convenient to install and maintain. No matter whether it is installation or maintenance, it does not need to cut off the fluid, and it will not affect the normal circulation of the fluid in the pipeline. It does not require valves, flanges, bypass lines, etc. during installation, so it is easy to install and cost. low.

 

(5) Good versatility. The instrument structure of other flowmeters is closely related to the size of the pipe diameter. When the diameter is changed, it is necessary to switch to different sizes of instruments. For ultrasonic flowmeters, no matter the pipe size changes, Or the flow measurement range changes, have greater adaptability.

 

Ultrasonic flowmeter

2. Ultrasonic flowmeter defects

 

Ultrasonic flowmeter is a high-integration and multi-function instrument based on integrated circuit, single-chip microcomputer and automatic control technology. The advanced control theory and mature circuit design lay the foundation for the realization of various functions of flowmeter. If the flow measurement accuracy is to be improved, The design of the measurement circuit must be further improved to improve the measurement of the sound velocity. The highly integrated circuit design of the ultrasonic flowmeter is bound to increase the cost and difficulty of design and manufacture, and in the event of a failure, the maintenance difficulty will increase accordingly. In addition, the ultrasonic transducer And the limitation of the temperature resistance of the coupling material between the sensor and the pipeline, and the influence of the original data of the measured sound velocity of the fluid at high temperature, it is difficult to apply the ultrasonic flowmeter to the flow measurement of the high temperature medium.

 

Second, the ultrasonic fault flow analysis of common faults

 

1. Instantaneous flow abnormality

The instantaneous flow abnormality may be caused by faults such as temperature change and voltage change, communication cable, surge protector, flow computer, etc. It needs to be carefully checked and confirmed. It can be exchanged by changing the temperature and pressure change of other flow meters. Computer problems or problems with field lines and meters. The following is an example of A and B ultrasonic flow meters. A is normal and B is abnormal.

 

1) Instrument and line fault judgment

 

After the temperature change and pressure change communication lines of A and B are swapped, the display of B is normal, A shows abnormality, and it can be judged that there is a problem with temperature change of A, pressure communication line or instrument. Both temperature change and pressure change support HART communication protocol. It is judged by the following method: disconnect the power supply of the secondary instrument, supply power with FLUK, and check the secondary instrument. If it is not detected or the data is abnormal, it is judged that the instrument is faulty, and the instrument is replaced to resume normal production.

 

If the meter value can be detected and the display is normal, then the 24V power supply to the field instrument is restored, and the communication cable of the flow computer is detected step by step. If no signal or signal abnormality is detected in a certain segment, it can be judged that the cable is damaged, and the cable is replaced. The segment cable resumes normal communication. If the signal is abnormal after passing the surge protector, it can be judged that the surge protector is faulty, and the replacement of the surge protector restores the normal transmission of data.

 

2) Flow computer Hart board failure

 

After the temperature change and pressure change communication lines of A and B are swapped, A shows normal, B shows abnormality, indicating that the flow computer of B is faulty. Check the status of the LED indicator of the flow computer Hart board. Under normal circumstances, it is green flashing, if it is displayed. Red is always on or off, and other states indicate that the temperature and pressure change correspond to the damage of the Hart board channel. There are two solutions. One is to directly replace the Hart board, no need to refresh the configuration; the second is to change the refresh configuration, please technology The engineer changes the configuration and changes the temperature and pressure change corresponding to the channel of the Hart board. Generally, in the summer thunderstorm weather, the flow computer is easily damaged by lightning, especially the Hart board, which is more likely to be damaged and needs special attention.

 

Ultrasonic flowmeter

 

2. No instantaneous flow

 

1) CPU board failure

 

The CPU board LED indicator is not lit or red. Check method: The laptop first installs the relevant configuration software and activates it. Connect with the network cable, change the IP address of the notebook, establish the communication LAN, download the configuration standby. Replace the standby CPU. Board, brush the configuration to the standby CPU board, cold start, check the status of the indicator.

 

2) Flowmeter failure

 

The LED indicator of the CPU board is not bright or red, indicating that the communication between the flowmeter and the flow computer is abnormal. You need to check the ultrasonic transducer and signal amplification unit of the flowmeter. Check method: Install the relevant software on the laptop, through the data cable and The ports of the flowmeter are connected, and the software checks whether there are alarms in the four channels under pressure. If it means that all the ultrasonic transducer signals are not uploaded, the head of the table needs to be removed, and the signal amplifier is tightened. Go up, indicating that the signal amplifier is damaged and needs to be replaced with a new one.

 

If the check finds that the channel alarm, signal-to-noise ratio is abnormal, and the channel gain signal is abnormal, the channel transducer may be damaged or the signal amplifier may be damaged. It can be judged by cross-wiring. The four pairs of channels of the ultrasonic flowmeter are A. , B, C, D, if the C channel is normal, the D channel is abnormal, and C1 and D1, C2 and D2 are swapped respectively. If the C channel is abnormal, the D channel is normal, indicating that the ultrasonic transducer is damaged and can be disassembled. If the signal amplifier is still abnormal, you need to replace the ultrasonic transducer of the same model. If the C channel is normal, the D channel is abnormal, indicating that the signal amplifier is damaged, you need to replace the same type of signal amplifier.

 

Ultrasonic flowmeter

Third, the conclusion and understanding of ultrasonic flowmeter

 

Most flow meters need suitable measuring media and working conditions to support the correct function. Only the flow meter performance with advantages and no disadvantages does not exist today. Reasonable selection instruments are crucial for field engineers and managers. To be properly and efficiently selected in industrial installations, using flow measurement methods and instruments, you must be familiar with both flow meter and production process fluid characteristics. Economic factors should also be considered.

 

Based on different principles, various forms of ultrasonic flowmeters have been developed for different occasions. The advantages of accurate measurement also make it the tool of choice for measuring flow rate in the chemical industry. Because ultrasonic flowmeter has strong anti-interference ability and measurement accuracy. High, no pressure loss, wide range ratio, small maintenance, etc., in recent years, has been vigorously applied. In actual production, ultrasonic flowmeters will have a variety of problems, we need to do the following Aspects: First, based on the actual situation, establish a reasonable maintenance system, check the ultrasonic transducer for half a year, use software inspection for half a year; second, reserve a proper amount of spare parts and configuration, and be able to replace the update in time when problems occur; Technicians need to continuously strengthen their learning, improve their skill level and ability to solve problems. Only in this way can we minimize the impact caused by ultrasonic flowmeter failures and reduce the operating costs of enterprises.