Fault analysis ideas and solutions for industrial automation instruments such as electromagnetic flow meters in practical applications

The process of industrial automation production involves many measurements and controls, which involve the application of various thermal control instruments. The accuracy and timeliness of these control instruments play a very important role in the stable operation of the entire production system. Therefore, maintaining good working conditions for on-site instruments is crucial for the stable and fast control effect. There are many types of thermal control instruments, which can be classified according to different principles. Generally, they can be divided into the following four categories: detection instruments, display instruments, regulating (control) instruments, and actuators. The detection instruments involve five categories: temperature, pressure, flow rate, level, and composition. This article discusses the common faults and solutions of industrial automation thermal control instruments in practical applications, including thermal resistors and thermocouples in temperature instruments, pressure/differential pressure transmitters in pressure instruments, level and liquid level instruments, and flow instruments. It is hoped that this can provide reference for related work,

1 Temperature Instrument

The temperature detection instruments used in factories are divided into contact and non-contact types, with contact types including expansion type, pressure type, thermocouple, and thermistor. Non contact methods include radiation and infrared. Due to the wide application of thermocouples and thermal resistors, which are mostly used in automatic interlocking control systems, only thermocouples and thermal resistances will be introduced here. Other instruments are relatively simple and the occurrence of faults is also more intuitive. I won't introduce it here.

The faults of temperature instruments mainly include the following situations: when measurement problems occur, either the instrument is damaged, the temperature instrument is damp and has poor insulation, or there is a problem with the settings of the upper computer. The thermocouple may have been affected by moisture, resulting in a low insulation resistance between the thermocouple wires or between the thermocouple wires and the outer shell, or the thermocouple wires inside may have broken. The thermocouple may be a broken resistance wire or wire. The setting issue is to select the appropriate scale number for the temperature instrument when configuring on DCS. The range should be set to the temperature measurement range corresponding to the scale number, rather than the range provided by the design institute, so that it can be measured and displayed correctly.

2 Pressure/Differential Pressure Transmitters

At present, most of the pressure/differential pressure transmitters used are intelligent transmitters. Compared with ordinary pressure/differential pressure transmitters, intelligent pressure/differential pressure transmitters have more complete functions, and the fault phenomena of the instrument itself are more intuitive and easy to judge. If there is a fault, there is a corresponding fault code on the LCD screen, which is very convenient to handle. The main problem with pressure/differential pressure transmitters in practical applications is the improper selection of the pressure port position during installation; Communication issues with the handheld device connection; Problem with instrument parameter settings; External malfunction, blockage or leakage of the pressure pipe. When the pressure pipe is blocked, remove the pressure pipe, clear it with steel wire, and blow it clean with compressed air or steam. These can be easily solved as long as the cause of the problem is identified.

When the pressure/differential pressure transmitter malfunctions, first check the on-site mining conditions and whether the pressure tap position can transmit the medium pressure to the pressure transmitter. Sometimes when communicating with the handheld device, it may not be possible to connect to the handheld device. This could be due to a mismatch between the handheld device and the pressure/differential pressure transmitter, inconsistent communication protocols, or mismatched software versions. These issues can be resolved by replacing the handheld device or upgrading the software; Another possibility is that the circuit resistance is too small to connect to the controller. This can be achieved by connecting a 250 ohm resistor in the circuit for normal communication. After normal communication with the handheld device, the parameters of the transmitter can be viewed. The main parameter here is the setting of the range, and accurate measurement requires an appropriate range, which is crucial. After these steps of inspection and processing, the malfunction of the pressure/differential pressure transmitter can be eliminated.

3. Level and liquid measurement

Level measurement can be divided into contact type and non-contact type. Contact type instruments include guided radar, radio frequency admittance level gauge, etc., while non-contact type instruments include radar, ultrasonic level gauge, etc. The main problem with these instruments is the interference of external signals on the sensor signal, which is the problem of indirect wiring or grounding between the sensor and transmitter. This can generally be solved by checking the wiring.

Secondly, if there are other obstacles in the measuring container that come into contact with the sensor or block the measurement signal emitted by the probe, it is necessary to carefully inspect the internal situation of the container being measured and avoid those obstacles when installing the probe or cable. If there is a four wire ultrasonic level gauge at a certain site that cannot measure correctly and the signal from the sensor displayed on the meter head is abnormal. After seeing the fault, first check the power supply and wiring. During the inspection of the power supply, it was found that the power supply was functioning properly; Upon rechecking the wiring, it was found that it was also functioning properly. But why can't it be measured normally? The other ones are all in good condition, but when I took off the sensors and looked at them, I found that the shielded wire of the sensor cable had poor contact, causing significant signal interference and preventing the transmitter from receiving feedback signals from the ultrasonic probe. After identifying the cause of the malfunction, the shielded wire of the sensor cable was reconnected and stabilized. After everything was restored, the ultrasonic wave was powered on and was able to work normally and measure accurately.

There is also a four wire ultrasonic level gauge, which can measure normally on site and display normally on the meter head. However, the level gauge cannot display normally on the DCS. After discovering the fault, the first thing that came to mind was the signal line issue. After checking the signal cable and wiring, it was found that there were no problems with the cable and wiring. Now we can only check if there is a problem with the 4-20mA signal output by the instrument. After measuring with a multimeter, both the meter end and the DCS end have no problem, and there is a 4-20mA signal. There is no problem with the signal, but the DCS cannot receive it. I suspect that there may be a problem with the AI card of the DCS. However, by connecting another signal to the channel corresponding to the card of the liquid level gauge, it can display normally again. At this point, the AC voltage of the signal was measured and it was found that both the positive and negative poles of the signal line had an AC voltage of about 7V to ground. It was the interference of this AC voltage that prevented the DCS from receiving 4-20mA signals normally. The solution to this fault was to ground the negative pole of the signal line to eliminate the influence of this AC voltage, and the DCS could display the liquid level change normally.

4 Flow meters

Flow meters can be divided into liquid flow meters and gas flow meters according to the measured medium. According to the measurement principle and method, they can be divided into rotor flow meters, throttling flow meters, electromagnetic flow meters, ultrasonic flow meters, mass flow meters, etc. The following will introduce the application faults and solutions of commonly used flow meters in industrial control.

The principle of throttling flowmeter is that when the medium flows through the throttling device, the flow velocity will form a local contraction at the throttling element, resulting in an increase in flow velocity and a decrease in static pressure, thus creating a pressure difference before and after the throttling element. The larger the fluid flow rate, the greater the pressure difference generated, which can be used to measure the flow rate. The main manifestation of this type of traffic failure is improper selection of installation location and whether the straight pipe section of the installation location meets the requirements; Whether the high-pressure side and low-pressure side are installed upside down during installation, and whether the calculation formula is correct, are all factors that affect the accurate measurement of the throttling flowmeter. The measurement principle of electromagnetic flowmeter is to measure the flow rate through electromagnetic induction. In practical applications, the main faults of electromagnetic flowmeter are as follows:

(1) The installation position does not meet the requirements, causing the medium inside the pipe to not be full or there to be bubbles inside. This problem can generally be solved by changing the installation position.

(2) The electrode of the electromagnetic flowmeter is corroded or scaled, causing it to be unable to measure. The solution to this type of fault is recommended to use pointed or hemispherical protruding electrodes that are not easily attached, replaceable electrodes, gua knife type cleaning electrodes, etc. The gua blade electrode can be manually scraped to remove sediment outside the sensor on a regular basis. In situations where adhesion layers are prone to occur, increasing the flow rate to achieve self-cleaning of the pipe wall is a relatively effective method. Of course, using easy to clean pipe connections is a more effective method.

(3) The low conductivity of the medium leads to abnormal measurement. In this case, the solution is to use other low conductivity electromagnetic flow meters that can meet the requirements (such as capacitive electromagnetic flow meters) or to use flow meters with other principles.

(4) External strong electromagnetic field interference prevents normal measurement. To prevent magnetic field interference in such abnormal situations, the installation position of the electromagnetic flowmeter sensor is usually kept away from the strong magnetic field source. Measures such as enhanced shielding can be taken to prevent strong electric field interference.

5 Conclusion

With the advancement of technology, the level of industrial production automation is increasing, and the production and application technology of thermal control instruments is also mature. The probability of instrument failure is very small, and most instruments include self diagnostic functions. The main faults are still in the installation and use of applications. The article discusses the common faults and solutions in the application of thermal control instruments. Mastering the above methods can solve most of the common faults in the practical application of thermal control instruments. Due to the limited level of the author, there may be omissions and shortcomings in the article. We hope for your understanding and valuable feedback.