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Description of the characteristics of the flow measurement instrument for process control

Process control is the field that uses flow meters (electromagnetic flowmeters, vortex flowmeters, orifice flowmeters). The role of flowmeters in process control is to monitor the flow of fluids in sealed pipelines. The instrument is equipped with an adjustment system such as a regulating instrument and an actuator to stabilize the flow at an appropriate value, thereby realizing the stabilization of the process. For such a specific task, the flow measurement instrument must have the following requirements.
(1) Reliability requirements
1 The meter should have a high degree of reliability. Modern industrial devices tend to be large-scale continuous processes, instrument failures can easily lead to process instability, and once the flowmeter installed on the pipeline fails, it is impossible to stop the process specifically for repair, so instrument manufacturing and system design Reliability must be considered first, including the reliability of the thermal resistance for temperature compensation.
Some instrument manufacturers have doubled the components that are prone to failure and inconvenient maintenance. For example, the 8800C vortex flowmeter has dual-sensor products. Under the control of the CPU, the signals of the two sensors can be automatically discriminated. If a fault occurs, discard it to improve the reliability of the transmitter. There are also manufacturers who have designed ways to replace sensors under constant flow conditions. Some electromagnetic flowmeter manufacturers have provided methods and tools for continuously changing electrodes, which have created favorable conditions for improving reliability.
2 Troubleshooting. In the event of a fault in the meter, the fault diagnosis section should automatically indicate the location of the fault and the nature of the fault so that the repairer can shorten the time to troubleshoot.
After the instrument uses the digital communication method to send the diagnostic information to the computer, the computer can monitor the running status of the meter, issue an alarm signal when the meter fails, display the fault content, and even take necessary safety measures.
(2) Stability requirements
1 The output of the flow measuring instrument should have good stability. If the flow signal itself has noise, it should be adjusted by the damping in the table to stabilize the indication to facilitate reading. When adjusting the system with the regulator, the regulator output should be free of obvious oscillations.
2 The flowmeter indicates that the ambient temperature effect of the value should be within the specified technical specifications.
3 meters should have good long-term stability. Most instrument companies now have such indicators for intelligent differential pressure transmitters. For example, the EJA-110A transmitter promises that its long-term stability is not more than ±0.1% per 24 months. FS, 2010 TD type The transmitter promises to drift no more than ±0.os%FS every 12 months.
(3) Response time requirements There are many flow measuring instruments together with regulators and the like to form an adjustment system, and the response time of the flow measuring instrument is required to be within Is. In the flow rate adjustment system, if the total time constant of the flow measurement link is greater than Is, it may have a significant impact on the adjustment quality. In severe cases, the guidance system oscillates and cannot work.
(4) Anti-interference ability requirements
1 anti-vibration interference ability. Most of the flow sensors are installed on the pipeline, and the environmental conditions on the site are harsh. The vibration is a big disturbance, so the flow sensor and converter should have strong anti-interference ability. Some models of vortex flowmeters and Coriolis mass flowmeters are not ideal because of their anti-vibration interference capability. They are not well used in the field, and there are phenomena such as “nothing to be born” and high values.
2 anti-radio interference capability. There are many sources of interference in the industrial field where the electromagnetic flowmeter is installed. For example, driving in the factory, driving the forklift or using a walkie-talkie nearby will cause some flowmeters to increase in value. This is because of the traffic. The radio frequency electromagnetic waves emitted by the spark plugs of electric appliances and forklifts, the radio frequency electromagnetic waves emitted by the antennas of the walkie-talkie enter the instrument through some means, and interfere with its work.
In recent years, people have begun to pay attention to the impact of radio frequency interference. Vortex flowmeters have added radio frequency interference impact indicators and adopted many measures to improve anti-interference ability.

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