There are many types of flow measurement technology and instruments , and the measurement objects are complex and diverse, which determines the complexity of flow measurement instruments in application technology. It is very different from the traditional application of measuring instruments in the traditional sense. It is not simply to install the flow meter , and it will definitely achieve the measurement purpose when the meter is put into operation. Two experts surveyed more than a thousand flow meters installed at the site, and found that about 60% of the selected measurement methods were not the most suitable or incorrect, and about half of the remaining 40%, although the measurement methods were suitable, existed in the field. Arrangement and installation are unreasonable. These cloths are suitable, incorrect, and irrational, which leads to corresponding measurement errors. Therefore, the flow measurement is a measurement that strongly depends on the use conditions. In the laboratory, the flowmeter can obtain extremely high accuracy, but at the use site, once the fluid conditions or environmental conditions have changed greatly, not only the accuracy cannot be guaranteed, even Normal measurement cannot be performed.
The calibration of a flow meter at the factory is better than ± 0.5%, but it is not uncommon for a new meter to be installed to the site to open the meter and the error may increase to ± 5% to ± 10%. There are a variety of reasons for this, such as irrational selection, inappropriate measuring range, insufficient length of upstream and downstream straight pipe sections, incorrect installation, fluid properties that deviate too much from the design state, working conditions exceed allowable values, and pulsating flow effects. Environmental conditions such as vibration are too severe, and many can be cited. Therefore, flow measurement is a system problem, including detection devices, display devices, straight pipe sections before and after, and auxiliary equipment. The study of applied technology also includes the measurement object itself. The good performance of the flow meter alone cannot guarantee the required measurement results.
The goal of the research on the application of flow measurement instrumentation technology is to use it correctly, which mainly includes the following specific contents.
(1) Increasing the meter opening rate In the management of instrumentation equipment, the meter opening rate is defined as: (total number of meters—the number of meters that are not used normally) / total number of meters . Therefore, increasing the meter opening rate is to reduce the number of meters that cannot be put into normal use. In the design institute, the measurement rate designed by the major of automatic control is one of the important indicators reflecting the quality and technical proficiency of the designer. Experienced and conscientious designers can make the measurement rate exceed 95%, or Over 95% through rectification. However, under the conditions of a market economy, engineering companies often implement key-key contracting practices for owners. The required opening rate is not 95% but 100%. If the designed instrument system cannot be used normally, it will be an engineering company. Responsibility, that is, rectification or instrument replacement, which means economic loss. Therefore, the research on instrument application technology has practical economic significance.
The open rate is the comprehensive performance of the technical level of the meter application and the quality of the meter itself. The measurement method and instrument's matching to the measurement object, the use environment, coordination, optimization, and prior design selection, installation and commissioning are all important factors affecting the open rate.
In recent years, the application technology of flow measurement instruments in China has made great progress, and the meter opening rate of flow measurement instruments has been greatly improved. This is due to the significant improvement in the overall technical level of instrumentation personnel and the increase in their sense of responsibility. It is important that the quality of the instrument has been greatly improved compared to the previous planned economic era. The proportion of imported instruments and instruments assembled with the introduction of foreign advanced technology is increasing. Especially after the intelligence of the instrument is generally realized, the adjustable range of the measurement range has been greatly expanded. The measuring range is not suitable for the instrument that can not be used normally. Generally, it can be used by changing the measuring range.
(2) Guaranteed measurement accuracy The accuracy of flow measurement refers to the accuracy obtained by the flow measurement system, which is different from the accuracy of the flow meter itself. Only the flow meter itself has good performance and high accuracy, and it may not necessarily obtain higher measurement accuracy.
To ensure the accuracy of the flow measurement system, in addition to reasonable selection, correct installation and commissioning, and timely maintenance and maintenance, it is also effective to apply intelligent technology to properly compensate and correct errors that may be introduced in the measurement part. method. For example, compensation for the temperature expansion coefficient of liquid, compensation for the temperature, pressure and compression coefficient of gas, compensation for the influence of Reynolds number and flow expansion coefficient of differential pressure flowmeter, and non-linearity of flow coefficient of various flowmeters Compensation, compensation for the temperature effect of volumetric flowmeters , vortex flowmeters , and compensation of the speed distribution of ultrasonic flowmeters . This compensation and correction uses a systematic method to process the errors inherent in the detection part and rely on the errors that cannot be overcome by itself to eliminate or substantially eliminate them. Practice has shown that this method is simple, effective and promising. This method is described in detail in Chapter 8 of this book.
Among many methods to ensure measurement accuracy, online real-flow calibration occupies an important position. In the past, most of the off-line method was used to calibrate the flowmeter . Although the accuracy of the flowmeter calibrated with this method is higher after the error correction, the reference conditions of the timing pipeline are different from the actual use. There are differences in the fluids, and the environmental conditions at the time of calibration are different from the actual environment at the place where the instrument is used, which causes additional usage errors and reduces measurement accuracy. The online real-flow calibration method is an effective method to solve this problem. For example, the oil metering station reserves the standard volume pipe connection port during the construction phase. After connecting to the standard volume pipe, the valve can be used to achieve online real-time calibration of each flow meter in the metering station. Nowadays, the method of online real-flow calibration is also required at natural gas distribution stations.
(3) Improve the reliability of the flow measurement system. If the flow meter used for the safety interlock alarm is unreliable, it should not be operated when the interlock action is performed, which is likely to cause an accident. When the action is not supposed to be chaotic, it may easily lead to an unnecessary stop And cause losses. If the fuel flow meter in the industrial furnace is unreliable, the flow path will be blocked, and the furnace will easily shut down and cause an accident. If the flow meter used for process control is unreliable, it is easy to send wrong information to the adjustment system, which will cause the adjustment system to be out of balance, disrupt the stability of the production process, affect product quality, output and material consumption, and cause losses. If the flowmeter used for financial settlement measurement is unreliable, it will easily cause measurement inaccuracy, cause measurement disputes and bring losses to the enterprise. It can be seen that the reliability of the flow meter is extremely important.
The way to improve the reliability of flow measurement is mainly to improve the reliability of the instrument itself, and select a highly reliable instrument for reliability design. In recent years, the reliability of flow measurement instruments has been significantly improved, mainly in the following aspects.
① The reliability of the meter itself has been significantly improved.
② The system reliability is improved by improving the structural design of the instrument. For example, the continuous flow plug-in structure can be used to replace the flow meter without affecting the process operation.
The clip-on structure of the ultrasonic flow sensor, the pressure-replacement structure of the electromagnetic flow sensor electrode, and the structure of the vortex flow sensor using an ultrasonic probe installed outside the tube can greatly shorten the repair time after the instrument is damaged.
③ Introduce redundant technology. If dual sensors are used, and the normality of the sensors is automatically judged, the faulty signal is eliminated.
④ Introduce self-diagnostic technology, and send the diagnosis result to the operation station or special equipment management system (AMS) through fieldbus to display alarms, find faults in time, and take early measures.
(4) Savings In addition to the instrument purchase fee, the cost mentioned here should also be included in the accessory purchase fee, installation and commissioning fee, operating fee, spare part fee, maintenance and regular calibration (verification) fee, and the average life of the meter The amortized depreciation charge is not negligible.
Although the purchase cost of some types of flowmeters is low, auxiliary equipment such as upstream and downstream shut-off valves and bypass valves must be added. Sometimes the cost of auxiliary equipment greatly exceeds the purchase cost of the flowmeter itself.
In the selection of instruments, one-sided pursuit of high performance and high accuracy should be avoided, because doing so not only increases the purchase cost, but also often increases the cost of spare parts. The optimal design and selection is the solution that has the highest reliability, convenient maintenance and the lowest cost on the premise of meeting the requirements for use.
(5) Safety Some tested fluids are flammable and explosive media, and some instrument installation sites are flammable and explosive sites. The selection of instrument, system design and installation should comply with explosion-proof regulations.
In addition to the above goals, other requirements for use, such as pressure loss requirements, sanitary requirements, and protection requirements, should also be met. Attention should also be paid to ease of maintenance, and some should also be considered to facilitate the implementation of compulsory verification.