In the flow measurement, compared with the differential pressure flowmeter, the bar flowmeter is widely used in the industrial field. According to statistics, it accounts for more than 50% of the whole industrial flow measurement. Its measurement system consists of throttling device (primary element), differential pressure transmitter and secondary instrument.
With the rapid development of electronic technology and the wide application of computer (DCS) system, the accuracy, sensitivity and other technical performance of Vespa flowmeter transmitter and secondary instrument have reached a very high level. The bottleneck of flow measurement is the development and application of primary element. For users, how to select the appropriate primary element is related to the measurement accuracy and even the normal use of the measurement system. Several kinds of throttling devices in common use have their own advantages and disadvantages, which limit their application scope to a certain extent.
(l) Throttling flowmeter
Compared with orifice plate, nozzle, venturi and other throttling flowmeter, Vespa flowmeter has a simple structure, a long history, wide adaptability to fluid and has been standardized, and has been recognized by international standards organization. Standard throttling elements can be put into use without real flow calibration. The design manual of throttling device for flow measurement details their design, manufacture, installation and operation Relevant specifications used.
However, the most prominent problem of throttling flowmeter is that the unrecoverable pressure loss is too large, and the use is limited under certain working conditions. If not, additional investment and energy consumption need to be increased. Although the pressure loss of Venturi tube is small, the investment is large, and these throttling devices have high requirements on the pipeline, the front and rear straight pipe sections should meet the requirements, and the measurement accuracy is affected by the use time and The working condition of medium has great influence, and it is difficult to clean up regularly, which is time-consuming and even affects production. In addition, its range ratio (usually 4:l) is difficult to adjust when the working condition changes greatly, which restricts its wide application to a large extent.
(2) Dynamic head flowmeter
The dynamic head flowmeter of Vespa flowmeter belongs to the plug-in flowmeter, which develops rapidly in recent years. Its outstanding advantages are: light weight, low price, convenient installation and maintenance, small pressure loss, etc. It is especially suitable for the application of large diameter flow measurement, but because the geometric dimension of this type of measuring piece is not standardized, the manufacturer generally needs to verify one by one according to the self-determined standard to accurately determine the flow coefficient. Due to the lack of calibration equipment for large diameter flow measurement, the manufacturer only conducts the typical verification.
The representative product of the averaging tube flowmeter is diamond 11 Annubar flowmeter of D.C company of the United States. Compared with the velcrometer, the flowmeter gets its name because the cross section of the detection rod is similar to diamond, which is widely used in China. However, the detection rod is easy to break when the vortex street frequency generated by the fluid flowing through the detection rod resonates with the natural frequency of the detection rod, and the detection rod is easy to break due to the upstream and downstream of the pressure tap At the back flow position, the impurities in the vortex generated by the resistance flow are easy to enter into the cavity of the detection rod, resulting in blockage or even unable to use.
By improving the structure of the detecting rod and the flow equation verified by the experiment, the accuracy, repeatability, reliability and anti blocking performance of the measurement are improved to a certain extent, and its application range is expanded. The structure, measurement principle and typical application in Shaoguan Iron and Steel Co. are introduced below.