特点靶式流量计是差压流量计的一个品种，它在工业上的开发应用已有数十年的历史。 Characteristics of target flowmeter Target flowmeter is a kind of differential pressure flowmeter. It has been developed and applied in industry for decades. In the 1970s, China developed electric and pneumatic target type flow transmitters. It is a testing instrument for electric and pneumatic unit combination meters. Since the force converter directly used the force balance mechanism of the differential pressure transmitter at the time, this flowmeter inevitably brought many defects caused by the force balance mechanism itself, such as easy zero drift, low measurement accuracy, and reliable lever mechanism. Poor sex, etc. Due to the drag of the poor performance of the force balancing mechanism, many advantages of the target type flowmeter itself have not been effectively used. So far, users' bad impressions of the old target flowmeter have not been eliminated.
The force converter of the new target type flowmeter uses a strain type force converter, which completely eliminates the shortcomings of the above-mentioned force balance mechanism. The new target type flowmeter also applies microelectronic technology and computer technology to the signal converter and display part. The meter has a series of advantages and I believe it will play an important role in many flowmeters in the future.
Second, the principle and structure
的原理在测量管(仪表表体)中心同轴放置一块园形靶板，当流体冲击靶板时，靶板上受到一个力F，它与流速V，介质密度ρ和靶板受力面积A之间关系式如式(1)所示。 1. Working principle The principle of the target type flowmeter is to place a circular target plate coaxially in the center of the measuring tube (instrument meter body). When the fluid hits the target plate, the target plate receives a force F, which is related to the flow velocity V and the medium density. The relationship between ρ and the target area A is shown in equation (1). (1) where F——force on the target, N; CD——resistance coefficient; ρ——fluid density, kg / m3; V——fluid flow velocity, m / s; A——force on the target Area, m2. After derivation and conversion, the flow calculation formula is as follows: where qm, qv——respectively are mass flow and volume flow, kg / h, m3 / h; α——flow coefficient; D——inner diameter of the measuring tube, mm; β ——Diameter ratio, β = d / D; d——diameter of target plate, mm. The remaining symbols are the same as above. The target board is transformed into an electric signal by a force converter, and after pre-amplification, AD conversion and computer processing, the corresponding flow and total amount can be obtained. 2. Structure The schematic diagram of the target type flowmeter is shown in Figure 2. It consists of a detection device, a force converter, a signal processing and a display instrument. The detection device includes a measuring tube and a target plate, the force converter is a strain gauge sensor, and the signal processing and display instrument can be directly read and displayed in situ or can transmit standard signals at long distances. The structure type of the target type flowmeter can be divided into pipeline type, clamp type and plug-in type, etc., and various types of structure types can also be divided into two types: integrated and separated. The integrated type is a direct reading display on the site, while the separate type separates the digital display from the detection device (generally no more than 100m).
Third, the main features of the flow meter
1. The sensing element has no moving parts, and its structure is simple and firm; 2. The application range and adaptability are very wide. The fluid medium in general industrial processes, including liquid, gas and steam, caliber range (above DN15), various tasks The state (high, low temperature, normal pressure, high pressure) can be applied. It can be said that its application range is more beautiful than that of the orifice flow meter. 3. High accuracy, total measurement can reach 0.2% R; 4. Wide range, 4: 1 ～ 15: 1 to 30: 1; 5. It can solve difficult flow measurement problems, such as measurement containing impurities (particles) Dirty fluids such as crude oil, sewage, high temperature residue, slurry, caustic soda, asphalt, etc. 6. High sensitivity, can measure small flow, and the flow rate can be as low as 0.08m / s;
7. For small caliber (DN15 ～ DN50) and low Reynolds number (Red = 103 ～ 5 × 103) fluid, it can make up for the occasions where standard throttling device is difficult to apply, such as small caliber steam flow measurement, etc .; 8. It is suitable for the measurement of high-parameter fluids with pressures up to tens of MPa and temperatures up to 450 ° C; 9. It can be used for the measurement of bidirectional flowing fluids; 10. Low pressure loss, which is about half that of standard orifice plates; Strong interference ability, the length of the straight pipe section on the upstream side is generally 5-10D; 12. Dry (hanging weight) calibration can be adopted to bring convenience to the user's periodic calibration; 13. Direct-reading meters do not require external energy and are clear It is clear and easy to operate, and can also output standard signals (pulse frequency or current signals); 14. The instrument has a high performance-price ratio and is an economical and affordable flow meter; 15. It is simple and convenient to install and easy to maintain.
越来越广泛地应用于工业生产中的各种流体介质的检测。 With the advancement and development of detection technology and manufacturing technology, pressure-sensitive strain gauge target flow meters are more and more widely used in the detection of various fluid media in industrial production. It has the characteristics of sturdy structure, small resistance drop, less leakage points than other flow meters, high measurement accuracy and sensitivity, and easy maintenance. The torch chemical plant has special process media and has high requirements for the reliability and leakage prevention of the flow meter. The plant used about 70 WFV series pipeline and plug-in target flow meters from the American Welkin Company in its expansion project in 2001. , Mainly used for the measurement of process media, water and steam and other media.
1 The cause of the error:
During the commissioning of the new device in 2002, the author summarized the experience of using some target flowmeters. After the target type flowmeter is installed in accordance with the technical conditions (the target type flowmeter has been calibrated according to the design requirements when it leaves the factory), it is put into operation according to the operating steps. It is found that a small number of meters display a large error and some alarms show full scale. Disassembled at the site and verified by removing the weight and finding that the flowmeter is good. After analysis, the main reasons are as follows. 1) Some large impurities in the fluid are entangled on the target rod, and it is easy to indicate the full range when there is a flow. This phenomenon mainly exists in the measurement of circulating water, which is due to the direct extraction of river water without filtering. 2) Due to installation errors, the target plate of all plug-in target flowmeters cannot be guaranteed to be located in the center of the pipeline, nor can the target plate be completely perpendicular to the pipeline. Therefore, there is a difference between the operating conditions and the calibration conditions before leaving the factory, which causes a measurement error of the target flow meter.
In view of the above reasons, considering that the plant's flow measurement points are not bypassed (reducing leakage points and reducing investment), the plant has formulated the following solutions. 1) In order to prevent larger debris in the fluid from becoming entangled in the target plate (rod), a filter is installed at the pump outlet to filter out these larger debris. 2) Use the comparison method to correct the error of the target flowmeter at the production site. The instrument used: a 4 and a half digital multimeter; a portable ultrasonic flowmeter; necessary wiring clamps and tools. The specific operation steps are as follows: First, install the portable ultrasonic flowmeter according to the requirements (pay particular attention to the electrodes installed), and calibrate and adjust the ultrasonic flowmeter. Then check the target flowmeter installed in place with a digital multimeter according to the calibration book provided by the factory to check and adjust the zero position, range, damping, etc., and finally set the work switch to the measurement position. 1) Set the valve openings at. , 25%, 5000, 750o, 1000o, record the display value of the two flow meters at the same time. 2) Set the valve openings at 10000, 7500, 5000, 25%, and 0 respectively, and record the displayed values of the two flow meters at the same time. 3) Analyze the error of the two flowmeters. If the indications of the two flowmeters are too different, you must find the cause separately, check whether there is a problem with the installation of the ultrasonic flowmeter (especially the two electrodes), and remove the target type flowmeter to perform Check (if there is any foreign object on the target rod, the resistance of the bridge, the connection of the bridge wiring, the setting is correct, etc.), and recalibrate and install. 4) If the difference between the two flowmeters is small or a constant value, the zero position and fullness of the target type flowmeter can be adjusted appropriately, and repeatedly adjusted until the measurement requirements are met. 5) Repeat the operations of 1) and 2), compare the values shown in the two tables, and calculate the material level by referring to the other flows of the production system. Repeat the adjustment until the production requirements are met. After adjustment by this method and more than 1 year of production operation, especially during the start-up of the device in 2004, it proved that the target flowmeter is stable and reliable in the production process, sensitive to response, less failure, less maintenance workload, inspection, adjustment The school is simple. The economic operation was realized in production and the expected purpose was completely achieved.
I hope the above is helpful to everyone. Maybe you are interested in our orifice flowmeter, electromagnetic flowmeter, pressure transmitter, paperless recorder. Welcome new and old customers to consult and order.
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