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Keywords: plug-in vortex flowmeter;
Comparison calibration calibration calibration method Introduction Vortex flowmeter with plug-in type is characterized by its low price, light weight, low pressure loss, easy installation and maintenance, etc. It has outstanding advantages in the measurement of large-diameter flow. Large-caliber water flow measurements commonly use plug-in vortex flowmeters. With the implementation of various energy-saving and consumption-reduction measures of the company in recent years, the accuracy requirements for water consumption measurement data of various users have become higher and higher. The accuracy of the company's internal data on large-volume flow measurement data has been highly controversial. Therefore, it must be Find a simple and effective method to compare the measurement data of the plug-in vortex flowmeter as soon as possible to correctly evaluate the accuracy of the metering data.
1 Introduction to plug-in vortex flowmeter The plug-in type vortex flowmeter is a point flowmeter type plug-in type flowmeter, which consists of a measuring head, an insertion rod, an insertion structure, a converter and a meter housing (measurement pipeline).
When the measuring head is inserted into a certain position in the pipeline (generally on the axis of the pipeline or the average flow velocity of the pipeline), the local flow velocity of the medium is measured, and then calculated based on the distribution of the medium flow velocity in the pipeline and the geometric parameters of the instrument and the pipeline. The value of the flow in the pipeline.
The measuring head of the plug-in vortex flowmeter is a pulse-frequency type. Its flow calculation formula is:
Qv=f/K(1)
Where: qv is the volumetric flow rate, m3/s; f is the frequency of the flowmeter, Hz; K is the meter factor of the flowmeter, 1/m3.
2 Plug-in vortex flowmeter calibration points The flowmeter-type plug-in flowmeter has two calibration methods: flowmeter method and flowmeter method.
The flowmeter method calibrates the entire flowmeter. The calibration equipment and method is the same as that of the full-tube flowmeter. However, since the point-flowmeter type plug-in flowmeter is often used for the measurement of large-diameter flow, the corresponding calibration equipment and Calibration is expensive and cannot be used universally. It is only used in certain special occasions (for example, the technical supervision department arbitrates the flowmeter measurement results, the flowmeter's stereotype test, etc.).
The flow meter method uses the flow meter's measuring head as a flow meter to calibrate. Firstly, the measuring coefficient K0 of the measuring head is measured. Then, the correction coefficient is determined according to the fluid and pipeline conditions at the site of use, and then the meter coefficient K of the entire flow meter is deduced from the cross-sectional area of ​​the pipe. For the standard calibration device used in the flow meter method, the medium is a straight groove and the gas is a low-speed wind tunnel.
Generally flowmeter manufacturers do not have the above two standard devices. Actually, more commonly used work-around method is to calibrate the measuring head with a round tube flow standard device to determine the meter coefficient K0 of the measuring head, but it is necessary to determine the test phase of the device at the same time. Some correction factors.
The meter factor K of the point flow meter type plug-in flow meter is usually calculated by correcting the meter coefficient K0 of the measuring head. The calculation formula for the meter factor K is:
K=K0/(αβA)(2)
Where: K0 is the meter coefficient of the measuring head, 1/m3; α is the velocity distribution coefficient; β is the blocking factor; A is the cross-sectional area of ​​the measuring pipe, m2.
3 Comparison method of measuring data of plug-in vortex flowmeter For a long time, the plug-in type vortex flowmeter used by our company has been using the factory data of the instrument since it was put into use. Changes in the process and field environment, the corrosion and wear of the measuring head on the medium, and the deterioration of the performance of the instrument's electronic converter may all affect the measurement accuracy of the instrument. Because the company does not have a standard device for the verification of plug-in vortex flowmeters, when the instrument measurement data error is large, the following methods are often used to simply compare the measurements.
3.1 Comparison of Pump Flow Rate Before, when the operator of the unit used doubts about the measurement data of the plug-in vortex flowmeter, the rated flow at the “specified performance point†on the pump nameplate or the typical pressure-flow characteristic curve of the pump was used. The corresponding flow rate readings are compared. If the two flow values ​​are inconsistent, the instrument is considered inaccurate. The instrument maintenance personnel are immediately notified of the check. However, after the instrument maintenance personnel check the instrument, no abnormality is found. Therefore, both parties have their own views and generate dispute.
Actually, misunderstanding is often caused by the ratio of the pump flow rate to the meter flow rate, because the pump delivery flow is determined by the intersection point of the pump characteristic curve and the piping load characteristic curve. It changes with the operating load characteristics and is indicated on the pump nameplate. The rated flow refers to the flow under a specified condition, and in most cases the actual flow and the rated flow, but in most cases the actual flow and the rated flow will not be consistent. In addition, the rated flow of the pump is also allowed to have a tolerance of 4% to 8%. The head-flow characteristic curve of each pump of the same specification will have a corresponding difference from the typical curve, and the delivery flow will not be the same, even if Is the actual measured pressure head - flow characteristics of the pump, the flow value and the true value may also have error of 2% ~ 3.5%, so can not use the flow rate of the pump as a basis to determine whether the accuracy of the flow meter. However, during daily operation, cross reference can be made. If there is an abnormal change in the difference, it can be used as a fault phenomenon to further inspect the pump, instrument and piping to determine the cause of the fault.
3.2 Comparison with Portable Ultrasonic Flowmeter In recent years, our company has successively introduced the American POLYSONICSDCT7088 type and Japan Fuji FUJIFLD/C type portable ultrasonic flowmeter to evaluate the flow conditions and energy/material balance of the pipe network, or to use Check the operating status of other flow meters installed on the pipeline. In order to resolve the controversy over the accuracy of the company's internal large-volume flow measurement data, we have tried to use a portable ultrasonic flowmeter to account for the amount of water, and further use it to compare with the measurement data of the plug-in vortex flowmeter. However, a portable ultrasonic meter that uses the transit time method for measurement needs to pass through the calibration calculation to intervene in the pipe flow area and the propagation distance. The measurement error is not only related to the clamping position, the characteristics of the pipe, such as the wall material and thickness, the rust condition, and the lining material. It is related to factors such as thickness and acoustic coupling changes. It is also related to the technical level of the installation and commissioning personnel. It is more complex to use and difficult for general personnel to grasp. In actual use, the measurement results of the instrument are not stable and the measurement accuracy cannot be guaranteed. It is difficult to provide effective The results of the data. Therefore, the conditions for on-site comparison between portable ultrasonic flowmeters and plug-in vortex flowmeters are currently immature, and further exploration is needed to accumulate experience.
3.3 Calibration method of plug-in vortex flowmeter Although the theory analysis, maintenance, and debugging can improve the field use of the plug-in vortex flowmeter, the accuracy of the measurement data of the flowmeter will eventually be solved. Controversial, the most convincing or effective data.
My company currently has a set of static volumetric water flow calibration device, the maximum working diameter of 300mm. We tried to remove the controversial plug-in type vortex flowmeter at the site and install it on the 300mm pipeline of the calibration device. The whole flowmeter was calibrated by the flowmeter method, and data was obtained by real flow calibration. Analyze the linearity, repeatability and accuracy of the instrument under this caliber condition, analyze whether the performance of the instrument is qualified, and further make relevant debugging to make the instrument reach the best use condition.
The question now is: How do you determine the usage data of the instrument in the field when the geometric parameters of the pipe installed on site are different from the experimental conditions? It can be known from the correction formula (formula 2) of the meter coefficient K calculation that for the same plug-in type vortex flowmeter, the K0 value is uniquely determined at the same time, and is calibrated and debugged after passing the flow calibration device (300mm diameter pipeline). The instrument itself is not a problem. The actual use factor of the instrument is K' because the diameter of the installation site is different, and K' is only related to the αβA parameter. Therefore, we can determine by calculation and complete the comparison.
3.4 Calibration of plug-in type vortex flowmeters compared with quick method Because computing K' through formulas is quite tedious and difficult to master, the author has compiled a quick check comparison chart based on his own long-term experience and experience. : The table is compiled with the data of D = 300mm as the benchmark for the ratio, so that it can be quickly converted), as listed in Table 1 and Table 2.
Using a flowmeter method to calibrate the plug-in vortex flowmeter on a 300mm pipeline and to pass the test, determine the calibration coefficient K of the instrument when D=300mm, and then check the table 1 and table 2 to find out the actual use of the diameter The ratio of αβA under the condition of 300mm pipe diameter is used to convert the meter coefficient K′ under the condition of the actual use of the meter, and then the calibration correction ratio of the plug-in type vortex flowmeter is completed.
3.5 Application example of plug-in type vortex flowmeter calibration and correction comparison method In our company's circulating water plant, SX88-900L1B2 plug-in type vortex flowmeter (4~20mA analog output type) is used for water quantity measurement. The measurement data of this watch is very controversial. If the flow value of the pump is used as a reference, the measurement error of the instrument is as high as 20%. Although it has been explained above that the pump's flow value cannot be used as the basis for determining the accuracy of the flow meter, they are used in cross-reference with each other in daily operation. After careful examination of the pumps, meters, and relationships by the technicians, it was confirmed that the apparent difference between the two was caused by the meter factor.
Remove the plug-in vortex flowmeter and perform a series of operations according to the calibration and correction comparison method. At the same time, debug the amplifier accordingly, and obtain valid data: The instrument calculates the maximum flow Qmax of the instrument according to the site conditions during use. = 6000m3/h, full-scale frequency fmax = 41.2Hz; use a flowmeter method to calibrate the entire flowmeter installed on a 300mm-diameter pipeline; calibrate the real flow to obtain the meter coefficient K=202.951, and then check the table. Table 2, the actual use of the diameter of 900mm under the condition of the diameter of 300mm and the diameter of the meter (usually calculated than the flow of the instrument), β, A ratio were 1.016,1.055,9.000, calculated and corrected the instrument in the 900mm aperture conditions The meter coefficient K'=K/(1.016*1.055*9.000)=21.038 is converted to the actual maximum flow rate Qmax=7050m<3>/h of the full-scale frequency fmax=41.2Hz to complete the calibration of the plug-in type vortex flowmeter. Corrected the comparison. After further evaluation, the error between the correction result and the traditional theoretical calculation is about 0.5%, which can meet the measurement needs of the site.
In addition, after the meter has been installed on the track, it can be properly debugged according to the actual conditions of the gain and sensitivity of the instrumentation amplifier so that the meter can achieve the best operating status. This is also very important.
4 Concluding remarks After a period of trials, it is proved that it is effective and feasible to use the calibration and correction comparison method to obtain actual usage data of the instrument. We applied this method to amend the instrument data of a large number of plug-in vortex flowmeters with large deviations in measurement data, which was approved by all parties and ensured the accuracy of the company's large-volume flow measurement data. Of course, since the measurement accuracy of the plug-in type vortex flowmeter is greatly affected by the installation of the field pipelines and meters, there is a certain error in the calculation of the correction coefficient itself. Therefore, there are many problems that need to be further explored.
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There are two kinds of calibration methods for the plug-in type vortex flowmeter. The composition of the plug-in vortex flowmeter and the calibration method of the plug-in type vortex flowmeter for the point velocity meter type are briefly described. A simple analysis of the disadvantages of using water pump flow ratio comparison and portable ultra flowmeter comparison method is presented. A quick look-up table method for calibration and correction of plug-in vortex flowmeter is proposed. Examples show that this method is effective and feasible.