1. The working principle of vortex flowmeter:
The principle of the vortex flowmeter is to install a stagnant part in the flowmeter pipe. When the fluid flows through the stagnant part, due to the stagnant effect on the surface of the stagnant part, two rows of asymmetric vortices will be generated downstream of the stagnant part. , These vortices are separated behind the side of the stagnant member to form a so-called Karman vortex column. The rotation directions of the two vortices are opposite. Karman theoretically proves that when h/L=0.281 (h is two vortex columns When L is the distance between two adjacent vortices), the vortex column is stable.
The vortex flowmeter is a fluid oscillating flowmeter made based on the Karman vortex principle. That is, placing a non-streamlined and symmetrical object in the flowing fluid (called the vortex generator in the vortex flow sensor) will produce two regular vortices on both sides of the downstream flow, namely the Karmen vortex street, the vortex The frequency is proportional to the incoming flow speed: F=Stu/d
In the formula, F-vortex street frequency d-vortex generator width u-incoming flow velocity St-Strouhal number St is related to the vortex generator width d and Reynolds number Re. When the Reynolds number Re<2×104, St is a variable: when Re is in the range of 2×104～7×106, the value of St remains basically unchanged, and this range is the basic measurement range of the flowmeter. Reynolds number Re is a dimensionless number that characterizes the flow characteristics of viscous fluids, and its physical meaning is the ratio of the inertial force to the viscous force of the fluid flow. The above formula shows that when d and St are fixed values, the frequency F generated by the vortex is proportional to the average flow velocity u of the fluid, and the flow rate of the fluid can be obtained by measuring the frequency of the vortex. This feature is used to make a vortex flowmeter.
2. Features of vortex flowmeter:
1. The vortex flowmeter can be used in almost all occasions where vortex columns can be formed, not only in closed pipelines, but also in open grooves.
2. Wide application range, gas, liquid and steam can be measured.
3. The vortex flowmeter has no movable mechanical parts, the maintenance workload is small, and the instrument constant is stable; compared with the orifice type flowmeter, the vortex flowmeter has a large measurement range, small pressure loss, high accuracy, and no need to be equipped with a guide. Pressure tube, easy to install and maintain.
4. However, there are many environment-related parameters of vortex flowmeters, which are easy to be ignored at the use site and affect the correct performance of the flowmeter.
5. The measuring range of vortex flowmeteris relatively large, generally 10:1
6. When using, pay attention to avoid mechanical vibration, especially the transverse vibration of the pipeline
7. The temperature of the medium also has a great influence on the performance of the vortex flowmeter
3. Common faults of vortex flowmeter:
①The indication is inaccurate for a long time; ②there is no indication; ③the indication fluctuates in a large range and cannot be read; ④the indication does not return to zero; ⑤there is no indication when the flow rate is small; When the indicator changes, the indicator cannot keep up; ⑧The K coefficient of the instrument cannot be determined, and the data in many places are inconsistent.
1. Problems in selection. Some vortex sensors have a larger choice in the selection of the diameter or after the design and selection due to changes in process conditions. The actual selection should be as small as possible to improve the measurement accuracy. The reasons for this are mainly the same. Questions ①, ③, and ⑥ are related. For example, a vortex pipeline is designed to be used by several equipment. Because some equipment in the process is sometimes not used, the current flow rate in actual use is reduced. Actual use causes the original design to select a diameter that is too large, which is equivalent to an increase in the measurable flow rate. The lower limit is not guaranteed when the process pipeline has a small flow rate, and it can be used when the flow rate is large, because it is sometimes too difficult to rebuild. Changes in process conditions are only temporary. It can be combined with the re-tuning of the parameters to improve the accuracy of the indication.
2. Problems with installation. The main reason is that the length of the straight pipe in front of the sensor is not enough, which affects the measurement accuracy. The reason for this is mainly related to the problem ①.
3. The reason for the parameter setting direction. Due to the parameter error, the indicator indication is wrong. The parameter error makes the calculation of the full-scale frequency of the secondary meter wrong. The reason for this is mainly related to the problems ① and ③. The same full-scale frequency makes the indication inaccurate for a long time, and the actual full-scale frequency is too much. The calculated full-scale frequency makes the indication fluctuate in a large range and cannot be read. The inconsistency of the parameters in the data affects the final determination of the parameters. Solution Is to confirm the parameters and re-calibrate.
4. The secondary instrument is faulty. There are many faults in this part, including: there is a disconnection on the circuit board of the instrument, the display of individual digits in the range setting is broken, and the display of individual digits in the K coefficient setting is broken, making it impossible to determine the range setting and K coefficient setting. Part of the reason is mainly related to problems ① and ②. The problem can be solved by fixing the corresponding fault.
5. The line connection problem. On the surface of some circuits, the circuit connection is very good. Check carefully. Some joints are actually loose and the circuit is interrupted. Although some joints are tightly connected, the fastening screws are fastened to the wire skin due to the secondary line problem, which also makes the circuit. Interruption, this part of the reason is mainly related to problem ②.
6. The connection problem between the secondary instrument and the follow-up instrument. The mA output loop of the secondary meter is interrupted due to the problem of the follow-up meter or the maintenance of the follow-up meter. For this type of secondary meter, this part of the reason is mainly related to the problem ②.
7. There is always no indication in the circuit due to the failure of the secondary instrument flat shaft cable. Due to the long-term operation and the influence of dust, the flat shaft cable fails. The problem can be solved by cleaning or replacing the flat shaft cable.
8. For the problem ⑦ is mainly due to the loosening of the fixing screw of the secondary meter display meter coil, which causes the meter head to sink, the pointer and the case of the watch are rubbed, and the movement is not working properly. The problem can be solved by adjusting the meter head and fixing it again.
9. Use environmental issues. Especially for the sensor part installed in the well, the circuit board is damp due to high environmental humidity. This part of the reason is mainly related to the problems ② and ②. The solution is to switch to a split flow meter.
10. Due to poor on-site adjustment, or due to further changes in the actual situation after adjustment. Due to the on-site vibration and noise balance adjustment and sensitivity adjustment is not good. Or due to the further change of the site conditions after a period of operation after the adjustment, the indication problem is caused. This part of the reason is mainly related to the problems ④ and ⑤. Use an oscilloscope and re-adjust according to the operating conditions of the process.