1 commonly used 4 kinds of flow meters
1.1 orifice plate, Venturi flow meter
The orifice plate [3] and venturi flow meter are flow meters manufactured based on the principle of Bernoulli equation, as shown in FIGS. 1 and 2 . It uses the pressure difference generated by the fluid flowing through the throttling device to measure the flow, so it is called differential pressure flowmeter. It is the most widely used type of flow measuring instrument. Its characteristic is that the measured medium is a single-phase, homogeneous Newtonian fluid, and no phase change and precipitated impurities occur when passing through the throttling device, and there must be no material adhesion or aggregation in the throttling device. Applicable to round pipe and straight pipe segment with long distance between upstream and downstream. The flow should be continuous and stable, with the streamline parallel to the axis of the tube. The throttling device has a simple structure, long service life and wide adaptability. It can measure fluids under various working conditions with an accuracy of ±1%. However, the pressure loss is large and the scale is nonlinear.
1.2 electromagnetic flowmeter
The electromagnetic flowmeter [4] is an instrument for measuring conductive liquids based on Faraday's law of electromagnetic induction, as shown in FIG. 3 . That is, the electromagnetic induction principle is used to measure the average flow rate of conductive liquid in the catheter.
Its characteristics are: the measurement channel is a smooth straight pipe, no obstruction, suitable for measuring liquid-solid two-phase fluid (such as pulp, mud, sewage, etc.) containing solid particles, without pressure loss, the measured volume flow is not affected by the fluid density, Viscosity, temperature, pressure and electrical conductivity (>10-5 Ω/cm) influence, large measuring range (flow rate 0.3 ~ 10m/s), wide aperture (3mm to 3m), high measurement accuracy (basic error value (±0.2% to ±0.5%), the output is proportional to the average flow velocity of the measured medium, regardless of the flow state, and can measure the instantaneous pulsation flow. However, it cannot measure liquids with low electrical conductivity (such as petroleum products), cannot measure gases, vapors, and liquids containing large bubbles, and cannot be used to measure mediums of higher temperature (subject to external electromagnetic interference).
1.3 Turbine flow meter
Turbine flow meter [5] is a speed flow meter based on the principle of conservation of momentum moment, that is, using the relationship between the rotational angular velocity of the impeller placed in the fluid and the flow velocity of the fluid, and measuring the volumetric flow through the pipeline by measuring the rotational speed of the impeller. It is a relatively mature and high-precision instrument in current flow meters. Its working principle is shown as 4.
Its characteristics are: simple structure, less processing parts, light weight, easy maintenance, large flow capacity (the same caliber can pass a large flow), easy to achieve pulse long-distance transmission, can adapt to high parameters (high temperature, high pressure and low temperature) Measurement needs, measurement accuracy is high, measurement range is wide, dynamic response is good, pressure loss is small; but some of the physical properties of the measured fluid have a certain influence on the measurement accuracy, can not maintain the calibration characteristics for a long time, clean the measured medium High degree of demand, the fluid temperature, viscosity, density have a greater impact on the indicator value of the instrument, due to wear of rotating parts will cause wear, the useful life of the instrument is affected. Suitable for measuring relatively clean and low viscosity liquids.
2 π Theorem Derivation of Hydraulic Characteristics
2.1 Orifice, Venturi Flowmeter Hydraulic Characteristics
For the orifice plate and venturi flowmeter, the influencing factors of the flow velocity v include: the inlet pipe diameter d1, the hole diameter (throat diameter) d2, the fluid density Ï, the dynamic viscosity coefficient μ, and the pressure difference between the sections Δp. According to Ï€ theorem:
Its dimension [v]=[LT-1];[d1]=[L];[d2]=[L];[Ï]=[L-3M];[μ]=[L-1MT-1] ; [Δp] = [L-1MT-2], choose Ï, v, d1 as the circulation quantity, Δp, μ, d2 are dimensionless numbers Ï€1, Ï€2, Ï€3.
Î 1=ΔpÏavbdc1;Ï€2=μÏavbdc1;Ï€3=d2Ïavbdc1 is substituted into each dimension:
Re, consider the loss, introduce the flow coefficient μQ, get the traffic expression:
From the formula (2), the flow rate Q is related to the flow coefficient μQ, the Reynolds number of the pipeline, and the ratio of pipe diameter to aperture ratio d2/d1.
2.2 Electromagnetic Flowmeter Hydraulic Characteristics
When the conductivity of the fluid is greater than 10-5Ω/cm, the flow rate of the electromagnetic flowmeter has nothing to do with the physical properties of the fluid. The influencing factors of the flow velocity v are: electromagnetic induction intensity e, magnetic field strength B, and pipe diameter D. According to π theorem:
f(v,D,e,B)=0(3)
Its dimension [ω] = [LT-1]; [D] = [L]; [e] = [L2MT-3I-1]; [B] = [MT-2I-1], elect v, D, B is the amount of circulation, e is the dimensionless number π0, π0 = evaDbBc.
Substitute into each dimension to sort out:
The traffic expression:
From equation (4), it can be seen that when the magnetic field strength B and the pipe diameter D are constant, the flow rate Q is only proportional to the electromagnetic induction intensity e.
2.3 Turbine Flow Meter Hydraulic Characteristics
When the turbine structure is constant, the turbine flowmeter velocity v is related to the rotation speed of the impeller, so the influencing factors are: the impeller rotation angular velocity ω, the fluid density Ï, the dynamic viscosity coefficient μ, the pipe diameter D, the resistance torque of the impeller rotation (including friction Resistance and magnetic damping torque, etc.). According to Ï€ theorem:
From Equation (6), we can see that when the pipe diameter D is constant, the flow rate Q is proportional to the turbine speed n, and the Reynolds number Re, the ratio of the turbine rotation resistance to the inertia force of the fluid, and the ratio of the turbine average radius to the pipe diameter.
3 test apparatus and method
The test and test device consists of four flowmeters (Φ25mm) with electromagnetic, turbine, venturi, and orifice plates to form a series pipeline, an automatic switching control device, a computer measurement and control system, a self-circulating water supply system, and an auxiliary replenishment mechanism. As shown.
Orifice plate flowmeter adopts standard orifice plate corner pressure, β=0.450; Venturi flowmeter adopts conical venturi tube, β=0.628; Electromagnetic flowmeter sensor model LDG225S, accuracy class 1; Turbine flowmeter sensor model LWGY225A , Accuracy 0.5. The role of the auxiliary replenishment mechanism is to replenish the water tank when measuring with the weighing method to keep the water level of the tank constant and to form constant motion at constant speed in the pipeline. System diameter Φ25mm, flow range 0 ~ 3m3 / h. Each flowmeter data is instantaneously collected by the computer. The weighing method records the time from the stopwatch, the electronic scale measures the weight, and uses the same flow rate to measure 3 times. The average value is the measured flow rate. Simultaneous measurement of the same flow 4 flowmeters and weighing methods, the measured flow values ​​are different, and the weighing method is used as the true value benchmark. Through analysis and comparison, the performance characteristics of each are revealed.
4 Test data and analysis
This experiment measured the experimental data of flowmeter and weighing method 5 sets, and calculated and summarized according to the test sequence as shown in Table 1 and Table 2. In order to facilitate the study, the weighing method measurement data is X-axis, the difference between the flowmeter data and the weighing method data is the Y-axis, and the relative error is the Y-axis. Flow curves plotted by using the Origin software are shown in FIGS. 6 and 7 . .
Figure 6 and Figure 7 are analyzed. When the flow rate is 0.451m3/h, the water velocity in the tube is 0.255m/s, Reynolds number Re=6336, and it is in a turbulent state. For the electromagnetic flowmeter, the measurement accuracy is mainly affected by the performance of the sensor. When the flow velocity is low, the charge accumulated on the electrode is less, and the longitudinal velocity is significantly affected, so a large deviation occurs; in the process of gradually increasing the flow rate, the longitudinal velocity The impact of the gradual decrease, the error decreases, accuracy, reliability increase, indicating that the electromagnetic flowmeter is less accurate at low flow. For turbine flowmeters, the flow rate is proportional to the turbine speed, which is related to the meter constant. When there is a small flow rate, the deviation occurs and the measured value is small. This is because the mechanical frictional resistance and the magnetic damping torque of the impeller are relatively large compared to the rotational torque. The measured flow rate is smaller; as the flow rate increases, the rotating torque increases, the error gradually decreases, and the accuracy and reliability increase. For the Venturi flowmeter, it belongs to smooth throttling. The vortex is less affected by the vortex in the measured process, and there are no rotating parts. The accuracy depends on the test calibration. Therefore, the accuracy and reliability are relatively high, which indicates that the Venturi meter has a wider adaptability. For orifice flowmeters, the measured data is large at low flow rates because the pressure difference Δp is small and susceptible to disturbances in the flow. There are vortices in the area before and after the orifice plate, so the measurement deviation is large; with the flow rate With the increase, the effect of vortexes has decreased, and accuracy and reliability have increased, indicating that orifice flowmeters are not suitable for measuring small flows. The accuracy of the electromagnetic flowmeter is slightly higher than that of the turbine, venturi, and orifice flowmeter during the flow rate increase of 1.799m3/h from 0.745m3/h, and the accuracy can reach Class 1. In the process of increasing the flow rate from 1.899m3/h to 2.963m3/h, the accuracy of the flowmeters is high, and the curve shown in Fig. 7 tends to be a little. The above analysis shows that electromagnetic flowmeters, turbines, and orifice flowmeters are not suitable for small flow rates, while venturi flowmeters are not sensitive to vortices because they have low pressure loss and are closer to the true value. At high flow rates, the flowmeters have some fluctuations, but the measured values ​​are close to the true value, indicating that their performance is reliable.
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