First, the flowmeter structure: Flowmeter consists of the following seven basic components:
1, vortex body
Made of aluminum alloy, with an angled spiral blade, which is fixed to the front of the housing contraction section, forcing the fluid to produce a strong swirling flow.
2, the shell
It has its own flange and a certain shape of fluid channel. According to different working pressure, the shell material can be cast aluminum alloy or stainless steel.
3, smart flow meter calculator
It is composed of temperature and pressure detection analog channels, flow detection digital channels, micro-processing units, liquid crystal drive circuits and other auxiliary circuits, and is equipped with an external transmission signal interface.
4, temperature sensor
The Pt100 platinum resistance is a temperature-sensitive element. In a certain temperature range, the resistance value and temperature have a corresponding relationship.
5, pressure sensor
A piezoresistive diffusion silicon bridge is used as a sensing element, and its bridge arm resistance will undergo expected changes under the action of external pressure. Therefore, under a certain excitation current, the potential difference between its two output ends is proportional to the external pressure.
6, piezoelectric crystal sensor
Installed in the throat near the expansion section of the housing, the frequency signal of the vortex precession can be detected.
7, derotator
Fixed in the outlet section of the housing, its role is to eliminate vortex flow to reduce the impact on the downstream instrument performance.
Second, the working principle:
Natural gas flowmeters For measuring gases, differential pressure flowmeters are the most widely used type of flowmeters, and their use accounts for the highest in all types of flow meters. In recent years, due to the advent of various new flowmeters, the percentage of its use has gradually declined, but it is still the most important type of flowmeter. The differential pressure flowmeter is based on the differential pressure generated by the standard orifice plate installed in the pipeline, the known fluid conditions and the geometry of the standard orifice plate and pipe to calculate the flow meter. The differential pressure flowmeter consists of a primary device (standard orifice flowmeter) and a secondary device (differential pressure transmitter, distributor, controller, and flow computer).
1, torque analysis
The torque acting on the turbine is:
a. The rotational torque Tr generated by the fluid flowing through the turbine, which is the active moment.
b. Mechanical frictional torque Trm generated by the friction between the turbine shaft and the bearing.
c. The flow resistance torque Trf to the turbine when the fluid flows through the turbine.
d. Torque of the electromagnetic resistance generated by the electromagnetic converter to the turbine.
Therefore, the rotational angular speed of the turbine. Can be expressed as:
In the formula, J is the rotational inertia of the turbine. Under normal circumstances, the electromagnetic resistance torque Tre is very small. The turbine rotates at a constant rotational speed, and therefore, the differential of the rotational angular velocity w with respect to time is zero. That is:
O=Tr-Trm-Trf
As shown in the figure, the angle between the guide vane and the axis of the rotor is θ, and the inlet and outlet flow rates of the fluid are u1 and u2. Their angles with the circumferential direction are α1 and α2, respectively. The rotational force produced by the action of the fluid on the rotor is circumferential. According to the principle of momentum, the force in the circumferential direction fr is equal to the change in the momentum of the fluid per unit mass in the circumferential direction, ie:
Fr=gvÏ(u1cosα1-u1cosα2)
In the equation, qv and Ï are the volume flow and density of the fluid.
Because the entrance and exit circular movement speed is equal, there is: ur1=ur2=ur=wr.
The angle between the relative velocity of the fluid leaving the blade and the direction of the circular motion is equal to the blade inclination angle θ. Therefore, there is: β2 = 90°-θ.
Due to no change in the axial component of the fluid flow velocity, there are: u1=u2sinα2
After simplifying, we get: fr=qvÏ(u1tanθ-wr). Therefore, the main thrust torque is: Tr=frr=rqvÏ(u1tanθ-wr)
Consider u1-qv/A. A is the cross-sectional area of ​​circulation. Then there are:
With the meter coefficient K, that is:
Where z is the number of turbine blades and f is the pulse rate generated by the turbine.
Based on the above, there are the following conclusions:
a. When Trm, and Trf are negligible compared to Tr, they can be approximated as zero. At this time, the natural gas flowmeter's volume flow qv is proportional to the frequency of the pulses generated by the turbine.
When the number of blades Z increases, K increases. At the same pulse frequency, the fluid volume flow decreases. In other words, the number of pulses at the same volume flow rate increases.
When the tilt angle θ increases, K increases, and the volumetric flow rate of the fluid at the same pulse frequency decreases. In other words, the number of pulses at the same volume flow rate increases.
When the blade radius r decreases or the flow cross-sectional area A decreases, K increases, and the number of pulses at the same volume flow increases.
When considering the practical application, the turbine needs to overcome the static friction torque before it can rotate. Therefore, Trm is not zero. It is still assumed that the fluid resistance torque Trf is ignored. The flow at the beginning of rotation is called the initial flow. At this time, the output pulse frequency is still zero, that is:
Therefore, the initial traffic qvmin is:
The starting flow rate is related to the mechanical friction torque Trm generated by the friction between the turbine shaft and the bearing, and the large starting torque is also large.
The angle of inclination θ increases. From the above formula, on the one hand, it reduces the starting flow; on the other hand, it increases the mechanical frictional torque and increases the initial flow. Therefore, the tilt angle θ has an optimum value.
The increase in the blade radius r or the decrease in the flow cross-sectional area A can reduce the initial flow. However, it also affects the meter factor K.
If the fluid density is large, the initial flow rate is small. Therefore, the initial flow rate changes when the fluid temperature changes and its density changes.
When the fluid flow is greater than the initial flow, the main thrust force of the fluid mainly overcomes the fluid resistance moment and the dynamic friction torque can be ignored. Therefore, it can be analyzed according to the fluid flow state.
Laminar flow. The resistive moment of fluid laminar flow can be expressed as a linear function of fluid volume flow: Trf = C2μqv
In the formula, C1 is the drag coefficient; μ is the fluid viscosity.
Turbulent flow. The resistance moment of fluid turbulent flow can be expressed as a quadratic function of the fluid volume flow: Trf = C2Ïqv
In the formula, C2 is the resistance coefficient. Ï is the fluid density.
Meter factor K. According to the above, draw the curve of the relationship between the meter coefficient and the fluid flow as shown in the figure.
Visible from the figure.
The flow rate at A corresponds to the initial flow rate. The flow state of the bd segment is in a laminar state, and c and d correspond to the linear minimum and maximum flow rates, respectively. e is the range of error. Natural gas flowmeters can range up to 20:1 or higher.
In general, when the fluid flow rate is greater than the c-point flow rate, it enters the linear operating region, and K can be considered as a constant.
In the figure, the differential pressure Δp is determined according to Bernoulli's equation, which changes with its square relationship as the flow rate increases.
2, the impact of fluid properties
a. Effect of fluid viscosity. The important characteristic of the natural gas flow meter is its high reproducibility, with a typical value of 0.02% and an accuracy of 0.25. According to equation (2-102), the viscosity of the fluid has an effect on the meter factor. Therefore, the viscosity of fluids with high viscosity, such as petrochemicals, should be considered.
Flat blade. Figure 2-51 (a) is the influence of the fluid viscosity on the meter coefficient when the blade is straight. It can be seen that the static friction torque increases with the increase of the viscosity. Therefore, the laminar flow section increases, the linear range section decreases, and the meter factor also increases.
Spiral blade. Figure 2-51 (b) is the effect of fluid viscosity on the coefficient of the meter when it is a spiral blade. It can be seen that as the viscosity increases, the static friction torque does not increase significantly. Therefore, the laminar flow section does not increase much, the linear range section decreases less, and the meter factor may remain substantially constant.
Instrument caliber. In general, the smaller the gauge diameter (<50mm), the more affected by the fluid viscosity.
The effect of fluid density. Fluid density affects the meter factor.
The effect of gas density. The gas density changes with temperature and pressure, because its value is a thousandth of the liquid density, so to obtain the same rotational torque, the gas flow rate should be increased by several dozen times. This results in the bearing friction torque with the use of time The increase has increased drastically. For this reason, in addition to the density compensation, the gas natural gas flowmeter is designed to have a smaller inclination to reduce the impeller rotation speed.
3, other influences
The effect of fluid flow conditions. The flow state of the fluid into the flow meter affects the meter factor. For this reason, there should be enough straight pipe length to fully develop the fluid and eliminate uneven and unstable flow velocity distribution at the inlet.
The effect of the installation method. Horizontal and vertical mounting will affect the meter factor of the flowmeter. The on-site installation method must be the same as the regular installation method, and the installation deviation should not exceed 5°.
4, the pressure loss of natural gas flowmeter
The pressure weight of a natural gas flow meter comes from the mechanical resistance of the turbine rotor to the kinetic energy of the fluid and the viscous drag caused by the fluid hysteresis. The higher the rotational speed, the greater the mechanical resistance; the greater the fluid viscosity, the greater the viscous resistance. According to the Bernoulli equation, the pressure drop of a natural gas flow meter is proportional to the square of the average flow velocity in front of the blade, and is proportional to the fluid density.
5, the impact of structural parameters of natural gas flowmeter on performance
a. Average blade angle θ. The inclination angle θ is small. Under the same size flow rate, the rotation speed of the impeller is increased and the sensitivity of the instrument is improved. However, the increased rotational speed increases the bearing wear and shortens the service life. Usually, the liquid has an inclination of 30°-45°. The gas is 10°-150°.
b. Number of leaves Z. The number of blades Z increases, the frictional resistance torque increases, and the moment of inertia of the impeller also increases. At the same time, the increase of Z increases the accuracy of the meter reading. Typically D=10mm, Z=2; D=15mm, Z=3; 25mm≤D<50mm, Z=6; 50mm≤D<100mm, Z=8; D≥100mm, Z=100.
c. The top diameter of the impeller and the gap between the inner wall of the flowmeter conduit δ. The clearance δ is too large and the fluid kinetic energy cannot be fully utilized. If δ is too small, the viscosity of the fluid increases. Usually D≤10mm, δ=(0.05, 0.07)D; 10mm
d. The ratio of the impeller root diameter to the inner diameter of the flowmeter conduit k, the greater the k, the higher the instrumental booming sensitivity; but it causes the impeller speed to increase, the wear increases, and the pressure loss increases.
E. Blade overlap P. If P is too large, not only will the weight of the impeller increase, but the wear will increase and the pressure loss will also increase. If it is too small, the fluid is easily leaked and the general sensitivity is reduced. Generally P=0.9~1.2.
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