Turbine flowmeter characteristics in pulsating flow
1 Introduction
Turbine flowmeter The so-called pulsating flow means that the flow rate of the fluid in the measurement area is a function of time (but there is a constant average value over a sufficiently long period of time. The presence of the pulsating flow causes the flowmeter to have a measurement error (not even normal). Work, how to correct or reduce the impact of pulsation on flow measurement characteristics (is an important topic in flow measurement.
2. Influence of pulsation on flow measurement of turbine flowmeter
2.1 Error equation and its calculation
The wing theory is applied to analyze the driving torque and the resisting torque acting on the turbine rotor (the equation of motion can be obtained: 
Where J is the blade moment of inertia θ is the angle between the blade and the axis r is the average radius of the turbine blade, A is the pipe runner area, Ï is the fluid density, ω is the rotation of the turbine, and the angular velocity Q is through the pipe flow.
If the pulsating flow is expressed as 
After analysis and analysis, the relationship between the rotational acceleration of the turbine and the parameters of the pulsating flow can be obtained: 
Where c is the value of ω at steady state. At this time, the display error can be expressed by the following formula: E=ω/C--1
For a specific turbine flowmeter and different pulsating flows, the ω(t) corresponding to each discrete point in the pulsation period can be calculated by (2), and the turbine flowmeter display error E can be calculated and drawn. Corresponding curve.
2.2 Results and analysis
After calculation and analysis, it is found that the main factor causing the error of the turbine flowmeter is the amplitude and frequency of the pulsating flow. By comparing several patterns, the following rules are found:
- From the quadrant of the curve distribution, the pulsating flow causes a positive error in the turbine flowmeter. When the fluid is pulsating, in the accelerating fluid, the moment of inertia of the blade can cause the rotor speed to slow down, which is slower than the steady-state flow; in the decelerating fluid, the moment of inertia of the blade can cause the rotor speed to increase, exceeding the steady flow. Rotating speed. Since the effect of acceleration is much smaller than the effect of deceleration, the average flow rate displayed by the flowmeter when the pulsating flow is present is much larger than the average flow rate, and a positive error occurs (this error sometimes reaches up to 50%.
- When the pulsation frequency fpx is smaller than the angular frequency ω of the turbine rotor, the flowmeter is similar to the input pulse, the measurement result is close to the true value, and the error caused by the pulsating flow is small. When the pulsation frequency fp is larger than the angular frequency ω of the turbine rotor, the response distortion will cause a large error, and as the frequency increases, the error will eventually stabilize (Fig. 1, Fig. 2).
- When the pulsation frequency is greater than the angular acceleration of the turbine rotor, the variation of the frequency pulsation amplitude can cause a large change in the measurement error of the turbine flowmeter, and the error increases as the pulsation amplitude increases, and the maximum
50%, but eventually stabilized (Figure 3, Figure 4, a is the ratio of pulsating amplitude to steady-state amplitude).
in conclusion
From the above analysis and calculation, the pulsating flow causes the turbine flowmeter to generate a positive systematic error, which is affected by the pulsation frequency and amplitude. When the pulsation frequency is less than the angular acceleration, the error can be considered as zero; when the pulsation amplitude is less than a certain amplitude value, the error can also be considered as not affecting the accuracy of the turbine flowmeter. There is a limit to the effect of pulsating flow on the measurement accuracy of the turbine flow meter.
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