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Gas Flowmeter Measurement Methods Commonly Used in Industrial Measurement

Gas Flowmeter Measurement Methods Commonly Used in Industrial Measurement

Views: Release Time:2022-02-24

Velocity Flowmeter


Velocity flowmeter is a type of flow meter based on the principle of directly measuring the flow velocity of a full pipe in a closed pipeline. The main industrial applications are:



Turbine flowmeter: 

When the fluid flows through the turbine flow sensor, the turbine is rotated by the thrust of the fluid, and its speed is proportional to the average flow velocity of the pipeline. The turbine rotation periodically changes the magnetic resistance value of the magneto electric converter, and the detection coil The magnetic flux changes periodically, generating periodic electrical pulse signals. Within a certain flow (Reynolds number) range, the electrical pulse signal is proportional to the volume flow of the fluid flowing through the turbine flow sensor. The theoretical flow equation of the turbine flow meter is:

 


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Where n is the turbine speed; 

qv is the volume flow; 

A is the fluid physical properties (density, viscosity, etc.), turbine structure parameters (turbine inclination, turbine diameter, flow passage cross-sectional area, etc.) related parameters; 

B is the turbine head clearance, the coefficient related to the fluid velocity distribution; 

C is the coefficient related to the friction torque.


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Vortex flowmeter: 

A non-streamline vortex generator is placed in the fluid, and the fluid is alternately separated on both sides of the vortex generator to release two regular alternating rows of vortex vortex streets. Within a certain flow (Reynolds number) range, the separation frequency of the vortex is proportional to the volume flow of the fluid flowing through the vortex flow sensor. The theoretical flow equation of the vortex flow meter is:


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In the formula, qf is the volume flow under working conditions, m3/s; D is the diameter of the surface body, mm; M is the ratio of the arc area on both sides of the vortex generator to the cross-sectional area of the pipe; d is the front face of the vortex generator width, mm; f is the frequency of the vortex, Hz; Sr is the Strouhal number, dimensionless.


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Precession turbine flowmeter: 

After the fluid passes through the swirler composed of spiral guide vanes, the fluid is forced to rotate strongly around the centerline to form a swirling turbine. The center of the vortex precesses along a conical spiral when passing through the expansion tube. Within a certain range of flow (Reynolds number), the precession frequency of the vortex flow is proportional to the volume flow of the fluid flowing through the precession vortex flow sensor. The theoretical flow equation of the precession vortex flow meter is:


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In the formula, qf is the volume flow under working conditions, m3/s; f is the vortex frequency, Hz; K is the meter coefficient of the flow meter, P/m3 (p is the number of pulses).


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Time-differential ultrasonic flowmeter: 

When the ultrasonic wave passes through the flowing fluid, within the same propagation distance, its propagation speed in the forward direction and in the reverse direction is different. In a wide range of flow (Reynolds number), the time difference is proportional to the volume flow (average flow velocity) of the measured fluid in the pipeline. The flow equation of the ultrasonic flow meter is:


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In the formula, 

qf is the volume flow rate under working conditions, m3/s; 

V is the channel length of the fluid through the propagation path between the ultrasonic transducers 1 and 2; 

L is the ultrasonic wave between the transducers 1 and 2. The length of the sound channel on the inter-propagation path, m; 

X is the axial component on the propagation path, m; 

t1 is the time for the ultrasonic wave to propagate downstream, s; 

t2 is the time for the ultrasonic wave to propagate backward, 

s. Velocity gas flow meters are generally composed of a flow sensor and a display instrument. 

For temperature and pressure changes, a pressure gauge (sensor or transmitter), thermometer (sensor or transmitter), and flow totalizer (temperature and pressure) are required. Compensation) or flow computer (temperature, pressure and compression factor compensation); for occasions with higher accuracy requirements (such as trade natural gas), an online chromatograph can be configured to continuously analyze the components or physical properties of the mixed gas to calculate the compression factor, density, Heat and so on.




Volumetric flowmeter:

Inside the volumetric flow meter, there is a large fixed space and a set of rotating bodies that divide the space into several small spaces of known volume, such as waist wheels, skins, drums, scrapers, elliptical gears, and pistons. , Screw, etc. The rotating body continuously rotates under the action of the fluid pressure difference, and continuously discharges the fluid from a small space with a known volume. According to the number of rotations of the rotating body within a certain period of time, the volume of fluid flowing through can be obtained. The theoretical flow calculation formula of positive displacement flow meter:


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In the formula, qf is the volume flow under working conditions, m3/s; n is the flow velocity of the rotating body, cycle/s; V is the volume of fluid discharged per one revolution of the rotating body, m3/cycle.    

In the standard state, the volume flow calculation formula of a positive displacement flowmeter is the same as that of a velocity flow meter. The gas volumetric flow meter is a mechanical instrument, which is generally composed of a measuring body and an integrator. For temperature and pressure changes, a pressure gauge (sensor or transmitter), thermometer (sensor or transmitter), and flow rate are required. Totalizer (temperature and pressure compensation) or flow computer (temperature, pressure and compression factor compensation).


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Differential pressure flow meter:

The differential pressure flow meter is based on Bernoulli equation and fluid continuity equation. According to the throttling principle, when the fluid flows through the throttling parts (such as standard orifice, standard nozzle, long diameter nozzle, classic venturi nozzle, venturi Li nozzle, etc.), a pressure difference is generated before and after it, and this difference pressure value is proportional to the square of the flow rate. Among the differential pressure flow meters, the standard orifice plate throttling device differential pressure flow meter is simple in structure, low in manufacturing cost, the most fully researched, and it has been standardized and has been the most widely used. The theoretical flow calculation formula of orifice flow meter is:

 


 

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In the formula, 

qf is the volume flow under working conditions, m3/s; 

c is the outflow coefficient, infinite steel; 

β=d/D, infinite steel; 

d is the inner diameter of the orifice under working conditions, mm; 

D is the upstream under working conditions Pipe inner diameter, mm; 

ε is the coefficient of expansion, infinite steel; 

Δp is the differential pressure before and after the orifice, Pa; 

ρ1 is the density of the fluid under working conditions, kg/m3.

For natural gas, the practical calculation formula for natural gas accumulation flow under standard conditions is:


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In the formula, 

qn is the natural gas volume flow rate under standard conditions, m3/s; 

As is the second measurement coefficient, depending on the measurement unit used, this formula As=3.1794×10-6; 

c is the outflow coefficient; 

E is the asymptotic velocity coefficient ; 

d is the inner diameter of the orifice plate under working conditions, mm; 

FG is the relative density coefficient, 

ε is the expansion coefficient; 

FZ is the super-compression factor; 

FT is the flow humidity coefficient; 

p1 is the absolute static pressure of the airflow at the upstream side of the orifice, MPa; 

Δp is the differential pressure generated when the air flows through the orifice plate.

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The above are several commonly used measurement methods for flow meters.

 



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