How Turbine Flow Meters Work？

Turbine flowmeter (turbine flowmeter) is a kind of velocity flowmeter. The structure is shown in the following figure:

It is mainly composed of turbine, deflector, casing and magnetoelectric sensor, etc. The bearing of the rotating shaft of the turbine is supported by the deflector fixed on the casing. The casing is made of non-magnetic stainless steel, and the turbine is magnetically conductive stainless steel, which usually has 4 to 8 helical blades.

 

As fluid passes through the flow meter, the turbine is driven to rotate at a speed that is a function of fluid flow. The frequency of the output pulse signal of the non-contact magnetoelectric rotational speed sensor installed outside the casing is proportional to the rotational speed of the turbine. Therefore, the flow rate of the fluid can be determined by measuring the output frequency of the sensor.

 

In order to reduce the axial thrust of the fluid acting on the turbine, the reverse thrust method is used to automatically compensate the axial thrust. It can be seen from the geometry of the turbine that when the fluid flows through the k-k section, the flow velocity increases and the static pressure decreases. An unequal static pressure field is generated between them. The pressure difference formed by it causes the force acting on the turbine rotor (the axial component of this force is opposite to the axial thrust of the fluid) to offset the axial thrust of a part of the fluid, thereby reducing the axial load of the bearing. The use of automatic axial thrust compensation can improve the life and accuracy of the instrument.

 

There is a deflector composed of a guide ring and a guide seat at the fluid inlet, which guides the fluid straight before it reaches the turbine and avoids changing the action angle between the fluid and the turbine blades due to the spin of the fluid, thereby ensuring the accuracy of the instrument. In order to further reduce the influence of fluid spin, a straight pipe section with the same diameter as its diameter should be installed before and after the flowmeter. Generally, thelength of the straight pipe section at the fluid inlet is more than 10 times the diameter of the pipe, and the length of the straight pipe section at the outlet is not less than 5 times the diameter.

 

If the friction of the bearing and the power loss of the turbine are ignored, the relationship between the fluid flow qv through the flowmeter and the frequency of the pulse signal output by the sensor is: 

 

The characteristics of the turbine flow sensor are: high precision, wide measurement range, good repeatability, high pressure resistance, corrosion resistance, digital signal output, simple structure, and convenient use and maintenance. Its limitations are: regular calibration is required, the ordinary type is not suitable for medium with higher viscosity, the fluid density and viscosity have a great influence on the flow characteristics, and the cleanliness of the measured medium is required to be high, and the performance of small-diameter instruments is seriously affected by the flow characteristics.