27.06.2022

A flow meter is an instrument that indicates the flow rate being measured and/or the total amount of fluid over a selected time interval. Simply put, it is an instrument used to measure the flow of fluid in a pipe or open channel.


Regarding the principle of the flow meter, one is more difficult to understand. How does it work?

1. It relies on the pressure head of the measured medium to push the oval gear to rotate for measurement.

2. The larger the viscosity of the medium, the smaller the leakage from the gear and the metering air gap. Therefore, the larger the sticky skin of the medium to be measured, the smaller the leakage error, and the better it is for the measurement.

3. It is suitable for measuring the flow of high-viscosity media, but not for fluids containing solid particles (solid particles will jam the gears, making it impossible to measure the flow). If the measured liquid medium contains gas, it will also cause measurement errors.


Different types of flowmeters

(1) Differential pressure flowmeter

Differential pressure flowmeter is an instrument that does not measure the differential pressure generated by the flow detection element installed in the pipeline, the known liquid conditions and the geometric dimensions of the detection element and the pipeline. Such as orifice plate flowmeter, Venturi tube flowmeter and uniform velocity tube flowmeter.


In recent years, within the range, the differential pressure flow accounts for about 50%-60% of the total number of flow meters. The accuracy of the differential pressure flowmeter is largely determined by the conditions of use in the field. The accuracy of the entire flowmeter also depends on the accuracy of the differential pressure transmitter and flow indicator. Therefore, the differential pressure flowmeter is a kind of instrument with strict requirements from design, manufacture to installation and use, and mistakes in any link will cause great errors. In addition, the output signal of the differential pressure flowmeter has a square relationship with the flow rate, which is a nonlinear instrument with a narrow range. Large pressure loss is also one of its weaknesses. In terms of installation conditions, like other inductive flows, a long straight pipe section is required.


(2) Positive displacement flowmeter

Positive displacement flowmeters use mechanical measurement files to continuously divide the liquid into a single known volume portion, and repeatedly fill and discharge the volume portion and the fluid according to the number of times the metering chamber does not measure the total volume of the liquid.


It is the class with the highest accuracy in the flow meter. The advantages of positive displacement flowmeter are as follows: high degree, the basic error is generally ±0.5%, and the special one can reach ±0.2%; there is no requirement for the straight pipe section in front, which is of great significance in field use; it can be used in high viscosity The measurement of fluids has a wide range, generally 10:1 to 5:1. It is a layer direct-reading instrument that requires no external energy and is easy to operate.


Any instrument has its advantages as well as its limitations. The disadvantages of positive displacement flowmeters are mainly manifested in: complex structure, large volume and heavy weight, so they are generally only suitable for small and medium calibers; Large, narrow range of application; poor safety, such as the detection of moving parts stuck, the fluid cannot pass; some forms of positive displacement flowmeters will bring pulsation to the flow during the measurement process. Due to the high precision, when metering expensive media in industrial sectors such as petrochemical, pharmaceutical, food and energy. However, due to the need for regular maintenance, it is not suitable for maintenance sites such as radioactive toxic fluids that do not allow access to people. At present, the positive displacement flowmeter is used as a trade settlement storage and transportation handover instrument.


(3) Float flowmeter

The float flow meter is a volume flow meter in which the float rises and falls with the flow change in the vertical conical tube, and the flow area between them is changed to measure. Also called rotameter.


Rotameters are suitable for small pipe diameters and low flow rates. Commonly used instruments have a diameter of 40-50mm or less, and the smallest diameter is 1.5-4mm. Float flowmeters do not have high requirements for straight pipe sections and have a wide flow range. At present, it is widely used in electric power, petrochemical, metallurgy and other process industries and public utilities such as sewage treatment. It is mainly used as a visual flow indication or a field indicating instrument that does not require high measurement accuracy.


Except for these there types, there are also many other flowmeters like electromagnetic flowmeter, vortex flowmeter, turbine flowmeter, ultrasonic flow meter, orifice flowmeter, metal tube rotameter, etc. And various of device is used for measuring as well, for example there are pressure transmitters and differential pressure transmitters, ultrasonic level gauge and radar level gauge and flow totalizer.

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