Introduce the working principles, advantages, disadvantages, causes of measurement inaccuracy, and corresponding solutions.
How Does a Turbine Flow Meter Work? What Are Its Advantages and Disadvantages?
A turbine flow meter measures flow based on the principle of fluid momentum torque. When the measured fluid passes through the sensor of the turbine flow meter, it impacts the turbine blades, causing the turbine to rotate. Within a certain flow range and specific fluid conditions, the turbine speed is proportional to the fluid flow rate. Therefore, the fluid flow rate can be obtained by measuring the turbine speed.
Advantages of Turbine Flow Meters
① High accuracy, typically available in two grades: ±0.5% and ±0.2%;
② Wide measuring range, with the ratio of maximum to minimum linear flow rate ranging from 6:1 to 10:1;
③ Low inertia and fast response, capable of measuring pulsating flow;
④ High pressure resistance and low pressure loss;
⑤ Compact instrument size, facilitating installation and maintenance.
Disadvantages of Turbine Flow Meters
① The instrument constant of a turbine flow meter is calibrated with water at normal temperature. If the operating conditions differ from the calibration conditions, errors will occur;
② During operation, the turbine rotates at a high speed continuously, leading to wear of the rotating parts and potential damage. If the rotating shaft or bearings are replaced, the instrument constant must be recalibrated, which is not feasible for most users due to the lack of calibration equipment;
③ Turbine flow meters cannot measure fluids containing impurities. Installing a filter in front of the flow meter will increase fluid resistance and the workload of disassembling and maintaining the filter;
④ Turbine flow meters measure volumetric flow. If the temperature, density, viscosity, or other properties of the fluid change during measurement, compensation measures must be adopted; otherwise, the measured data will be meaningless.
What Are the Requirements for Installing a Turbine Flow Sensor?
① The sensor should be installed horizontally.
② There must be straight pipe sections or flow straighteners before and after the sensor. The upstream straight pipe section should be no less than 15D, and the downstream straight pipe section should be 5D (where D is the nominal diameter of the sensor).
③ The sensor must be coaxially installed with the front and rear straight pipe sections, and the gasket must not protrude into the pipe.
④ If the measured medium contains impurities, a filter should be installed before the straight pipe section to prevent damage to the sensor's bearings and blades. If the medium contains free gas, a deaerator should also be installed.
⑤ The output signal of the sensor should be transmitted via shielded cables, and reliable grounding must be ensured.
What Precautions Should Be Taken When Using a Turbine Flow Meter?
The following points should be particularly noted during the operation and use of a turbine flow meter:
① When starting the turbine flow meter, the process must be gradual. First, open the upstream valve of the flow meter, then the downstream valve. This avoids water hammer and ensures that the internal cavity of the flow meter is filled with liquid;
② When measuring easily vaporizable liquids, sufficient back pressure must be maintained on the downstream side of the sensor to ensure that all fluid passing through the instrument is in a liquid state;
③ Turbine flow meters are highly sensitive to fluid viscosity. The viscosity of the measured fluid should not exceed 10 cSt (some sources specify 5 cSt). Otherwise, the instrument constant must be recalibrated (1 cSt = 1 mm²/s).
Why Is a Filter Generally Installed Before a Turbine Flow Meter?
Turbine flow meters operate by the fluid pushing the turbine to rotate as it flows through. The entry of solid particles can easily jam the turbine. Fibrous substances in the fluid can become entangled on the blades, leading to measurement inaccuracies.
For turbine flow meters used to measure gas flow, particles such as welding slag entrained in the fluid can damage the blades, resulting in instrument failure.
How Does the "Runaway" Phenomenon of Turbine Flow Meters Occur? What Are Its Hazards? How to Prevent It?
When a turbine flow meter is used to measure gas flow, if the sensor operates under low-pressure conditions; or when used to measure liquid flow, if the sensor outlet is vented to the atmosphere, the impeller is prone to ultra-high-speed rotation, a phenomenon known as "runaway."
Prolonged operation of the turbine at excessive speeds will accelerate the wear of the shaft and bearings.
Methods to prevent "runaway" are as follows:
① For high-pressure gas measurement applications, before switching from the bypass to the main line, close the downstream shut-off valve of the flow meter, fully open the upstream shut-off valve, then slowly open the downstream shut-off valve and gradually close the bypass valve. Continuously monitor the turbine speed until the bypass valve is fully closed.
② For liquid measurement applications, if overspeed is detected, reduce the valve opening or install a restrictor orifice to limit the flow velocity within a safe range.
In addition to turbine flow meters, other rotary flow meters such as positive displacement flow meters, vane-type water meters, and propeller-type water meters also prohibit overspeed rotation. The same preventive measures apply.
