Classification and selection of flowmeters

1. Classification according to measurement principle 1. Mechanics principle: Instruments belonging to this kind of principle include differential pressure type and rotor type using Bernoulli's theorem; impulse type and movable tube type using momentum theorem; direct mass type using Newton's second law; Target type; turbine type using the theorem of angular momentum; vortex type and vortex street type using the principle of fluid oscillation; 2. Electrical principle: The instruments used for this kind of principle include electromagnetic type, differential capacitance type, inductive type, strain resistance type, etc. 3. Acoustic principle: There is an ultrasonic type for flow measurement using the acoustic principle. Acoustic type (shock wave type), etc. 4. Thermal principle: There are calorimetric, direct calorimetric, and indirect calorimetric methods for measuring flow using thermal principles. 5. Optical principle: type, photoelectric type, etc. are instruments belonging to this type of principle. 6. Originated from physical principles: nuclear magnetic resonance type, nuclear radiation type, etc. are instruments belonging to this type of principle. 7. Other principles: There are labeling principles (tracing principles, nuclear magnetic resonance principles), related principles, etc. 2. Classification according to the structural principle of the flowmeter According to the actual situation of the current flowmeter products, according to the structural principle of the flowmeter, it can be roughly classified into the following types: (1) Volumetric flowmeter: Volumetric flowmeter is equivalent to a standard A volumetric container that continuously measures a flowing medium. The larger the flow, the more measurements are taken and the output is more frequently. The principle of positive displacement flowmeter is relatively simple, suitable for measuring fluid with high viscosity and low Reynolds number. According to the shape of the rotator, the current products are divided into: oval gear flowmeter, waist wheel flowmeter (Roots flowmeter), rotary piston and scraper flowmeter suitable for measuring liquid flow; servo flowmeter suitable for measuring gas flow Volumetric flowmeter, membrane type and simple flowmeter, etc. (2) Impeller type flowmeter: The working principle of the impeller type flowmeter is to place the impeller in the fluid to be measured and rotate under the impact of the fluid flow, and reflect the flow rate by the speed of the impeller rotation. Typical impeller flowmeters are water meters and turbine flowmeters, and their structures can be mechanical transmission output or electric pulse output. Generally, the accuracy of the water meter output by mechanical transmission is low, and the error is about±2%, but the structure is simple and the cost is low. It has been mass-produced in China and has been standardized, generalized and serialized. The accuracy of the turbine flowmeter output by the electric pulse signal is relatively high, and the general error is±0.2% to 0.5%. (3) Differential pressure flowmeter (variable pressure drop flowmeter): The differential pressure flowmeter consists of a primary device and a secondary device. The primary device is called a flow measuring element, which is installed in the pipeline of the measured fluid to generate a pressure difference proportional to the flow (flow rate) for the secondary device to display the flow. The secondary device is called display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display. The primary device of the differential pressure flowmeter is usually a throttling device or a dynamic pressure measurement device (pitot tube, uniform velocity tube, etc.). The secondary device is a variety of mechanical, electronic and combined differential pressure gauges with flow display instruments. The differential pressure sensitive elements of the differential pressure gauge are mostly elastic elements. Since the differential pressure and the flow rate are in the square root relationship, the flow display instruments are equipped with square root devices to linearize the flow scale. Most of the instruments are also equipped with a flow integration device to display the accumulated flow for economical calculation. This method of using differential pressure to measure flow has a long history and is relatively mature. It is generally used in more important occasions in countries all over the world, accounting for about 70% of various flow measurement methods. This meter is used for flow measurement of main steam, feed water, condensate, etc. in power plants. The current products are divided into: orifice flowmeter, wedge flowmeter, Venturi tube flowmeter, average Pitot tube. (4) Variable area flowmeter (equal pressure drop flowmeter): The float placed in the conical flow channel with a large upper part and a smaller one at the lower part moves under the force of fluid flowing from bottom to top. When this force and the float's“When the display weight' (the weight of the float minus the buoyancy of the fluid it is subjected to) is balanced, the captive is at rest. The rest height of the float can be used as a measure of the flow rate. Since the cross-sectional area of ​​the flowmeter varies with the height of the float, and the pressure difference between the upper and lower parts of the float is equal when the float is stable, this type of flowmeter is called a variable area flowmeter or an equal pressure drop flowmeter. A typical instrument of this type of flowmeter is a rotor (float) flowmeter. (5) Momentum flowmeter: A flowmeter that uses the momentum of the measured fluid to reflect the flow rate is called a momentum flowmeter. Since the momentum P of the flowing fluid is proportional to the density of the fluid and the square of the flow velocity v, that is, pv2, when the flow cross section is determined, v is proportional to the volume flow Q, so p Q2. Assuming that the proportional coefficient is A, then Q=A. Therefore, the measured P can reflect the flow Q. This type of flowmeter mostly uses detection elements to convert momentum into pressure, displacement or force, etc., and then measure the flow. Typical instruments for such flowmeters are target and rotating vane flowmeters.

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