Classification of flow meters and flow instruments

What is a flow meter or flow meter? Flow meter or flow instrument refers to a measuring instrument for fluid flow. Flow meter is one of the important instruments in industrial measurement. With the development of industrial production, the requirements for the accuracy and range of fluid flow measurement are getting higher and higher. Flow meter Technology is changing with each passing day. In order to adapt to various uses, various types of flow meters have been developed one after another. There are currently more than one hundred types of flow meters in use. From different perspectives, flow meters have different classifications. There are two commonly used classifications, one is based on the measurement principle used by the flow meter; the other is classified based on the structural principle of the flow meter.

1. Classification by measurement principle

(1) Mechanical principle: Instruments based on this principle use the differential pressure type and rotor type of Bernoulli's theorem; they use the impulse type of Dongliang's theorem. , movable tube type; the direct mass type using Newton's second law; the target type using the Dongliang principle of fluids; the turbine type using the angular momentum theorem; the vortex type and vortex type using the principle of fluid vibration; using the total static pressure difference Pitot tube type as well as volumetric and weir trough types, etc.

(2) Electrical principle: Instruments based on this principle include electromagnetic type, differential capacitance type, inductance type, strain resistance type, etc.

(3) Acoustic principle: There are ultrasonic, shock wave, etc. that use acoustic principles to carry out flow.

(4) Thermal principle: There are thermal type, direct calorimetric type, indirect calorimetric type, etc. that use thermal principles to measure flow.

(5) Optical principle: laser type, photoelectric type, etc.

(6) Principles of atomic physics: nuclear magnetic resonance formula, nuclear radiation formula, etc.

2. Classification according to the structural principles of flow meters

According to the actual situation of current flow meter products and the structural principles of flow meters, they are divided into several categories:

1 , Positive displacement flow meter

A positive displacement flow meter is equivalent to a container with a standard volume and continuously measures the flowing medium. The greater the flow rate, the greater the number of measurements and the higher the output frequency. The principle of positive displacement flow meter is relatively simple. Suitable for measuring media with high viscosity and low Reynolds coefficient. Depending on the shape of the rotary body, the products currently produced are: oval gear flowmeter, waist wheel flowmeter (Roots flowmeter), rotating piston and scraper flowmeter suitable for measuring liquid flow; servo volumetric flowmeter suitable for measuring gas flowmeter meter, diaphragm type and rotary flow meter, etc.

2. Impeller flow meter

The working principle of the impeller flow meter is to place the impeller in the fluid to be measured, and rotate under the impact of the fluid flow. The speed of the impeller rotation is determined by the speed of the impeller rotation. Reflects the size of the flow. Typical impeller flowmeters are water meters and turbine flowmeters, and their structures can be mechanical rotation output or electrical pulse output. Generally, water meters with mechanical transmission output have low accuracy, but have simple structure and low cost. They have been mass-produced in China and are standardized, universal and serialized. Turbine flow meters with electrical pulse signal output have higher accuracy.

3. Differential pressure flowmeter

The differential pressure flowmeter consists of a primary device and a secondary device. The primary device is calledThe flow measuring element is installed in the pipeline of the fluid to be measured, and produces a pressure difference proportional to the flow rate (flow rate) for flow display by the secondary device. The secondary device is called a display instrument. It mainly receives the differential pressure signal generated by the measuring element and converts the corresponding flow rate for display. The primary device of a differential pressure flowmeter is often a throttling device, and the secondary device is a variety of mechanical, electronic, and combined differential pressure meters equipped with flow display instruments. The differential pressure sensitive components of differential pressure meters are mostly elastic components. Since differential pressure or flow rate has a square root relationship, flow display instruments are equipped with a square root device to linearize the flow scale. Most instruments are not equipped with a flow accumulation device to display the accumulated flow rate for economical calculation. This method of using differential pressure to measure flowmeters has a long history and is relatively mature. It is generally used in important occasions in all countries around the world and accounts for the mainstream of flow measurement methods. Power plants use this method for flow measurement of main steam, feed water, condensate water, etc. flow meter.

4. Area flow meter

The float placed in the tapered flow channel with a large upper and smaller size moves due to the force of the fluid flowing from bottom to top. When this force is balanced by the "revealed weight" of the float (the weight of the float itself minus the buoyancy of the fluid on which it is acting), the float is at rest. The height of the float at rest can be used as a measure of the flow rate. Since the flow cross-sectional area of ​​the flow meter 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 flow meter is called a variable area flow meter or a constant pressure drop flow meter. A typical instrument for this flow meter is a rotor (float) flow meter.

5. Momentum flow meter

A flow meter that uses the momentum of the fluid to reflect the flow is called a momentum flow meter. Since the momentum P of the flowing fluid is related to the density and flow rate of the fluid Proportional to the square of v, and p=V?, when determined by the flow cross-section, v is proportional to the volumetric flow rate Q, so p=Q?. Suppose the proportional coefficient is A, then Q=A. Therefore, if P is obtained, the flow rate Q can be reflected. Most of this type of flowmeter uses detection elements to convert momentum into pressure, displacement or force, etc., and then measures the flow rate. Typical instruments for this type of flow are target and rotating vane flowmeters.

6. Impulse flowmeter

A flowmeter that uses the impulse theorem to measure flow is called an impulse flowmeter. It is mostly used to measure granular solid media and is also used to measure mud. , crystalline liquid and abrasive materials and other flows. Flow measurement ranges from a few kilograms to nearly 10,000 tons per hour. A typical instrument is a horizontal component impulse flowmeter. Its measurement principle is that when the measured medium falls freely from a certain height to a measuring plate with an inclination angle, an impulse is generated. The impulse is proportional to the mass flow rate of the horizontal component force, so the measurement This horizontal component can reflect the size of the mass flow rate. According to the signal detection method, the flow meter is divided into displacement detection type and direct force measurement type.

7. Electromagnetic flowmeter

The electromagnetic flowmeter uses the movement of a conductor in a magnetic field to produce an induced electromotive force, and the induced electromotive force is proportional to the flow rate. It reflects the pipeline by measuring the electromotive force. Made on the principle of flowof. During the measurement process, it is necessary to ensure that the liquid passes through the tube in a full state, so that the measurement accuracy and sensitivity are high. It is often used in industry to measure the flow rate of water, slurry and other media. The measurable* pipe diameter is up to 3000mm, and the pressure loss is extremely small. However, media with low conductivity, such as gas, steam, etc., cannot be used. The cost of electromagnetic flowmeters is high, and their models are susceptible to interference from external magnetic fields, which affects their wide application in industrial pipe flow measurement. To this end, the product has been continuously improved and updated, adding anti-interference modules and developing towards microcomputerization.

In modern industrial production, the operating parameters such as temperature and pressure of the flowing medium continue to increase. Under high temperature and high pressure, the application of direct mass flow meters encounters difficulties due to reasons such as material and structure. , and the indirect mass flow meter is often not suitable for practical application because the density meter is limited by the applicable range of humidity and pressure. Therefore, temperature and pressure compensated mass flow meters are widely used in industrial production. It can be regarded as an indirect mass flow meter. It is not equipped with a density meter, but uses the relationship between temperature, pressure and density. The temperature and pressure signals are calculated into density signals through functions, and the mass flow rate is obtained by multiplying the volume flow rate. . Although the temperature-pressure compensated mass flowmeter has been put into practical use at present, when the measured medium parameters change in a large or rapid range, it is difficult or impossible to compensate correctly. Therefore, further research on mass flowmeters and devices suitable for mobile phone production is required. Density meter is still a topic.

In summary, there are many flowmeters with common structural principles than flowmeters with various structures. For example, they are suitable for open channel flowmeters, various weir flowmeters, and trough flowmeters; they are suitable for large-diameter measurements. Insertion flowmeter for flow; laminar flowmeter for measuring laminar flow; correlation method flowmeter suitable for two-phase flow measurement; as well as laser method, nuclear magnetic resonance method flowmeter and various tracer methods, dilution method flow measurement, etc. With the development of science and technology and the needs of practical applications, new flow meters will continue to emerge, and the types of flow meters will become more complete.