Classification of flow meters.

Classification of flow meters.

Instruments that measure fluid flow are collectively called flowmeters or flowmeters. Flowmeter is one of the important instruments in industrial measurement. With the development of industrial production, the accuracy and scope of flow measurement are increasing, and the flow measurement technology is changing with each passing day. In order to adapt to various USES, various types of flowmeters have been published. More than 100 flowmeters have been used.

Each product has its specific applicability and its limitations. According to the principle of measurement, the principle of power, thermal principle, acoustic principle, electrical principle, optical principle, atomic physics principle, etc.

Classify according to the structure principle of flow meter. Volumetric flowmeter, differential pressure flowmeter, flow meter, turbine flowmeter, electromagnetic flowmeter, flow oscillation flowmeter of vortex flowmeter and mass flowmeter and inserted into the meter.

There are two main types of closed pipe and open channel according to the measurement object. According to the purpose of measurement, it can be divided into total measurement and flow measurement, which are called total meter and flowmeter respectively. Total table measure the rate of flow of a period of time through the pipeline, based on the amount of flow in a short time, actually flowmeter is usually also have accumulated flow device, do the total table is used, and total table equipped with flow transmission device. Therefore, it is of no practical significance to divide the flowmeter and the total meter strictly.

1. Classification according to the principle of measurement.

1. Mechanics principle: the instrument that belongs to such principle has the differential pressure type and rotor type of Bernoulli's theorem; The impulse type and movable tube of momentum theorem are used. Using the direct mass of Newton's second law; Using the target type of fluid momentum principle; Turbine type using angular momentum theorem; The vortex type and vortex street type of the principle of fluid oscillation; Using the pitot tube of total static pressure and volume type and weir, groove type and so on.

2. Electrical principle: the instruments used for such principles are electromagnetic, differential capacitance, inductance, strain resistance, etc.

3. Acoustic principle: ultrasonic wave is used to measure flow rate by using acoustic principle. Acoustic (shockwave) and so on.

4. Thermal principle: using thermal principle to measure the calorific, direct calorific and indirect calorific value of the flow.

5. Optical principle: laser type and photoelectric type are the instruments of this kind of principle.

6. Physics principle: nuclear magnetic resonance, nuclear radiation and so on are the instruments of this kind of principle.

7. Other principles: marked principle (tracer principle, nuclear magnetic resonance principle), relevant principles, etc.

2. Classification by flow meter structure principle.

According to the actual situation of current meter products, according to the structure principle of flowmeter, the following types can be summarized as follows:

1 differential pressure flowmeter.

The differential pressure flowmeter is based on the differential pressure generated by the flow detection in the pipeline, the known fluid condition and the measuring instrument and the geometrical ruler of the pipe to calculate the flow meter.

The differential pressure flowmeter is composed of a unit (detection unit) and a secondary device (differential pressure conversion and flow display instrument). The differential pressure flowmeter is usually classified in the form of test piece, such as orifice flowmeter, venturi flowmeter, uniform velocity tube flow meter, etc.

The secondary device is a variety of mechanical, electronic and electromechanical differential pressure gauge, differential pressure transmitter and flow display instrument. It has developed into three conversions (seriation, generalization and standardization) of high level, type specification of confused kind instrument, it can measure flow parameters, other parameters can be measured, such as pressure, level, density, etc.).

The test pieces of differential pressure flowmeter can be divided into: throttling device, hydraulic resistance type, centrifugal type, dynamic pressure head type, dynamic pressure head gain type and jet flow type.

The test pieces can be divided into two categories according to their standardization: standard and non-standard.

The so-called standard test pieces are designed, manufactured, installed and used according to the standard document, and the flow value and estimated measurement error can be determined without the actual flow calibration.

Non-standard test pieces are less mature and have not been included in international standard.

Differential pressure flowmeter is a kind of widely used flowmeter which occupies the first place in all kinds of flow meters. In recent years, with the advent of various new flowmeters, the percentage of its use has gradually declined, but it is still an important type of flowmeter.

Advantages:

(1) the more porous plate flowmeter structure is strong, the performance is stable and reliable, and the service life is long;

(2) there is a wide range of applications, so far there is no one type of flowmeter that can be compared with it;

(3) the test piece and the transmitter and the display instrument are produced by different manufacturers to facilitate the production of economies of scale.

Disadvantages:

(1) the measurement accuracy is generally low;

(2) narrow range, generally only 3:1~4:1;

(3) high requirements for site installation;

(4) large pressure loss (finger orifice, nozzle, etc.).

Application overview:

The application range of differential pressure flowmeter is very wide, and all kinds of objects have applications in the flow measurement of closed pipelines, such as: single phase, miscible phase, clean, dirty, viscous flow, etc. Working status: normal pressure, high pressure, vacuum, room temperature, high temperature, low temperature, etc. Pipe diameter: from a few mm to a few m; Flow conditions: subsonic, sonic, pulsating flow, etc. It accounts for about a quarter to a third of the total amount used in the industrial sector.

2 orifice flowmeter.

Advantages:

Standard throttling is universal in the world and is recognized by the international standards organization, which can be used without real flow calibration and is unique in the flow meter. The structure is easy to copy, simple, firm, stable and reliable, low price;

Disadvantages:

The repeatability and accuracy of measurement are moderate in the flowmeter. Due to the complexity of many factors, accuracy is difficult to raise. The range is narrow because the flow coefficient is related to the Reynolds number. The general range is only 3:1 ~ 4:1. It is difficult to meet the requirement of long straight pipe length. Especially for larger pipe diameter, the problem is more prominent; Great pressure loss; Normally, the pump needs additional power to overcome the pressure loss of the orifice plate to maintain the normal operation of a hole plate flowmeter.

The hole sharp Angle line inside the hole board ensures the precision, so it is sensitive to corrosion, abrasion, scaling and dirty. It is difficult to guarantee the long-term use accuracy, so it needs to be removed and checked once a year. Using flange connection, easy to produce running, run, drop, leakage problem, greatly increased maintenance work.

Application overview:

Orifice meter is the standard orifice differential pressure transmitter with multiple parameters (or differential pressure transmitting, temperature transmitter and pressure transmitter) form a complete set of high contrast range pressure flow device, can measure the rate of flow of gas, steam, liquid and gas. It is widely used in the process control and measurement of petroleum, chemical, metallurgy, electric power, heating and water supply.

Float flowmeter.

Flow meter, also called the rotor flow meter, is a kind of variable area flowmeter, under a main scale upward vertical cone tube, the circular cross section of float is under the liquid force of gravity, which can float freely in the cone tube rises and falls.

Float flowmeter is a kind of flowmeter with wide application range after differential pressure flowmeter, especially in small and micro flow.

In the mid-1980s, sales in Japan, Western Europe and the United States accounted for 15% to 20% of the flow meter. In 1990, China's production was estimated to be between 12 and 140,000, of which more than 95% were glass cone-tube float flowmeter.

Features:

(1) the glass cone-tube float flowmeter is simple in structure and easy to use. The disadvantage is that it has low pressure resistance and high risk of brittle glass tube.

(2) suitable for small pipe diameter and low flow velocity;

(3) low pressure loss.

4 positive displacement meter.

The volumetric flowmeter, also known as the constant displacement flowmeter, is called the PD flowmeter, which is a kind of high precision in the flow meter. It USES mechanical measure component to divide fluid continuously into the volume of a single known part, according to the measuring chamber of successive repeatedly the number of full of emissions and the volume of fluid to measure the total fluid volume.

Displacement meter according to the measuring element classification, can be divided into oval gear flowmeter, scraper flowmeter, double rotor flowmeter, rotary piston flow meter, reciprocating piston meter, flow meter, liquid seal disks rotary flowmeter, wet gas meter and diaphragm gas meter, etc.

Advantages:

(1) high measurement accuracy;

(2) the installation pipeline condition has no effect on the measurement accuracy;

(3) it can be used for measurement of high viscosity liquid;

(4) wide range;

(5) direct reading instrument requires no external energy to achieve cumulative, total, clear and simple operation.

Disadvantages:

(1) the results are complex and bulky;

(2) there is a large limitation in the type, caliber and medium working state of the tested media;

(3) not suitable for high and low temperature occasions;

(4) most meters only apply to clean single phase fluid;

(5) generate noise and vibration.

Application overview:

The volumetric flowmeter and differential pressure flowmeter and float flowmeter are classified into three types of flowmeter, which are often used in the measurement of the total amount of expensive medium (oil, natural gas, etc.).

In recent years, the sales amount of PD flowmeter (excluding domestic gas meter and household water meter) in industrialized countries accounts for 13% to 23% of the flow meter. Our country accounts for about 20%, and the annual output of 1990 (excluding domestic gas meters) is estimated to be 340,000, of which the elliptic gear type and the waist wheel are about 70% and 20% respectively.

Sewage flowmeter.

The sewage flowmeter is classified according to the principle of measurement:

1 a nodal flow meter, flowmeter, pitot tube flowmeter, velocity-equalizing tube flowmeter, the rotor flowmeter, the target flowmeter, the flow meter is a principle of using the Bernoulli equation, differential pressure signal reflected by measuring the fluid flow;

2. The flowmeter has turbine flowmeter, vortex flowmeter, electromagnetic flowmeter, doppler ultrasonic flowmeter and hot-wire measuring flow meter. These measures are measured by measuring fluid flow velocity.

3. The flow meter has a gear flow meter, a scraper type flow meter and a rotating piston type flowmeter, which can reflect the flow by measuring the small volume of each standard volume.

4, flowmeter with thermal mass flowmeter, the differential pressure type flowmeter, the impeller type mass flowmeter, the elder brother force type mass flowmeter, indirect mass flowmeter, which is by measuring the fluid quality to reflect the flow;

5. The flowmeter has a weir type flow meter, which reflects the flow by measuring the liquid level.

Characteristics of sewage flowmeter:

1. The sewage flowmeter structure is simple, solid and reliable, and the service life is long.

2, the measuring tube has no moving parts and resistance parts, no pressure loss, won't produce congestion measurement, reliable and strong anti-jamming capability Small volume, light weight, easy installation, maintenance, wide measuring range, low volume measurement is not affected by the fluid temperature, pressure, density, viscosity, conductivity, such as the impact of the change, can install on servant road opening reformation, construction installation is simple, small quantities.

Turbine flowmeter.

The turbine flowmeter is the main type in the velocity flowmeter. It USES the multi-blade rotor (turbine) to feel the average flow velocity of the fluid, and the flow or total amount of the meter is derived.

Generally, it consists of two parts: the sensor and the display instrument.

Turbine flowmeter and volumetric flowmeter, coriolis mass flowmeter is called flowmeter in three kinds of repeatability, high precision products, as one of the top ten type flowmeter, its products has developed series of multiple varieties, more than the size of the batch production.

Advantages:

(1) high precision, which is the most accurate flowmeter in all flowmeters;

(2) good repeatability;

(3) zero point drift, good anti-interference ability;

(4) wide range;

(5) compact structure.

Disadvantages:

(1) the calibration characteristics cannot be maintained for a long time;

(2) fluid property has a great influence on flow characteristics.

Application overview:

The turbine flowmeter is widely used in the following measurement objects: petroleum, organic liquid, inorganic liquid, liquefied gas, natural gas and cryogenic fluid.

In Europe and the United States, the turbine flowmeter in dosage is the natural next to orifice meter measuring instrument, only the Netherlands on the gas pipeline is adopted more than 2600 sets of various sizes, from 0.8 to 6.5 MPa pressure of gas turbine flowmeter, which has become an excellent natural gas meters.

7 vortex street flowmeter (USF)

Vortex street flowmeter is a non-streamline vortex generator in the fluid, and the fluid is separated by alternately on both sides of the body. When the flow section is certain, the flow rate is proportional to the volume flow. Therefore, the measurement of the oscillation frequency can be measured. Vortex street flowmeter can be divided into: stress, strain, capacitor, thermal, vibration, photoelectric and ultrasonic. This flowmeter was developed and developed in the 1970s. Due to its advantages of non-rotating parts and pulse digital output, it has a promising future.

advantages

(1) vortex street flowmeter has no moving parts. The measuring element is simple in structure, reliable in performance and long in service life.

(2) the measuring range of vortex street flowmeter is wide. The range is about 1:10.

(3) the volume flow of vortex flowmeter is not affected by thermal parameters such as temperature, pressure, density or viscosity of the measured fluid. It does not need to be individually calibrated. It can measure the flow of liquid, gas or steam.

(4) it causes less pressure loss.

(5) the accuracy is high, the repeatability is 0.5%, and the maintenance volume is small.

disadvantages

Vortex flowmeter are (1) the working status of the volume flow is not affected by measurement of body temperature, pressure, density, the influence of thermal parameters, such as the liquid or steam, mass flow measurement results should be for gas, should be a standard volume flow measurement results. Mass flow or standard volume flow must be converted through fluid density, and fluid density changes caused by fluid working conditions must be considered.

(2) the factors that cause the error of flow measurement are mainly: measurement errors caused by uneven flow velocity of pipelines; The dielectric density in the fluid condition can not be determined accurately. The wet saturated steam is measured as dry saturated vapor. If these errors are not limited or eliminated, the total measurement error of vortex flowmeter will be very large.

(3) poor vibration resistance. The external vibration will cause the vortex flowmeter to generate measurement error and not even work normally. The high velocity impact of channel fluid can cause additional vibration to the cantilever of vortex street and reduce the measurement accuracy. The influence of large pipe diameter is more obvious.

(4) poor adaptability to measurement of dirty media. Vortex flowmeter's occurrence body is very easy to be polluted by the media or fouled by filth, changing geometry size, which has a great impact on the measurement accuracy.

(5) high requirement of straight pipe section. The expert points out that the vortex flowmeter straight pipe section must ensure the front 40D 20D, to meet the measurement requirements.

(6) poor temperature resistance. Normally only vortex flowmeter measurement under 300 ℃ medium fluid flow.

USF entered the industrial application in the late 1960s, and has accounted for 4%~6% of the sales amount of the flow meter in each country since the late 1980s. In 1992, the worldwide estimated sales volume was 3.548 million, while the domestic product was estimated at 8000~9000.

8 electromagnetic flowmeter (EMF)

The electromagnetic flowmeter is a kind of instrument for measuring the conductivity of the liquid according to the law of Faraday's electromagnetic induction.

The electromagnetic flowmeter has a series of excellent characteristics, which can solve the problems that other flowmeters are not easy to apply, such as the measurement of dirty flow and corrosion flow.

In the 1970s and 1980s, there was a major breakthrough in the technology of electromagnetic flow, making it a widely used type of flowmeter, and its usage percentage increased steadily in the flow meters.

Advantages:

(1) the measurement channel is a smooth straight pipe, which is not obstructed. It is suitable for measuring liquid solid two-phase fluid containing solid particles, such as pulp, mud, sewage, etc.

(2) there is no flow detection.