Nouns and terms
Instantaneous flow: the amount of fluid flowing through the cross section of the pipeline per unit time (m3/h, t/h).
Cumulative flow: the total amount of fluid flowing through the cross section of the pipeline in a period of time (m3, t).
Flowmeter: the measuring instrument used to measure the flow in the pipeline is called flowmeter.
Main quality indicators
Flow range: the maximum and minimum range that can be measured, and the error does not exceed the allowable value.
Range and range ratio: the range is the difference between the maximum flow and the minimum flow; Range ratio is the ratio of maximum flow to minimum flow, also known as range.
measurement error
Basic error:
Accuracy: the ability of flowmeter to indicate the true value close to the measured flow is called the accuracy of flowmeter.
The accuracy levels are: 0. 1, 0.2, 0.5, 1.0, 1.5, 2.5 and 4.0.
Repeatability: the consistency of the indicated value of the flowmeter after repeated measurements under the same working condition reflects the random error of the instrument.
According to the measurement object, there are two types: closed pipeline and open channel;
According to the purpose of metering, it can be divided into total metering and flow metering, and their instruments are called total meter and flowmeter respectively.
According to the measurement principle, there are mechanical principle, thermal principle, acoustic principle, electrical principle, optical principle, atomic physics principle and so on.
Brief introduction of flowmeter
There are many methods and instruments for flow measurement. There are more than 100 kinds of industrial flowmeters. The reason why there are so many varieties is that a flowmeter suitable for any fluid, any range, any flow state and any use condition has not been found so far.
According to the most popular and extensive classification at present, the principle, characteristics, application and development at home and abroad of various flowmeters are introduced respectively.
Serial number flowmeter type global output
per cent
1 differential pressure flowmeter (orifice plate, venturi tube) 45 ~ 55%
2 float flowmeter (also called glass rotor flowmeter) 13 ~ 16%
3 positive displacement flowmeter (ellipse, waist wheel and spiral) 12 ~ 14%
4 Turbine flowmeter 9 ~ 1 1%
5 electromagnetic flowmeter 5 ~ 6%
6 fluid oscillating flowmeter (vortex street, precession) 2.2 ~ 3%
7 ultrasonic flowmeter (time difference, Doppler) 1.6 ~ 2.2%
8 Thermal flowmeter 2 ~ 2.5%
9 Coriolis mass flowmeter 0.9 ~ 1.2%
10 other flowmeter (plug-in flowmeter 1.6 ~ 2.2%)
1. 1 differential pressure flowmeter
Differential pressure flowmeter is an instrument to calculate the flow according to the differential pressure generated by the flow detector installed in the pipeline, the known fluid conditions and the geometric dimensions of the detector and the pipeline.
The differential pressure flowmeter consists of primary equipment (test piece) and secondary equipment (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified in the form of test pieces, such as orifice flowmeter, venturi flowmeter and average tube flowmeter.
Secondary equipment is a variety of mechanical, electronic, electromechanical integration of differential pressure gauge, differential pressure transmitter and flow display instrument. It has developed into a large class of instruments with three types (serialization, generalization and standardization), which can measure both flow parameters and other parameters (such as pressure, liquid level and density).
According to its working principle, the detection components of differential pressure flowmeter can be divided into throttling device type, hydraulic resistance type, centrifugal type, dynamic head type, dynamic head gain type and injection type.
Specimens can be divided into standard and non-standard according to their standardization degree.
The so-called standard specimen is designed, manufactured, installed and used according to standard documents, and its flow value can be determined and measurement error can be estimated without actual flow calibration.
Non-standard specimens are not mature enough to be included in international standards.
Differential pressure flowmeter is the most widely used flowmeter, and its usage ranks first among all kinds of flowmeters. In recent years, due to the advent of various new flowmeters, the percentage of their use has gradually decreased, but it is still the most important type of flowmeter.
Advantages:
(1) The most widely used orifice flowmeter has firm structure, stable and reliable performance and long service life.
(2) The application range is wide, so far no flowmeter can match it;
(3) Specimen, transmitter and display instrument are produced by different manufacturers, which is convenient for scale economic production.
Disadvantages:
(1) measurement accuracy is generally low;
(2) The range is narrow, generally only 3:1~ 4:1;
(3) High requirements for site installation conditions;
(4) Large pressure loss (orifice plate, nozzle, etc. ).
Application overview:
Differential pressure flowmeter has a wide range of applications, which can be used to measure the flow of various objects in closed pipelines, such as fluid: single phase, mixed phase, clean, dirty and viscous flow. Working conditions: normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc. Diameter: from several millimeters to several meters; Flow conditions: subsonic, sonic, pulsating flow, etc. Its consumption in various industrial sectors accounts for about 1/4~ 1/3 of the total consumption of flowmeter.
1.2 Float flowmeter
Float flowmeter, also known as rotary flowmeter, is a variable area flowmeter. In the vertical conical tube that expands from bottom to top, the gravity of the float with circular cross section is borne by the hydraulic power, so that the float can freely rise and fall in the conical tube.
Float flowmeter is the most widely used flowmeter after differential pressure flowmeter, especially in small flow and micro flow.
In the mid-1980s, the sales amount of Japan, Western Europe and the United States accounted for 15%~20% of the flowmeter. The output of 1.990 in China is estimated to be1.2 ~1.4000 units, of which more than 95% are glass conical tube float flowmeters.
Features:
(1) glass cone tube float flowmeter is simple in structure and convenient to use, but its disadvantages are low pressure resistance and high risk of glass tube breakage.
(2) Suitable for small diameter and low flow;
(3) Low pressure loss.
1.3 positive displacement flowmeter
principle
Structural positive displacement flowmeter can be divided into elliptical gear flowmeter, scraper flowmeter, double-rotor flowmeter, rotary piston flowmeter, reciprocating piston flowmeter, disk flowmeter, liquid-sealed rotary cylinder flowmeter, wet gas flowmeter and membrane gas flowmeter according to its measuring elements.
Features (1) high measurement accuracy;
(2) Pipeline installation conditions have no influence on measurement accuracy;
(3) It can be used to measure high viscosity liquid;
(4) wide range;
(5) The direct reading instrument can directly obtain the cumulative amount and total amount without external energy, which is clear and easy to operate.
Disadvantages:
(1) The result is complicated and huge;
(2) The type, caliber and working state of the measured medium have great limitations;
(3) Not suitable for high and low temperature occasions;
(4) Most instruments are only suitable for clean single-phase fluids;
(5) Noise and vibration.
Volumetric flowmeter, differential pressure flowmeter and float flowmeter are listed as the three most used flowmeters, which are often used to measure the total amount of precious media (petroleum products, natural gas, etc.). ).
In recent years, the sales amount of PD flowmeter (excluding domestic gas meter and domestic water meter) in industrialized countries accounts for13% ~ 23% of flowmeter; China accounts for about 20%, and the output of 1990 (excluding domestic gas meters) is expected to be 340,000 sets, of which elliptical gear type and waist wheel type account for about 70% and 20% respectively.
Advantages:
Application overview:
1.4 turbine flowmeter
Turbine flowmeter is the main type of velocity flowmeter. It is an instrument that uses a multi-blade rotor (turbine) to feel the average velocity of fluid and infer the flow or total amount.
Generally, it consists of a sensor and a display, or it can be made into a whole.
Turbine flowmeter, volumetric flowmeter and Coriolis mass flowmeter are called the three products with the best repeatability and accuracy. As one of the top ten flowmeters, its products have developed to the mass production scale of many varieties and series.
Advantages:
(1) high precision, which is the most accurate flowmeter among all flowmeters;
(2) Good repeatability;
(3) Zero drift and strong anti-interference ability;
(4) wide range;
(5) Compact structure.
Disadvantages:
(1) cannot keep the calibration characteristics for a long time;
(2) The physical properties of the fluid have great influence on the flow characteristics.
Application overview:
Turbine flowmeter is widely used in the following measurement objects: petroleum, organic liquid, inorganic liquid, liquefied gas, natural gas and cryogenic fluid are all in Europe and America. The consumption of turbine flowmeter is second only to orifice flowmeter. In the Netherlands alone, there are more than 2,600 gas turbine flowmeters of various sizes and pressures ranging from 0.8 to 6.5MPa to 6.5 MPa, which have become excellent natural gas metering instruments.
1.5 electromagnetic flowmeter
Electromagnetic flowmeter is an instrument for measuring conductive liquid according to Faraday's law of electromagnetic induction.
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 corrosive flow.
In 1970s and 1980s, there was a great breakthrough in electromagnetic flow, which made it a widely used flowmeter, and its percentage of use in flowmeter was increasing.
Advantages:
(1) The measuring channel is smooth and straight, and will not be blocked. It is suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, mud and sewage.
(2) no pressure loss caused by flow detection, and good energy-saving effect;
(3) The measured volume flow is not obviously affected by the changes of fluid density, viscosity, temperature, pressure and conductivity;
(4) Large flow range and wide aperture range;
(5) Corrosive fluids can be used.
Disadvantages:
(1) cannot measure liquids with low conductivity, such as petroleum products;
(2) Gas, steam and liquid with big bubbles cannot be measured;
(3) It cannot be used at higher temperature.
Application overview:
Electromagnetic flowmeter is widely used, and large-caliber instruments are mostly used in water supply and drainage projects; Small and medium caliber is often used in places with high requirements or difficult measurement, such as cooling water control of blast furnace tuyere in iron and steel industry, measurement of pulp liquid and black liquor in paper industry, measurement of highly corrosive liquid in chemical industry and pulp in non-ferrous metallurgy industry; Small caliber and micro caliber are often used in places with health requirements such as pharmaceutical industry, food industry and biochemistry.
1.6 vortex flowmeter
Vortex flowmeter is an instrument that places a streamlined vortex generator in the fluid, and the fluid alternately separates and releases two series of regularly staggered vortex generators on both sides of the generator.
Vortex flowmeter can be divided into stress type, strain type, capacitance type, thermal type, vibration type, photoelectric type and ultrasonic type according to frequency detection methods.
Vortex flowmeter is the youngest flowmeter, but it has developed rapidly and has become a general flowmeter.
Advantages:
(1) simple and firm structure;
(2) There are many kinds of applicable fluids;
(3) high accuracy;
(4) wide range;
(5) The pressure loss is small.
Disadvantages:
(1) is not suitable for low Reynolds number measurement;
(2) Long straight pipe sections are required;
(3) Low instrument coefficient (compared with turbine flowmeter);
(4) The instrument lacks application experience in pulsating flow and multiphase flow.
1.7 ultrasonic flowmeter
Ultrasonic flowmeter is an instrument to measure the flow by detecting the influence of fluid flow on ultrasonic beam (or ultrasonic pulse).
According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam migration method, Doppler method, cross correlation method, spatial filtering method and noise method.
Ultrasonic flowmeter, like electromagnetic flowmeter, is a barrier-free flowmeter because there are no obstacles in the instrument flow channel. It is a kind of flowmeter suitable for solving the problem of flow measurement, especially for large-diameter flow measurement, and it is one of the flowmeters that have developed rapidly in recent years.
Advantages:
(1) can be used for non-contact measurement;
(2) No flow hinders measurement and no pressure loss;
(3) It can measure non-conductive liquid, which is a supplement to the electromagnetic flowmeter without hindrance.
Disadvantages:
(1) The propagation time method can only be used for cleaning liquids and gases; Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles;
(2) The measurement accuracy of Doppler method is not high.
Application overview:
The (1) propagation time method is suitable for clean single-phase liquids and gases. Typical applications include liquid discharged from factories, strange liquid, liquefied natural gas, etc.
(2) Good gas application experience in the field of high-pressure natural gas;
(3) Doppler method is suitable for two-phase fluids with low heterogeneous content, such as untreated sewage, factory discharge liquid, dirty process liquid, etc. Usually not suitable for very clean liquids.
1.8 coriolis mass flowmeter
Coriolis mass flowmeter (hereinafter referred to as CMF) is a direct mass flowmeter, and its principle is that fluid flows in a vibrating tube to generate Coriolis force proportional to mass flow.
The application of CMF in China started late. In recent years, several manufacturers (such as Taihang Instrument Factory) have developed their own supply markets. Several other manufacturers have established joint ventures or introduced foreign technology to produce series of instruments.
1.9 open channel flowmeter
Different from the past, it is a flowmeter used to measure the free surface natural flow in an open channel with incomplete pipes.
The channel with incomplete pipe flow is called open channel, and the one that measures the flow in open channel is called open channel flowmeter.
Open channel flowmeter has many shapes besides circular, such as U-shaped, trapezoidal and rectangular.
The application place of open channel flowmeter is urban water supply and diversion channel; Water diversion and drainage channels, sewage treatment inlet and drainage channels of thermal power plants; Drainage of industrial and mining enterprises, water conservancy projects and agricultural irrigation channels. Some people estimate 1995 sets, accounting for 1.6% of the total flowmeter, and there is no estimated data for domestic application.
Research and development of working principle of new flowmeter
2. 1 electrostatic flowmeter
Tokyo institute of technology has developed an electrostatic flowmeter suitable for measuring the flow rate of low conductive liquid in oil pipelines.
The metal measuring tube of the electrostatic flowmeter is insulated from the pipeline system, and the charge in the measuring tube can be known by measuring the electrostatic charge on the capacitor. They tested the actual flow of copper, stainless steel, plastic and other measuring tube instruments with inner diameter of 4 ~ 8 mm. The experiment shows that the relationship between flow and charge is close to linear.
2.2 comprehensive effect table
The working principle of the instrument is based on the deformation of the instrument cavity caused by the momentum and pressure of the fluid, and the deformation of the composite effect is measured to obtain the flow rate. The instrument was developed by GMI Engineering Management College and has applied for two patents.
2.3 tachometer flow sensor
It was developed by the Industrial Instrument Company of Russian Science and Engineering Center according to the principle of levitation effect. This instrument has been successfully applied in many fields (for example, more than 2,000 nuclear power plants have been installed and hot water flow has been measured for 8 years), and it is still being continuously improved to expand its application fields.
Application and Development Trend of Several Flowmeters
3. 1 coriolis mass flowmeter (CMF)
More than 30 series of CMF have been developed abroad, and the technical focus of each series is: structural design innovation of flow detection metering tube; Improve the zero stability and accuracy of the instrument; Increase the deflection of the measuring tube and improve the sensitivity; Improve the stress distribution of the measuring tube, reduce fatigue damage and enhance the anti-vibration interference ability.
3.2 Electromagnetic Flowmeter (EMF)
Since EMF entered the industrial application in the early 1950s, its application field has been expanding day by day. Since the late 1980s, EMF has accounted for 16%~20% of flowmeter sales in various countries.
China has developed rapidly in recent years, and the sales volume of 1994 is estimated at 6500~7500 units. ENF with a maximum diameter of 2~6m has been produced in China, and it has the ability to verify the equipment with a diameter of 3 m.
3.3 Vortex Flowmeter (USF)
USF entered industrial application in the late 1960s, and has accounted for 4%~6% of flowmeter sales in various countries since the late 1980s. 1992 global sales are estimated to be 354,800 units, while domestic products are estimated to be 8,000-9,000 units in the same period.
4 conclusion
As can be seen from the above, although the flowmeter has become increasingly mature today, its types are still extremely diverse, and no flowmeter is suitable for any occasion.
Each flowmeter has its scope of application and limitations. This requires us to:
(1) When choosing an instrument, you should be familiar with both the instrument and the measured object, and other factors should be considered, so that the measurement will be accurate;
(2) Strive to develop new instruments to make them more perfect on the existing basis.
Physical parameters related to flow
In flow measurement and calculation, some physical properties of fluid (fluid properties) are used, which have great influence on the accuracy of flow measurement and the selection of flowmeter. We only introduce the basic concepts and some simple formulas of these physical parameters, and you need to consult relevant manuals for detailed data.
1. Fluid density
The density of a fluid is defined by the following formula
Rho-fluid density, kg/m3;
M—— mass of fluid, kg;
V—— fluid volume, m3.
(1) Density of liquid
When the pressure is constant, the calculation formula of liquid density is:
ρ-density of liquid at temperature t, kg/m3;
ρ Density of liquid at 20-20℃, kg/m3;
μ-volume expansion coefficient of liquid, 1/℃;
T-temperature of liquid,℃.
When the temperature is constant, the calculation formula of liquid density is:
ρ1-density of liquid under pressure P 1, kg/m3;
ρ 0-density of liquid under pressure P0; Kg/m3;
β-volume compressibility of liquid1/MPa;
P0, p 1- pressure of liquid, Mpa.
Usually, the change of pressure has little effect on the density of liquid, which can be ignored below 5Mpa, but for hydrocarbons, pressure correction should be carried out even at low pressure.
(2) Density of gas
The formula for calculating the density of dry gas under working conditions is:
ρ-density of dry gas under working condition, kg/m3;
ρn—— density of dry gas in standard state (293. 15k, 10 1.325kPa), kg/m3;
P—— absolute pressure of gas under working conditions, kPa；;
PN—— absolute pressure in standard state,101.325kpa;
T—— absolute temperature of gas under working condition, k;
TN—— absolute temperature in standard state, 293.15k; ;
Zn-compressibility of gas in standard state;
Z is the compressibility of gas under working conditions.
2. Fluid viscosity
The viscosity of a fluid is called the property that the fluid itself hinders the relative sliding of its particles. The viscosity of a fluid is measured by viscosity. The viscosity of the same fluid changes with the temperature and pressure of the fluid. Generally, when the temperature increases, the viscosity of the liquid decreases and the viscosity of the gas increases. The viscosity of liquid only needs pressure correction under very high pressure, while the viscosity of gas is closely related to pressure and temperature. There are two common methods to characterize fluid viscosity:
(1) dynamic viscosity
η —— hydrodynamic viscosity, Pa? s；
τ-internal friction per unit area, Pa;
—velocity gradient, 1/ sec;
U—— fluid velocity, m/s;
H- the distance between two fluids, m.
(3) The ratio of the dynamic viscosity of a fluid with kinematic viscosity to its density is called kinematic viscosity.
V—— kinematic viscosity m2/s
3. Thermal expansion rate
The coefficient of thermal expansion refers to the fluid temperature change 1? C when the relative change rate of its volume, namely:
β-coefficient of thermal expansion of fluid, 1/℃;
V—— original volume of fluid, m3;
V refers to the volume of fluid expanded due to temperature change, m3;
T- fluid temperature change value,℃.
4. Compression coefficient
Compressibility coefficient refers to the rate of change of fluid volume when the temperature is constant and the pressure changes, that is:
K-compressibility of fluid,1/pa;
V is the fluid volume m3 when the pressure is p;
P- increased pressure? Change of fluid volume under pressure, m3.
5. Reynolds number
Reynolds number is a dimensionless quantity representing the ratio of fluid inertia force to viscous force, which is defined as:
V-average velocity of fluid, m/s;
L refers to the characteristic length of flow velocity, such as the value of the inner diameter of the pipeline in a circular pipeline, m;
Independent variable-kinematic viscosity of fluid, m2/s
Reynolds number can be used to judge the flow state. Generally speaking, Reynolds number Re < 2300 is laminar, Re = 2000 ~ 4000 is transition, and Re > 4000 is turbulent.
I hope it can be used.