1 Overview The instrument used to measure fluid flow (volume of fluid passing per unit time) in a pipeline. There are rotor flow meters , throttle flow meters , fine slit flow meters , volume flow meters , electromagnetic flow meters , ultrasonic flow meters and weirs.
There are many types of flow measurement methods and instruments, and many classification methods. So far, there are as many as 60 types of flow meters available for industrial use. The reason why there are so many types is that no flow meter has been found that is suitable for any fluid, any range, any flow state, and any use conditions.
These more than 60 kinds of flow meters, each product has its specific applicability, but also has its limitations. According to the measurement object, there are two categories: closed pipes and open channels. According to the purpose of measurement, it can be divided into total measurement and flow measurement. The instruments are called total meters and flow meters.
The total meter measures the flow through the pipeline over a period of time.It is expressed by the quotient of the total amount flowing in a short time divided by the time.In fact, the flow meter usually also has a cumulative flow device for the total meter. The total meter is also equipped with a flow signalling device. Therefore, it is no longer practical to distinguish flow meters and total meters in a strict sense.
According to the measurement principles, there are mechanical principles, thermal principles, acoustic principles, electrical principles, optical principles, and atomic physics principles.
According to the most popular and widely used classification method at present, it is divided into: volumetric flowmeter, differential pressure flowmeter, float flowmeter , turbine flowmeter , electromagnetic flowmeter , vortex flowmeter in fluid oscillation flowmeter , quality Flowmeters, plug-in flowmeters, and probe-type flowmeters are used to explain the principles, characteristics, application overview, and domestic and foreign developments of various flowmeters.
1.1 Differential pressure flowmeter Differential pressure flowmeter is a meter that calculates the flow rate based on the differential pressure generated by the flow detection element installed in the pipeline, the known fluid conditions and the geometric dimensions of the detection element and the pipeline.
The differential pressure flowmeter is composed of a primary device (detection piece) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified in the form of test pieces, such as orifice flowmeters , venturi flowmeters , and averaging tube flowmeters.
The secondary devices are various mechanical, electronic, electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments. It has developed into a large type of instrument with a high degree of tri-chemical (serialization, generalization, and standardization) and a variety of specifications.It can measure both flow parameters and other parameters (such as pressure, level, density, etc.). ).
According to their working principle, the differential pressure type flowmeter can be divided into: throttle device, hydraulic resistance type, centrifugal type, dynamic head type, dynamic head gain type and jet type.
Test pieces can be divided into two categories according to their degree of standardization: standard and non-standard.
The so-called standard test parts are designed, manufactured, installed and used in accordance with standard documents, and their flow values and estimated measurement errors can be determined without actual flow calibration.
Non-standard test pieces are of poor maturity and have not been included in international standards.
Differential pressure flowmeter is the most widely used type of flowmeter, and its usage ranks first in various flow meters. In recent years, due to the advent of various new flowmeters, its usage percentage has gradually decreased, but it is still the most important type of flowmeter.
(1) The most widely used orifice plate type flowmeter has a solid structure, stable and reliable performance, and long service life;
(2) It has a wide range of applications, and so far no other type of flowmeter can be compared with it;
(3) Detection parts, transmitters, and display instruments are produced by different manufacturers, which is convenient for economies of scale.
(1) The measurement accuracy is generally low;
(2) The range is narrow, generally only 3: 1 ~ 4: 1;
(3) High requirements for on-site installation conditions;
(4) Large pressure loss (referring to orifice plate, nozzle, etc.).
Note: A new type of product: the intelligent probe type flowmeter, which has taken care of the above disadvantages, has almost no pressure loss, and has an accuracy of 0.2.
Differential pressure flowmeters have a wide range of applications.They are used in various objects in flow measurement of closed pipelines, such as fluids: single phase, mixed phase, clean, dirty, viscous flow, etc .; working conditions: atmospheric pressure, high pressure. , Vacuum, normal temperature, high temperature, low temperature, etc .; tube diameter: from several mm to several m; flow conditions: subsonic, sonic, pulsating flow, etc. Its use in various industrial sectors accounts for about 1/4 to 1/3 of the total use of the flow meter.
1.2 Float Flowmeter
Float flowmeter, also known as rotor flowmeter, is a type of variable area flowmeter.In a vertical cone tube that expands from bottom to top, the gravity of the circular cross section of the float is carried by the liquid power, so that the float It can rise and fall freely within the cone.
Float flowmeter is the second most widely used type of flowmeter 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 flow meters. China's output in 1990 was estimated at 120,000 to 140,000 units, of which more than 95% were glass cone-tube float flow meters.
(1) The glass cone tube float flow meter has a simple structure and is easy to use.The disadvantage is that the pressure resistance is low, and there is a greater risk that the glass tube is fragile.
(2) Suitable for small diameter and low flow rate;
(3) Low pressure loss.
1.3 positive displacement flowmeter
Positive displacement flowmeter, also known as fixed displacement flowmeter, referred to as PD flowmeter, is the most accurate type in flow meters. It uses a mechanical measuring element to continuously divide the fluid into a single known volume part, and measures the total volume of the fluid based on the number of times the measuring chamber is filled and discharged repeatedly.
Volumetric flowmeters are classified according to their measuring elements and can be divided into elliptical gear flowmeters, scraper flowmeters, double rotor flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disc flowmeters, and liquid-sealed barrel flowmeters , Wet gas meter and film gas meter.
(1) High measurement accuracy;
(2) The installation pipeline conditions have no effect on the measurement accuracy;
(3) can be used for the measurement of high viscosity liquids;
(4) Wide range;
(5) Direct-reading meters can be directly accumulated without external energy. The total amount is clear and easy to operate.
(1) The results are complex and bulky;
(2) The type, diameter, and working status of the medium being tested are limited;
(3) Not suitable for high and low temperature occasions;
(4) Most instruments are only suitable for clean single-phase fluid;
(5) Generate noise and vibration.
Positive displacement flowmeters, differential pressure flowmeters, and float flowmeters are listed as the three most used flowmeters. They are often used for the total measurement of expensive media (oil, natural gas, etc.).
In recent years, the sales volume of PD flow meters (excluding domestic gas meters and domestic water meters) in industrial developed countries accounts for 13% ~ 23% of flow meters; China accounts for about 20%. The output in 1990 (excluding domestic gas meters) is estimated to be 340,000 Platforms, of which oval gear type and waist wheel type account for about 70% and 20% respectively.
1.4 Turbine flowmeter
Turbine flowmeter is the main type of speed flowmeter. It uses a multi-blade rotor (turbine) to sense the average flow velocity of the fluid, and derives the flow rate or the total amount of the meter.
Generally, it consists of two parts: the sensor and the display, and it can also be made as a whole.
Turbine flowmeters, volumetric flowmeters, and Coriolis mass flowmeters are called the three types of repeatable and most accurate products in the flowmeter. As one of the ten types of flowmeters, their products have been developed into multiple varieties and multiple The scale of series production.
(1) High accuracy, among all flowmeters, it is the most accurate flowmeter;
(2) Good repeatability;
(3) Element zero drift, good anti-interference ability;
(4) Wide range;
(5) Compact structure.
(1) Cannot maintain calibration characteristics for a long time;
(2) The fluid physical properties have a great influence on the flow characteristics.
Turbine flowmeters are widely used in the following measurement objects: petroleum, organic liquids, inorganic liquids, liquefied gases, natural gas and low temperature fluids are all in Europe and the United States.Turbine flowmeters are second only to natural flow metering in meter usage For meters, the Netherlands alone has used more than 2,600 gas turbine flow meters of various sizes and pressures from 0.8 to 6.5 MPa. They have become excellent natural gas metering meters.
1.5 electromagnetic flowmeter
The electromagnetic flowmeter is a meter for measuring conductive liquids made 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 and corrosive flows.
In the 1970s and 1980s, electromagnetic flow made major breakthroughs in technology, making it a widely used type of flowmeter, and its percentage of use in flow meters has continued to rise.
(1) The measurement channel is a smooth straight tube that will not block, and is suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, mud, sewage, etc .;
(2) No pressure loss caused by flow detection, and good energy saving effect;
(3) The measured volume flow is practically not significantly affected by changes in fluid density, viscosity, temperature, pressure, and conductivity;
(4) Large flow range and wide caliber range;
(5) Corrosive fluids can be used.
(1) Cannot measure liquids with very low conductivity, such as petroleum products;
(2) Cannot measure gases, vapors and liquids containing large bubbles;
(3) Cannot be used at higher temperatures.
Electromagnetic flowmeters have a wide range of applications, and large-caliber meters are often used in water supply and drainage projects; small- and medium-caliber meters are often used in high-demand or difficult-to-measure applications, such as cooling water control for blast furnace tuyeres in the iron and steel industry, measuring pulp and black liquor in the paper industry, Strong corrosive liquid, non-ferrous metallurgy industry slurry; small and small calibers are often used in places with health requirements such as the pharmaceutical industry, food industry, and biochemistry.
1.6 Vortex Flowmeter
A vortex flowmeter is a meter in which a non-streamline vortex generator is placed in a fluid. The fluid is alternately separated on both sides of the generator to release two series of regularly staggered vortex generators.
Vortex flowmeters can be divided into stress type, strain type, capacitive type, thermal type, vibrating body type, photoelectric type and ultrasonic type according to the frequency detection method.
Vortex flowmeters belong to the youngest type of flowmeters, but they have developed rapidly and have now become a universal type of flowmeters.
(1) The structure is simple and firm;
(2) There are many types of applicable fluids;
(3) high accuracy;
(4) Wide range;
(5) Pressure loss is small.
(1) Not applicable to low Reynolds number measurement;
(2) A longer straight pipe section is required;
(3) low meter coefficient (compared to turbine flowmeter);
(4) The instrument lacks application experience in pulsating flow and multiphase flow.
1.7 Ultrasonic flowmeter
An ultrasonic flowmeter is an instrument that measures the flow by detecting the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse).
According to the principle of signal detection, ultrasonic flowmeters can be divided into propagation velocity difference methods (direct time difference method, time difference method, phase difference method, and frequency difference method), beam shift method, Doppler method, cross correlation method, and spatial filtering method. And noise method.
Ultrasonic flowmeters are the same as electromagnetic flowmeters.Because there are no obstructions in the flow channel of the instrument, they are all unobstructed flowmeters.They are a type of flowmeter suitable for solving difficult problems in flow measurement. In recent years, it is one of the fastest-growing types of flowmeters.
(1) Non-contact measurement;
(2) No flow obstruction measurement, no pressure loss;
(3) It can measure non-conductive liquid, which is a supplement to the electromagnetic flowmeter without obstruction measurement.
(1) The propagation time method can only be used to clean liquids and gases; the Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles;
(2) The measurement accuracy of the Doppler method is not high.
(1) The propagation time method is applied to clean, single-phase liquids and gases. Typical applications are factory discharge fluid, weird fluid, liquefied natural gas, etc .;
(2) Good experience in gas applications in the field of high pressure natural gas;
(3) The Doppler method is suitable for two-phase fluids with a low heterogeneous content, such as: untreated sewage, factory discharge fluid, dirty process fluid; it is generally not suitable for very clean liquids.
1.8 Coriolis mass flow meter Coriolis mass flow meter (hereinafter referred to as CMF) is a direct-type Mass flow meter.
The application of China's CMF started relatively late. In recent years, several manufacturers (such as Taihang Instrument Factory) have developed their own supply markets; there are also several manufacturers that have established joint ventures or produced a series of instruments using foreign technology.
1.9 Open channel flowmeter is different from the previous ones. It is a flow meter that measures the free surface natural flow in non-full tubular open channels.
Waterways with non-full pipe flow are called open channels, and open channel flowmeters are used to measure water flow in open channels.
In addition to circular channels, open channel flow meters also have various shapes such as U-shaped, trapezoidal, and rectangular.
Open channel flowmeter application site for all urban water supply diversion channels; water diversion and drainage channels of thermal power plants, sewage treatment inflow and discharge channels; water discharge from industrial and mining enterprises, and channels for water conservancy projects and agricultural irrigation. Some people estimate that 1995 sets account for about 1.6% of the total flow meters, but there is no estimated data for domestic applications.
2 Research and development of new working principle flow meter
2.1 Electrostatic flowmeter
Tokyo Institute of Technology in Japan developed an electrostatic flowmeter suitable for low-conductivity liquid flow measurement of petroleum transmission pipelines.
The metal measuring tube of the electrostatic flowmeter is connected to the piping system with insulation, and the electrostatic charge on the measuring capacitor can be used to know the charge in the measuring tube. They have conducted actual flow tests on metal and plastic measuring tubes such as copper and stainless steel with an inner diameter of 4 ~ 8mm. The tests show that the flow and charge are close to linear.
2.2 Composite effect flow meter
(combined effects meter)
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. This instrument was developed by the American GMI School of Engineering and Management and has applied for two patents.
2.3 Tachometer flow sensor
(tachmetric flowrate sensor)
It was developed by the Russian Scientific Engineering Center Industrial Instrumentation Corporation and is based on the theory of suspension effects. The instrument has been successfully applied in several sites (for example, more than 2,000 units were installed in nuclear power plants to measure hot water flow for 8 consecutive years), and it is still being improved to expand the application area.
3 Application and Development Trends of Several Flow Meters
3.1 Coriolis Mass Flow Meter (CMF)
CMF abroad has developed more than 30 series. The development of each series is technically focused on: design innovation in the structure of flow detection and measurement tubes; improving the zero point stability and accuracy of the instrument; increasing the deflection of the measurement tube and improving the sensitivity; improving the stress of the measurement tube Distribution, reduce fatigue damage, and strengthen anti-vibration interference ability.
3.2 Electromagnetic Flowmeter (EMF)
Since entering into industrial applications in the early 1950s, EMF has been used in an increasingly expanding field. Since the late 1980s, it has accounted for 16% to 20% of the sales of flow meters in various countries.
China has developed rapidly in recent years, with sales estimated at 6,500 to 7,500 in 1994. China has produced ENF with a maximum diameter of 2 ~ 6m, and has equipment capacity of 3m for real flow calibration.
3.3 Vortex Flowmeter (USF)
The USF entered industrial applications in the late 1960s, and has accounted for 4% to 6% of the sales of flow meters in various countries since the late 1980s. Worldwide sales in 1992 were estimated to be 3.548 million units, while domestic products were estimated to be 8,000 to 9,000 units during the same period.
4 Conclusions From the above, we can see that although the development of flowmeters has matured today, there are still many types of flowmeters. There is no flowmeter that is suitable for any occasion.
Each type of flowmeter has its application scope and also has limitations. This requires us:
(1) When selecting a meter, you must be familiar with the situation of the meter and the measured object, and consider other factors so that the measurement will be accurate;
(2) Efforts to develop new instruments to make them more perfect on the existing basis.