The correct choice of flowmeter is very important for accurate measurement of flow.Xi'an Huaheng Instrument explains that the choice of different flowmeters under the same working conditions is certainly quite different for the final measurement results.
If the flowmeter is not selected properly at the beginning,it may even lead to the failure of quantitative measurement and instrument damage.Therefore,it is necessary for instrument maintenance personnel to have a correct understanding of how to select the appropriate flowmeter and to understand some basic selection methods,which will help to make mistakes in the selection of instruments.
In order to ensure the best and most accurate use of flow meters in the process of production site,the choice of flow meters,in general,must be based on the medium of measurement needed in the production site.
Common working conditions can be referred to as follows:
First,sewage,pulp and other turbid liquid media,the optional flowmeter varieties are:ultrasonic flowmeter and intelligent electromagnetic flowmeter.But when choosing electromagnetic flow rate,we should consider that there are no more air or bubbles in the liquid.
2.For petroleum,diesel and other oil media,the optional flowmeters are ultrasonic flowmeters.
3.For medium with large concentration and solid particle content,such as mortar and electro-powder slurry,the optional flowmeters are electromagnetic flowmeters.
4.For the medium with large flow of tap water,the optional flowmeters are intelligent electromagnetic flowmeter and ultrasonic flowmeter.Other flow meters such as vortex flowmeter and orifice plate flowmeter can also be used.
5.Gas medium,the optional flowmeter varieties are:1.ultrasonic flowmeter.2.Vortex Street Flowmeter.If the gas temperature exceeds 300 C,the air pressure flowmeter can be selected.
6.For liquid media with more bubbles,the optional flowmeters are ultrasonic flowmeters,which can be used to measure the fluid with bubbles,and the effect is very good.
Seventh,pure water,desalted water and other low conductivity media,the optional flowmeter varieties are:ultrasonic flowmeter is very suitable for measuring such fluids.
Eighth,acid,alkali and other strong corrosive media,the optional flowmeter varieties are:1,acid and alkali resistant lined electromagnetic flowmeter.2.External clip type ultrasonic flowmeter.
Afterwards,attention should be paid to other requirements and limitations of measurement site.
Solution 1:If your water temperature is below 80 degrees,it is not pure water(not conductive),and if there is not much sewage slurry,it will not deposit in the pipeline.Electromagnetic flowmeter can be the first choice with mature technology,strong anti-interference,little maintenance and moderate price in flowmeter.
Solution 2:If you are a non-conductive liquid,the vortex flowmeter depends on your flow rate,which is too low for him to measure,and if the sewage can not be measured,the accuracy is not so high,you can hit 1%accuracy,if you are an expert,please 1:Don't fool people,many uprisings do not have to die,2:If you do not know to learn first.
Scheme 3:If it is not sewage,and the diameter of pipeline is less than 100 mm:glass rotor flowmeter(float flowmeter)is the most affordable,hundreds of yuan,but only can see the instantaneous flow;turbine flowmeter,with moderate accuracy,is a wheel rotating inside,which may be changed once in a year or two.The quicker the flow rate,the shorter the time interval,because there will be wear;in addition,the highest accuracy is still gear.Flowmeter,similar to water meter,but the price is higher,slightly cheaper than electromagnetic flowmeter,large caliber may be more expensive.
The difference between different kinds of flowmeters is that the measuring principle is different and the working conditions are different.
Flowmeters are commonly used in industry to measure the flow of water,slurry and other media.The maximum diameter of the tube can be measured up to 2m,and the pressure loss is very small,but the medium with low conductivity,such as gas and steam,can not be applied.There are many kinds of flowmeters used in industrial field.The measuring principle is different,and the working conditions are different too.
Electromagnetic flowmeter:According to Faraday's principle of electromagnetic induction,when conductive fluid flows through electromagnetic field,the velocity of fluid can be obtained by measuring voltage.Electromagnetic flowmeter has no moving parts and is not affected by fluid.The accuracy of measuring conductive liquid is very high when the pipe is full.It can be used to measure the velocity of slurry fluid.Like other velocity meters,it is used to measure volume.Flow meter.
Ultrasound Flowmeter:Propagation time method and Doppler effect method are commonly used in ultrasonic flowmeter to measure the average velocity of fluid.It is also an unobstructed flowmeter.If the ultrasonic transmitter is installed outside the pipeline to measure,it does not need to be inserted.
Turbine flowmeter:When the flow of fluid through the turbine is timed,the fluid makes the rotor rotate.The rotating speed of the rotor is related to the speed of the fluid.Through the average flow rate of the fluid perceived by the rotor,the flow rate or total amount can be deduced,and the clean liquid and gas can be accurately measured.Aberrant pressure throttle flowmeter,turbine flowmeter will also produce irreversible pressure error,also need to move parts.
Differential pressure flowmeter:The most common flow measurement techniques,including orifice plate flowmeter,V-cone flowmeter,venturi tube and sonic nozzle,and the equalizing tube flowmeters of Wilba flowmeter,Anuba flowmeter and Deltaba flowmeter,can be used to measure the flow rate of most liquids,gases and vapors.Differential pressure flowmeter has no moving part and is widely used and easy to use.But after blockage,it will cause pressure loss and affect the accuracy.The accuracy of flowmeter precision measurement depends on the accuracy of pressure measurement instrument.
Different flowmeters have different advantages and disadvantages according to their own measuring principles.
Turbine flowmeter is the main type of speed flowmeter.It uses multi-blade rotors(turbines)to sense the average flow velocity of fluid,and derives the flow or total amount of instruments.Generally,it consists of sensors and display devices,and can also be made into a whole.
Turbine flowmeter,volumetric flowmeter and Coriolis mass flowmeter are three kinds of products with the best repeatability and accuracy.As one of the ten types of flowmeters,their products have been developed into multi-variety and multi-series batch production scale.
Advantages:(1)High accuracy,among all flowmeters,belongs to the most accurate flowmeter;(2)good repeatability;(3)good zero disturbance-free ability;(4)wide range;(5)compact structure.
Disadvantages:(1)the calibration characteristics can not be maintained for a long time;(2)the physical properties of the fluid have a greater impact on the flow characteristics.
Applications:Turbine flowmeters are widely used in the following measurement objects:petroleum,organic liquids,inorganic liquids,liquefied gas,natural gas and cryogenic fluids in Europe and the United States.Turbine flowmeters are second only to orifice flowmeters in consumption of natural metering instruments.In the Netherlands alone,more than 2600 gas flowmeters of various sizes are used on natural gas pipelines with pressure ranging from 0.8 to 6.5 MPa.Turbine flowmeters have become excellent natural gas meters.
Vortex flowmeter is an instrument that installs a non-streamlined vorticity generator in the fluid,and the fluid separates and releases two series of vortices in regular staggered arrangement alternately on both sides of the vorticity generator.
Vortex flowmeter can be divided into stress type,strain type,capacitive type,heat sensitive type,vibration type,photoelectric type and ultrasonic type according to frequency detection mode.
Vortex flowmeter is the youngest type of flowmeter,but its rapid development has become a general type of flowmeter.
Advantages:(1)simple and firm structure;(2)applicable to a variety of fluids;(3)high accuracy;(4)wide range;(5)low pressure loss.
Disadvantages:(1)not suitable for low Reynolds number measurement;(2)long straight pipe section;(3)low instrument coefficient(compared with turbine flowmeter);(4)lack of application experience in pulsating flow and multiphase flow.
Electromagnetic flowmeter is a kind of instrument for measuring conductive liquid,which is based on 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 use,such as dirty sewage flow and corrosion flow measurement.
In the 1970s and 1980s,electromagnetic flow has made great breakthroughs in technology,making it a widely used type of flowmeter.The percentage of its usage in flowmeter has been increasing.
(1)The measuring channel is a smooth straight pipe,which is not blocked.It is suitable for measuring liquid-solid two-phase fluids containing solid particles,such as pulp,slurry,sewage,etc.
(2)There is no pressure loss caused by flow detection,and the effect of energy saving is good.
(3)The measured volume flow rate is not obviously affected by the changes of fluid density,viscosity,temperature,pressure and conductivity;(4)the flow rate range is large and the caliber range is wide;(5)corrosive fluid can be used.
Disadvantages:(1)can not measure very low conductivity liquid,such as petroleum products;(2)can not measure gas,steam and liquid containing larger bubbles;(3)can not be used at higher temperatures.
Applications:Electromagnetic flowmeters are widely used in water supply and drainage engineering;small and medium-sized instruments are often used in high-demand or difficult-to-measure occasions,such as blast furnace tuyere cooling water control in iron and steel industry,pulp and black liquor measurement in paper industry,strong corrosive liquor in chemical industry,pulp in non-ferrous metallurgical industry;small and micro-caliber instruments are often used in pharmaceutical industry and food industry.Areas with hygienic requirements such as industry,biochemistry,etc.
4.Differential pressure flowmeter
Differential pressure flowmeter is an instrument which calculates the flow rate according to the differential pressure produced by the flow detector installed in the pipeline,the known fluid condition and the geometric size of the flow detector and the pipeline.
Differential pressure flowmeter is composed of primary device(detection device)and secondary device(differential pressure conversion and flow display instrument).Differential pressure flowmeters,such as orifice plate flowmeters,Venturi flowmeters,and average speed tube flowmeters,are usually classified in the form of detectors.
The secondary devices are all kinds of mechanical,electronic and electromechanical integrated differential pressure gauges,differential pressure transmitters and flow display instruments.It has developed into a large class of instruments with high degree of serialization,generalization and standardization,and a large variety of specifications.It can measure not only flow parameters,but also other parameters(such as pressure,level,density,etc.).
According to its working principle,the detection parts of differential pressure flowmeter can be divided into throttle device,hydraulic resistance type,centrifugal type,dynamic pressure head type,dynamic pressure head gain type and jet type.
Tests can be divided into two categories according to their standardization:standard and non-standard.
The so-called standard detectors are designed,manufactured,installed and used in accordance with standard documents,and their flow values and measurement errors can be determined without real-flow calibration.
Non-standard test pieces are less mature and have not yet been included in international standards.
Differential pressure flowmeter is one of the most widely used flowmeters,which occupies the first place in all kinds of flowmeters.In recent years,due to the advent of various new types of flowmeters,its percentage of usage has gradually declined,but it is still the most important type of flowmeters.
(1)The orifice plate type flow meter with the most application has firm structure,stable and reliable performance and long service life.
(2)It has a wide range of applications,so far no flowmeter of any kind can be compared with it.
(3)Testing parts,transmitters and display instruments are manufactured by different manufacturers,which is convenient for large-scale economic production.
Disadvantages:(1)measurement accuracy is generally low;(2)narrow range,generally only 3:1-4:1;(3)high site installation conditions;(4)high pressure loss(finger orifice plate,nozzle,etc.).
Overview of application:
Differential pressure flowmeter has a wide range of applications.It is widely used in the flow measurement of closed pipelines for various objects,such as fluid:single-phase,miscible,clean,dirty,viscous flow;working conditions:atmospheric pressure,high pressure,vacuum,normal temperature,high temperature and low temperature;pipe diameter:from several m m to several m;flow conditions:subsonic,sonic,pulsating flow,etc.It accounts for about 1/4 to 1/3 of the total amount of flowmeters used in various industrial sectors.
Float flowmeter,also known as rotator flowmeter,is a kind of variable area flowmeter.In a vertical cone tube enlarged from bottom to top,the gravity of a circular cross-section float is borne by the hydrodynamic force,so that the float can rise and fall freely in the cone tube.
Float flowmeter is the most widely used type of flowmeter after differential pressure flowmeter,especially in small and small flow.
In the mid-1980s,Japan,Western Europe and the United States accounted for 15%to 20%of the sales of flow meters.In 1990,China's output was estimated at 120,000 to 140,000 units,of which more than 95%were glass cone tube float flowmeters.
Features:(1)Glass cone tube float flowmeter is simple in structure and easy to use,but its shortcomings are low pressure resistance and high risk of glass tube fragility;(2)suitable for small diameter and low flow rate;(3)low pressure loss.
Volumetric flowmeter,also known as fixed displacement flowmeter,or PD flowmeter for short,is one of the most accurate flowmeters.It uses mechanical measuring elements to continuously divide the fluid into a single known volume part,and measures the total volume of the fluid according to the number of times the measuring chamber repeatedly fills and discharges the fluid of the volume part.
Volumetric flowmeters can be divided into elliptical gear flowmeters,scraper flowmeters,double rotors flowmeters,rotary piston flowmeters,reciprocating piston flowmeters,disc flowmeters,liquid seal rotary cylinder flowmeters,wet gas flowmeters and membrane gas flowmeters.
Advantages:(1)High measurement accuracy;(2)Installation of pipeline conditions has no effect on measurement accuracy;(3)It can be used for measuring high viscosity liquids;(4)Wide range;(5)Direct reading instrument can be directly accumulated without external energy,total amount,clear and easy to operate.
Disadvantages:(1)the results are complex and the volume is huge;(2)the type,caliber and working state of the medium are limited;(3)they are not suitable for high and low temperature situations;(4)most instruments are only suitable for clean single-phase fluid;(5)noise and vibration are generated.
Overview of application:
Volumetric flowmeter,differential pressure flowmeter and float flowmeter are the three most used flowmeters.They are often used to measure the total amount of expensive media(oil,natural gas,etc.).
In recent years,the sales amount of PD flowmeters(excluding household gas meters and water meters)in industrial developed countries accounts for 13%~23%of the flow meters;China accounts for about 20%.In 1990,the output(excluding household gas meters)is estimated to be 340,000 units,of which the elliptical gear type and the waist wheel type account for about 70%and 20%respectively.
Ultrasound flowmeter is an instrument to measure flow rate by detecting the effect of fluid flow on ultrasonic beam(or ultrasonic pulse).
According to the principle of signal detection,the ultrasonic flowmeter can be divided into three methods: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.
Ultrasound flowmeter,like electromagnetic flowmeter,is a kind of flowmeter which is suitable for solving the difficult problem of flow measurement because there is no hindrance in the flow passage of the instrument.It has prominent advantages especially in large caliber flow measurement.In recent years,it is one of the rapidly developing flowmeters.
(1)Non-contact measurement;(2)non-flow obstruction measurement,no pressure loss;(3)non-conductive liquid measurement,non-obstruction measurement of electromagnetic flowmeter is a supplement.
1)The propagation time method can only be used to clean liquids and gases,while the Doppler method can only be used to measure liquids containing suspended particles and bubbles;(2)The accuracy of Doppler method is not high.
Overview of application:
(1)The propagation time method is applied to clean,single-phase liquids and gases.Typical applications include factory discharge liquid,strange liquid,liquefied natural gas,etc.
(2)Gas application has good 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 effluent and dirty process fluids;it is usually not suitable for very clean fluids.
8.Coriolis mass flowmeter
Coriolis mass flowmeter(hereinafter referred to as CMF)is a kind of direct mass flowmeter,which is based on the principle of Coriolis force,which is proportional to mass flow when fluid flows in a vibrating tube.
9.Open Channel Flowmeter
Unlike the above-mentioned ones,it is a flow meter for measuring free surface natural flow in non-full tubular open channels.
An open channel flowmeter is used to measure the flow in an open channel.
The open channel flowmeter has many shapes,such as U-shaped,trapezoidal,rectangular and so on,besides the circular one.
Open channel flowmeters are used in urban water supply and diversion canals;thermal power plant water diversion and drainage canals,sewage treatment inflow and discharge canals;industrial and mining enterprises water discharge and water conservancy projects and agricultural irrigation channels.