More than 60 kinds of old flowmeter, each product has its specific applicability, but it also has its limitations. 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.
In addition, according to the measurement principle, it can be divided into the following categories:
1, mechanical principle: the instruments belonging to this principle include differential pressure type and rotor type using Bernoulli theorem; Pulse type and moving tube type using momentum theorem; Using the direct mass formula of Newton's second law; Target types using the principle of fluid momentum; Turbine formulas using angular momentum theorem: vortex type and vortex street type using fluid oscillation principle; Pitot tube type, volumetric type, weir type, trough type, etc. use the total static pressure difference.
2. Electrical principle: The instruments used in this principle include electromagnetic, differential capacitance, inductance and strain resistance.
3. Acoustic principle: Ultrasonic waves and sound waves (shock waves) are used to measure the flow.
4. Thermal principle: there are calorimeters, direct calorimeters and indirect calorimeters. Measure the flow by thermal principle.
5. Optical principle: both laser and photoelectric instruments belong to this principle.
6. Principle of atomic physics: Both nuclear magnetic resonance vibration type and nuclear radiation type are instruments belonging to this principle.
7. Other principles: labeling principle (tracer principle, nuclear magnetic resonance principle), correlation principle, etc.
According to the most popular and extensive classification at present, this paper expounds the principles, characteristics, applications and applications at home and abroad of various flowmeters:
Target type
Target flowmeter is a kind of flowmeter based on mechanical principle, which has been developed and applied in industry for decades. The new SBL target flowmeter is a new capacitive force sensing flowmeter based on the traditional target flowmeter and developed with the development of new sensors and microelectronics technology. It has the characteristics of orifice plate, vortex street and other flowmeter without moving parts, and at the same time, it has high sensitivity, accuracy comparable to that of positive displacement flowmeter and wide measuring range.
In 1970s, China developed the electro-pneumatic target flow transmitter, which is an electric and qdz testing instrument. At that time, the force converter directly adopted the force balance mechanism of differential pressure transmitter, which inevitably brought many defects brought by the force balance mechanism itself, such as easy drift of zero position, low measurement accuracy and poor reliability of lever mechanism. Due to the poor performance of the force balance mechanism, many advantages of target flowmeter itself have not been effectively brought into play, and users' bad impression of old target flowmeter has not been eliminated.
The force converter of the new SBL target flowmeter adopts strain type force converter, which completely eliminates the shortcomings of the above force balance mechanism. The new target flowmeter also applies microelectronics and computer technology to the signal converter and display part. The flowmeter has a series of advantages and is believed to play an important role in many flowmeters in the future.
Differential pressure type
Differential pressure flowmeter is an instrument to calculate the flow according to the differential pressure generated by the interaction between the flow detector installed in the pipeline and the fluid, the known fluid condition and the geometric dimensions of the detector and the pipeline.
The differential pressure flowmeter consists of primary equipment (detector) and secondary equipment (differential pressure converter and flow indicator). Differential pressure flowmeters are usually classified in the form of test pieces, such as orifice flowmeter, Venturi flowmeter, averaging tube flowmeter, Pitot tube principle-Bitoba flowmeter, etc.
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. Due to the advent of various new flowmeters, the percentage of their use has gradually decreased, but it is still the most important flowmeter type at present.
The fluid volume flow formula of differential pressure flowmeter is:
v=aA √2/j(p-q)
volume
J- liquid density
A—— Flow coefficient, which is related to the size of the flow channel, the pressure taking method and the flow release.
Hole area of orifice plate
pressure differential
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. ).
Note: New product: Balance flowmeter developed by NASA. The measuring accuracy of this flowmeter is 5- 10 times that of the traditional throttling device, and the permanent pressure loss is 1/3. The pressure recovery is twice as fast, and the smallest straight pipe section can be as small as 1.5D, which is convenient to install and use and greatly reduces the capacity consumption of fluid operation.
Application overview:
Differential pressure flowmeter has a wide range of applications. All kinds of objects have applications in the flow measurement of closed pipes. Such as fluid: single phase, miscible, clean, dirty, viscous flow, etc. 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. Common standard throttling devices (orifice plate), (nozzle) and (venturi tube).
2. The commonly used non-standard throttling devices are (double orifice plate), (circular orifice plate), (1/4 circular nozzle) and (venturi nozzle).
3. The commonly used methods of orifice pressure are (angular pressure) and (flange connection pressure), and other methods are (theoretical pressure), (radial distance pressure) and (pipe joint pressure).
4. With the standard orifice flange pressure method, the distance between the center of the upstream and downstream pressure holes and the front and rear end faces of the orifice plate is (25.4±0.8)mm, also called 1 inch flange pressure.
5. 1 15 1 The working power supply range of the transmitter is (12)vdc to (45)vdc, and the load is (0) ohm to (1650) ohm.
6. The range of1151dp4e transmitter is (0 ~ 6.2) to (0 ~ 37.4) kPa.
7. 1 15 1 The maximum positive migration of differential pressure transmitter is (500%), and the maximum negative migration is (600%).
8. Generally speaking, the velocity of the fluid in the pipeline is maximum at (the center line of the pipeline), and the velocity at (the wall of the pipeline) is equal to zero.
9. If (Reynolds number) is the same, the motion of fluid is similar.
10. When the fluid filled with pipeline flows through the throttling device, there will be flow (local contraction) at (contraction), so that (flow) will increase and (static pressure) will decrease.
1 1, 1 15 1 The differential pressure transmitter uses a variable capacitor as a sensitive element. When the pressure difference increases, the measuring diaphragm moves, so the capacitance on the low-voltage side (increases) and the capacitance on the high-voltage side (decreases).
When the minimum adjustment range of 12, 1 15 1 differential pressure transmitter is used, the maximum load transfer is (600%) of the range, and the maximum forward transfer is (500%). If the maximum adjustment is 1 15 1,
The accuracy of 13, 1 15 1 differential pressure transmitter is (0.2%) and (0.25%) respectively. Note: Large differential pressure transmitter is 0.25%.
14, common flow units, volume flow (m3/h), (t/h), mass flow (kg/h), (t/h) and gas volume flow (nm3/h) in standard state.
15. Use orifice flowmeter to measure steam flow. In the design, the steam density is 4.0kg/m3, and the actual indicated flow rate in actual work is (0.866) times of the designed flow rate.
16. measure ammonia flow with orifice flowmeter. The design pressure is 0.2mpa (gauge pressure) and the temperature is 20℃, while the actual pressure is 0. 15mpa (gauge pressure) and the temperature is 30℃, so the actual indicated flow is (0.897) times of the design flow.
17, straight pipe section in front of orifice plate generally requires (10)d, and straight pipe section behind orifice plate generally requires (5) d. For correct measurement, straight pipe section in front of orifice plate is preferably (30 ~ 50) d, especially when there is a pump or regulating valve in front of orifice plate.
18. In order to make the flow coefficient α of orifice flowmeter tend to a constant value, the Reynolds number of fluid should be greater than the critical Reynolds number.
19. In the technical requirements of orifice plate processing, the upstream plane should be perpendicular to the center line of orifice plate without visible scars, the upstream surface and the downstream surface should be parallel, and the upstream inlet edge should be sharp without burrs and scars.
flotage
Float flowmeter, also known as rotor flowmeter, metal rotor flowmeter and wind glass rotor flowmeter, is a variable area flowmeter. In a vertical conical tube that expands from bottom to top, the gravity of a float with a circular cross section is borne by fluid 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 1990 in China is estimated to be12 ~140,000 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.
Positive displacement
Volumetric flowmeter, also known as constant displacement flowmeter, or PD flowmeter for short, is the most accurate one among flow meters. It uses mechanical measuring elements to continuously divide the fluid into a known volume part, and measures the total volume of the fluid according to the number of times the measuring chamber fills and discharges the fluid in this volume part.
Volumetric flowmeter can be divided into elliptic 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.
Advantages:
(1) has 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 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.
Application overview:
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.). ).
1990 (excluding household gas meters) produced 340,000 sets, of which elliptical gear type and waist wheel type accounted for 70% and 20% respectively.
Electromagnetic flowmeter
1, advantages
(1) electromagnetic flowmeter can be used to measure industrial conductive liquid or slurry.
(2) No pressure loss.
(3) The measuring range is large, and the diameter of the electromagnetic flow transmitter ranges from 2.5 mm to 2.6 mm. ..
(4) The electromagnetic flowmeter measures the volume flow of the measured fluid under working conditions, and the measuring principle does not involve the influence of temperature, pressure, density and viscosity of the fluid.
2. Deficiencies
The application of (1) electromagnetic flowmeter has some limitations. Only the liquid flow of conductive media can be measured, but the flow of non-conductive media, such as water-treated gas and hot water, can not be measured. In addition, its lining should consider high temperature.
(2) The electromagnetic flowmeter determines the volume flow in the working state by measuring the velocity of conductive liquid. According to the measurement requirements, for liquid media, mass flow should be measured, and the measurement of media flow should involve the density of fluid. Different fluid media have different densities and change with temperature. If the electromagnetic flowmeter converter does not consider the fluid density, it is not appropriate to give only the volume flow at room temperature.
(3) The installation and debugging of electromagnetic flowmeter is more complicated and stricter than other flowmeters. Transmitter and converter must be used together, and two different types of instruments cannot be used together. When installing the transmitter, from the selection of installation location to the specific installation and debugging, it must be carried out in strict accordance with the requirements of the product manual. There should be no vibration and strong magnetic field at the installation site. During installation, the transmitter and pipeline must be in good contact and well grounded. The potential of the transmitter is equal to the potential of the measured fluid. When in use, the residual gas in the measuring tube must be exhausted, otherwise it will cause great measurement error.
(4) When the electromagnetic flowmeter measures the viscous liquid with dirt, the viscous substance or sediment adheres to the inner wall of the measuring tube or the electrode, which changes the output potential of the transmitter and brings measurement errors. When the dirt on the electrode reaches a certain thickness, the instrument may not be able to measure it.
(5) Scaling or abrasion of water supply pipeline changes the inner diameter, which will affect the original flow value and cause measurement error. If the inner diameter of an instrument with a diameter of 100mm changes 1mm, it will bring about an additional error of about 2%.
(6) The measuring signal of the transmitter is a very small millivolt potential signal. In addition to the flow signal, there are some signals unrelated to the flow, such as phase voltage, quadrature voltage and * * * mode voltage. In order to accurately measure the flow, it is necessary to eliminate all kinds of interference signals and effectively amplify the flow signal. The performance of the flow converter should be improved, and it is best to control the excitation voltage by using a microcomputer-based converter, and select the excitation mode and frequency according to the properties of the measured fluid to eliminate in-phase interference and orthogonal interference. However, the improved instrument has complex structure and high cost.
(7) the price is higher
supersonic flowmeter
1, advantages
(1) Ultrasonic flowmeter is a non-contact measuring instrument, which can be used to measure fluid flow and large pipeline runoff that are difficult to contact and observe. It will not change the flow state of fluid, will not produce pressure loss, and is convenient to install.
(2) It can measure the flow of strongly corrosive medium and non-conductive medium.
(3) The measuring range of ultrasonic flowmeter is large, and the pipe diameter varies from 20mm~5m to 5 m. 。
(4) Ultrasonic flowmeter can measure the flow of various liquids and sewage.
(5) The volume flow measured by ultrasonic flowmeter is not affected by the thermophysical parameters such as temperature, pressure, viscosity and density of the measured fluid. Can be made into fixed and portable.
2. Deficiencies
(1) The temperature measuring range of ultrasonic flowmeter is not high, and generally it can only measure fluids with temperature lower than 200℃.
(2) Poor anti-interference ability. It is easy to be disturbed by bubbles, scaling and ultrasonic noise mixed with sound sources such as pumps, which affects the measurement accuracy.
(3) Straight pipe section is strictly required, 20 days before and 5 days after. Otherwise, the discreteness is poor and the measurement accuracy is low.
(4) The uncertainty of installation will bring great error to the flow measurement.
(5) Scaling in the measuring pipeline will seriously affect the measurement accuracy and bring great measurement error, even if the instrument has no flow display.
(6) The reliability and accuracy are not high (generally around 1.5 ~ 2.5) and the repeatability is poor.
(7) Short service life (general accuracy can only be guaranteed for one year).
(8) The ultrasonic flowmeter determines the volume flow by measuring the velocity of the fluid, and the mass flow of the liquid should be measured. The mass flow measured by the instrument is obtained by multiplying the volume flow by the artificially set density. When the fluid temperature changes, the fluid density changes, and the artificially set density value cannot guarantee the accuracy of mass flow. Only by measuring the fluid velocity and density can the real mass flow value be obtained through operation.
(9) The price is higher.
vortex shedding flowmeter
1, advantages
(1) Vortex flowmeter has no moving parts, and the measuring element has simple structure, reliable performance and long service life.
(2) The vortex flowmeter has a wide measuring range. The range ratio can generally reach 1: 10.
(3) The volume flow of vortex flowmeter is not affected by the thermal parameters such as temperature, pressure, density or viscosity of the measured fluid. Generally speaking, no separate calibration is required. It can measure the flow of liquid, gas or steam.
(4) The pressure loss caused is small.
(5) High accuracy, repeatability of 0.5% and little maintenance.
2. Deficiencies
(1) The volume flow of vortex flowmeter in working state is not affected by the thermal parameters such as temperature, pressure and density of the measured fluid, but the final measurement result of liquid or steam should be mass flow, and for gas, the final measurement result should be standard volume flow. Both mass flow and standard volume flow must be converted by fluid density, and the change of fluid density caused by the change of fluid working conditions must be considered.
(2) The main factors causing the flow measurement error are: the measurement error caused by the uneven flow velocity in the pipeline; The medium density cannot be accurately determined when the fluid working condition changes; Wet saturated steam is assumed to be dry saturated steam for measurement. If these errors are not limited or eliminated, the total measurement error of vortex flowmeter will be great.
(3) Poor anti-vibration performance. External vibration will cause the measurement error of vortex flowmeter, and even it can't work normally. The high-speed impact of channel fluid will cause additional vibration of the cantilever of vortex generator, which will reduce the measurement accuracy. The influence of large diameter is more obvious.
(4) Poor adaptability to measuring dirty media. The generator of vortex flowmeter is easily polluted by medium or entangled by dirt, which changes the geometric size and greatly affects the measurement accuracy.
(5) The straight pipe section has high requirements. Experts pointed out that the straight pipe section of vortex flowmeter must meet the measurement requirements in the first 40 days and the last 20 days.
(6) Poor temperature resistance. Generally, vortex flowmeter can only measure the fluid flow of medium below 300℃.
orifice meter
1, advantages
(1) standard throttling element is universal all over the world and has been recognized by the international organization for standards. It can be put into use without actual flow calibration, and it is unique in flowmeter.
(2) the structure is easy to copy, simple and firm, stable and reliable in performance and low in price;
(3) The application range is wide, including all single-phase fluids (liquid, gas, steam) and some mixed-phase fluids. The products can be used under the pipe diameter and working conditions (temperature and pressure) of general production processes.
(4) Specimens and differential pressure display instruments can be produced by different manufacturers, so they can be produced on a professional scale;
2. Deficiencies
The repeatability and accuracy of (1) measurement are at a medium level in the flowmeter, and it is difficult to improve the accuracy due to the complex influence of many factors.
(2) The scope is narrow. Because the discharge coefficient is related to Reynolds number, the general range is only 3 ∶ 1 ~ 4 ∶ 1.
(3) There is a long straight pipe length requirement, which is generally difficult to meet. Especially for the large pipe diameter, the problem is more prominent;
(4) great pressure loss;
Usually, in order to maintain the normal operation of orifice flowmeter, the water pump needs extra power to overcome the pressure loss of orifice plate. Additional power consumption can be directly determined by pressure loss and flow calculation. Tens of thousands of kilowatt hours of electricity are needed for more than one year, which is equivalent to tens of thousands of RMB. See the table below for the calculation results of energy consumption of orifice plate under normal pressure loss. The operation days are calculated as 350 days, and the electricity price is calculated as 0.35 yuan/kWh. As can be seen from the power consumption data calculated in the table, the additional operating cost of orifice plate is extremely high, while the operating cost of elbow flowmeter is zero!
(5) The accuracy of the orifice plate is guaranteed by the acute angle line of the inner hole, which is sensitive to corrosion, wear, scaling and dirt, and it is difficult to guarantee the accuracy of long-term use, so it needs to be removed and inspected once a year.
(6) Flange connection is prone to run, run, drip and leak, which greatly increases the maintenance workload.
Thermal mass flowmeter (constant temperature difference)
-Advantages
1. The ball valve is easy to install and disassemble. And can be installed under pressure.
2. Based on the Golden Law, directly measure the mass flow. The measured value is not affected by pressure and temperature.
3. Quick response.
4. The measuring range is large, the minimum flow of 8.8mm pipeline can be measured by pipeline installation, and the maximum flow can be measured by 30''.
5. Plug-in flowmeter, one flowmeter can be used to measure various pipe diameters.
-Disadvantages
1. The accuracy is not as good as other types of flowmeters, generally 3%.
2. The scope of application is narrow, and it can only be used to measure dry non-explosive gases, such as compressed air, nitrogen, argon and other neutral gases.