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In the past 30 years, with the rapid development of the international economy and science and technology, the process automation flowmeter has received increasing attention. In order to meet the characteristics of different kinds of fluids and solve the problem of flow measurement under different flow conditions, there are dozens of types of process automation flowmeters developed and put into use in various countries, and hundreds of process automation flowmeters are listed at home and abroad.
The industrial production industry pays more attention to energy conservation and environmental protection, and the accurate measurement of water, oil, gas, and steam in the production process is also increasingly important. The measurement of medium flow in industrial production is affected by many factors such as its state, type, temperature, flow rate, flow rate, pressure, etc., which determines the complexity of flow measurement. Therefore, the selection of a flowmeter has strong technicality and practicability.
At present, although the development of flowmeter measurement technology has become increasingly mature, there is a wide range of flowmeters, and there is no flowmeter that can be applied to any occasion, and each flowmeter has its scope of application and limitations.
Due to the complexity and importance of the process and the measured object in industrial production, it is important to select the type of on-site flowmeter. Therefore, to select a flowmeter correctly and effectively, one must be familiar with various characteristics of the process and the fluid to be measured in industrial production and the measurement method to be adopted. At the same time, other relevant influencing factors, such as flowmeter performance indicators (flow range, extreme withstand voltage, error, repeatability, etc.), price, use requirements, installation environment, and maintenance process, must be considered Overall planning, careful design, and reasonable selection.
Flowmeter selection refers to the selection according to the industrial production requirements and the principles of fluid type and instrument performance, fluid characteristics, installation conditions, environmental conditions, and economic factors.
Instrument performance quality, stability, accuracy, range, linearity, signal parameters, response time, pressure loss, and other factors.
Fluid characteristics - temperature, pressure, density, viscosity, plugging, phase change, corrosivity, electrical conductivity, sound velocity, thermal conductivity, and other indexes.
Installation conditions - pipeline layout direction, flow direction, length of straight pipe section at upstream and downstream sides of the test piece, pipe diameter, maintenance space, vibration, power supply, and other conditions.
Environmental conditions - environmental temperature, humidity, electromagnetic interference, safety, explosion-proof and lightning protection, pipeline vibration, etc.
Economic factors - purchase cost, installation cost, calibration cost, maintenance cost, service life, spare parts, and other factors.
Applicable materials -- mass flowmeter is applicable to multiple media
Principle and structure of mass flowmeter
The mass flowmeter is based on Coriolis force (a force generated on the tube wall when the fluid flows in a rotating tube). There are two parallel flow tubes inside the sensor, with a drive coil in the middle and a detection coil at both ends. When the excitation voltage provided by the transmitter is applied to the drive coil, the vibration tube vibrates in a reciprocating cycle. When the fluid medium in the industrial process flows through the vibration tube of the sensor, the Coriolis force effect will be generated on the vibration tube, The detection coils installed at both ends of the vibration tube will generate two groups of signals with different phases when the two vibration tubes are torsionally vibrated. The phase difference between the two signals is proportional to the fluid mass flow passing through the sensor. The mass flow through the vibrating tube is calculated by computer. When different medium flows through the sensor, the main vibration frequency of the vibration tube is different, and the medium density is calculated accordingly. The platinum resistance installed on the vibration tube of the sensor can indirectly measure the temperature of the medium.
Measurement characteristics of mass flowmeter
The mass flowmeter can directly measure the mass flow of the medium passing through the flowmeter, and can also measure the density of the medium and indirectly measure the temperature of the medium.
Precautions for use of mass flowmeter
1. When measuring gas, some models are only applicable to the measurement of air and other gases close to ideal, while some can use natural gas and steam. Consult the manufacturer in detail.
2. In case of air entrainment, gasification, or two-phase flow in the measuring medium, the transmitter will display an alarm, and the sensor will stop working in serious cases.
Applicable materials - orifice flowmeter is widely used for flow measurement of gas, steam, and liquid.
The orifice flowmeter is more suitable for measuring saturated steam, superheated steam, gas, cooling water, and other dirty fluids.
An orifice plate flowmeter is more suitable for flow measurement of high-temperature and high-pressure fluids.
Principle and structure of orifice flowmeter
LG-type orifice flowmeter, also known as differential pressure flowmeter, is composed of a primary detection piece (throttling piece) and a secondary device (differential pressure transmitter and flow indicator). The pressure-equalizing ring and one-piece structure are adopted.
Measuring characteristics of orifice flowmeter
Precautions for use of orifice flowmeter
The orifice flowmeter shall be installed with a section of measuring pipe, which shall at least keep the first 10DN and the last 5DN straight pipe sections of equal diameter to improve the measurement accuracy;
Single-phase fluid must always be maintained in the impulse piping. When the measured fluid is gas, the pressure pipeline (including the pressure chamber of the differential pressure gauge) is full of gas phase; When the fluid to be measured is liquid, the pressure pipeline is full of the liquid phase, and there must be no bubbles. For this purpose, a drain valve shall be installed at the lowest point of the impulse piping or an exhaust valve shall be installed at the highest point.
Applicable materials
The glass rotameter has strong corrosion resistance and can detect the flow of acid (except hydrofluoric acid), alkali, oxidant, and other corrosive gases or liquids. It is suitable for chemical, pharmaceutical, paper making, sewage treatment, and other industries.
Principle and structure of rotameter
A float flowmeter, also known as a rotameter, is a device that calculates the flow by measuring the (position) of rotating parts in the DC pipeline. It is a variable area flowmeter. In a vertical tapered tube expanded from bottom to top, the gravity of the float with a circular cross-section is borne by the hydrodynamic force, and the float can rise and fall freely in the tapered tube. Under the action of velocity and buoyancy, it moves up and down, balances with the weight of the float, and transmits to the dial through magnetic coupling to indicate the flow. Metal tube float flowmeter is mainly composed of an indicator, float, and conical measuring chamber.
Measurement characteristics of rotameter
Precautions for use of rotameter
Applicable materials
It is used to measure the volume flow and cumulative amount of low-viscosity, non-corrosive and clean liquids in closed pipelines. It can be widely used in the petroleum, chemical industry, metallurgy, organic liquid, inorganic liquid, liquefied gas, urban gas pipe network, pharmacy, food, paper making, and other industries. Water, petroleum, diesel, and other liquids.
Principle and structure of turbine flowmeter
The liquid turbine flowmeter is composed of a turbine and an external pulse detector. The liquid flows into the turbine, causing the rotor to rotate. The specific inner diameter makes the rotor speed directly proportional to the flow.
Measurement characteristics of turbine flowmeter
With strong anti-impurity ability, the tangential impeller can release suspended impurities in the fluid at any time when rotating, so that it does not wind on the blade of the tangential impeller.
Strong anti-electromagnetic interference and anti-vibration capability.
Precautions for use of turbine flowmeter
Applicable materials (media)
It can accurately measure the volume flow of conductive fluid and solid-liquid two-phase bodies such as various acids, alkalis, salt solutions, raw water, cooling water, sewage, coal slurry electromagnetic flowmeter, mineral slurry, slurry, cement slurry, pulp, syrup, coal slurry, corn slurry, fiber slurry, grain slurry, lime milk, hydrogen peroxide, beer, wort, black liquor, green liquor, and various beverages.
Principle and structure of electromagnetic flowmeter
The (integrated) electromagnetic flowmeter consists of a sensor and a converter. It is based on Faraday's electromagnetic induction law. It is used to measure conductivity greater than 5 μ The volume flow of S/cm conductive liquid is an inductive instrument for measuring the volume flow of the conductive medium. In addition to measuring the volume flow of general conductive liquids, it can also be used to measure the volume flow of strong corrosive liquids such as strong acids and alkalis and even liquid-solid two-phase suspension liquids such as mud, pulp, and paper pulp.
Measuring characteristics of electromagnetic flowmeter
Measurable fluid: according to the working principle of the electromagnetic flowmeter, the fluid that can be used to measure the flow with the electromagnetic flowmeter must be conductive. Strictly speaking, except for high-temperature fluid, as long as the conductivity is greater than 5 μ/ Any fluid in cm shall be measured by a corresponding electromagnetic flowmeter, so non-conductive gas, steam, oil, acetone, and other substances cannot be measured by the electromagnetic flowmeter.
Determination of sensor diameter: the flow rate of the flowmeter should be within 0.3-15m/s. At this time, the flowmeter diameter can be selected to be consistent with the user's pipe diameter. When the flow rate is lower than 0.3m/s, it is better to increase the flow rate locally at the instrument part, and the pipe shrinking method is adopted.
Precautions for use of electromagnetic flowmeter
Select the appropriate type according to the characteristics of integrated and separated electromagnetic flowmeter. The all-in-one electromagnetic flowmeter is easy to install and lays lines with medium accuracy, and should not be installed below the ground to prevent the converter from being flooded. The separated electromagnetic flowmeter has high accuracy, and the converter and sensor are installed in different places, which is more suitable for the situation where the site environment is relatively poor (such as harmful, toxic, easily flooded, and other places), but the installation and wiring are strict, otherwise, it is easy to introduce interference signals.
Applicable materials
Like the ultrasonic flowmeter, the ultrasonic flowmeter is an unhindered flowmeter because there are no obstructions in the instrument flow channel. It is a type of flowmeter suitable for solving the problem of difficult flow measurement, especially in large-diameter flow measurement.
Principle and structure of ultrasonic flowmeter
The ultrasonic flowmeter adopts the time difference measurement principle: the signal transmitted by one probe passes through the pipe wall, medium, and pipe wall on the other side, and then is received by another probe. At the same time, the signal transmitted by the second probe is also received by the first probe. Due to the influence of the medium flow rate, there is a time difference between the two Δ t. Flow velocity V and time difference can be obtained according to calculation Δ Conversion relation between t V=(C2/2L) ×Δ t. Then the flow value Q can be obtained.
Measurement characteristics of ultrasonic flowmeter
Advantages: Ultrasonic flowmeter is a non-contact instrument, which can not only measure the medium flow of large pipe diameter but also measure the medium that is not easy to contact and observe. Its measurement accuracy is very high, and it is almost free from the interference of various parameters of the measured medium. In particular, it can solve the flow measurement problems of highly corrosive, non-conductive, radioactive, flammable, and explosive media that other instruments cannot.
Precautions for use of ultrasonic flowmeter
Applicable materials
The oval gear flowmeter is a kind of volumetric flowmeter, which is used for precise continuous or intermittent measurement of liquid flow or instantaneous flow in pipelines. It is especially suitable for flow measurement of heavy oil, polyvinyl alcohol, resin, and other media with high viscosity.
Principle and structure of oval gear flowmeter
The oval gear flowmeter (also known as displacement flowmeter, or gear flowmeter) is a kind of volumetric flowmeter, which is of high accuracy in flow meters. The measuring box and a pair of elliptical gears installed in the measuring box form a sealed initial moon-shaped cavity (not absolutely sealed due to the rotation of the gear) with the upper and lower cover plates as the calculation unit of the primary displacement. When the measured liquid enters the flowmeter through the pipe, the pressure difference at the inlet and outlet drives a pair of gears to rotate continuously, and the liquid metered by the first lunar cavity is continuously delivered to the outlet. The product of the number of revolutions of the elliptical gear and four times each displacement is the total flow of the measured liquid (see the figure for the principle). The flowmeter is mainly composed of housing, counter, oval gear, and coupling (including magnetic coupling and axial coupling).
Measurement characteristics of oval gear flowmeter
Oval gear flowmeter, also known as constant displacement flowmeter, or PD flowmeter for short, is a type of flowmeter with high accuracy, especially suitable for the measurement of high viscosity medium flow.
Features: The flow measurement is independent of the flow state of the fluid because the oval gear flowmeter measures the flow by driving the oval gear to rotate with the pressure head of the measured medium.
The greater the viscosity of the medium, the smaller the leakage error, and the more favorable the measurement. And the oval gear flowmeter has high measurement accuracy and is suitable for measuring the flow of high viscosity medium, but it is not suitable for the fluid containing solid particles (solid particles will jam the gear and make it impossible to measure the flow). If there is gas in the measured liquid medium, the measurement error will also be caused.
Precautions for use of oval gear flowmeter
Applicable materials
Measurable conductivity ≥ 1 μ S/cm of the fluid flow of acid, alkali, salt solution, water, sewage, corrosive liquid, mud, and slurry. However, the airflow meter cannot measure the flow of gas, steam, and purified water.
Principle and structure of air flow meter
The airflow meter is used to measure the flow according to Faraday's law of electromagnetic induction
Measuring characteristics of air flow meter
The advantage of an airflow meter is that the pressure loss is very small and the flow range can be measured as large. The ratio of the maximum flow to the minimum flow of the airflow meter is generally more than 20:1. The applicable industrial pipe diameter range is wide, up to 3m. The output signal and the measured flow are linear, with high accuracy.
In a word, no measurement method or flowmeter can adapt to all kinds of fluids and flow conditions. Different processes and environments, different measurement methods, and structures require different measurement operations, use methods, and use conditions. After the type of flowmeter is determined, the flowmeter shall be designed and accurately calculated according to the process conditions and fluid parameters in industrial production. Comprehensively analyze and comprehensively consider various influencing factors, and conduct scientific and effective analysis and research. Finally, we will make a choice that is safe, reliable, economical and durable, and suitable for production requirements. If you need to purchase a large number of automatic process control instruments such as flow meters, please send an inquiry to leave your contact information, and our professional technical engineers will contact you.
