Flow measurement technology of the hottest special

2022-08-05
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Flow measurement technology for special fluids

flow measurement involves a wide range of application fields. The demand for flow measurement in process measurement, energy measurement, environmental protection, transportation and other high energy consuming fields is growing rapidly, which puts forward new requirements for flow measurement technology. The flow measuring instrument is not only required to be resistant to high temperature and high pressure, but also can automatically compensate the impact of parameter changes on the measurement accuracy. Considering the special requirements of energy conservation, cost accounting, trade and medical health, it is required to have high flow measurement accuracy, small pressure loss and high reliability

the development and application of new technologies, new devices, new materials, new processes and new software make the measurement accuracy of flowmeter higher and higher, and the measurement range of flow wider and wider. At the same time, the requirements of flowmeter for measuring medium are decreasing, the application range is becoming wider and wider, and the degree of intelligence and reliability have been greatly improved

in the last article, we introduced the measurement technology of small flow and large flow fluids. Today, we will continue to introduce the flow measurement technology of corrosive media and multiphase fluids

flow measurement of corrosive media

corrosion is a phenomenon that metals are destroyed due to chemical action in their environment. All metals and alloys can be resistant to corrosion in certain environments, but are sensitive to corrosion in other environments. Generally speaking, industrial metallic materials that are resistant to corrosion in all environments do not exist

corrosion can be divided into uniformcorrosion, general corrosion and localized corrosion. The corrosion rate of overall corrosion can be expressed in mm/a (mm of corrosion per year). Generally, materials with corrosion rate below 0.1mm/a are regarded as corrosion-resistant materials. The corrosion rate is one order of magnitude higher than this, that is, the material with corrosion rate of 1mm/a can sometimes be used for general equipment. For the measuring elements of flow instruments, it is not allowed. According to the corrosion rate, the service life of metal can be predicted

1. Damage of corrosive medium to flow measuring instruments

the corrosivity of medium is a serious threat to flow measuring instruments. Only some kinds of flow meters such as clip on ultrasonic flow meters are less affected by corrosion

a. the corrosive medium corrodes the key parts of the flow measuring instrument in direct contact with the medium, causing them to be damaged and lose their functions. For example, the diaphragm of differential pressure transmitter is damaged due to corrosion, and the silicon oil leaks out, resulting in complete failure. Medium leakage caused by corrosion of electromagnetic flowmeter electrode leads to burning of outer ring of excitation line, etc

b. shorten instrument life. For example, the conical tube and other parts in the metal tube rotameter are worn out after several years of use

c. the key components of the flow measuring instrument are corroded by corrosive medium for a long time, which changes the geometric dimensions, resulting in the reduction of the accuracy of the instrument

for example, after the rotor in the rotameter is corroded by fluid, the overall dimension is reduced, resulting in low flow indication. Another example is that the vortex generator in the vortex flowmeter is corroded by the fluid and the width size decreases, and the surface of the upstream surface becomes rough, resulting in the change of the flow coefficient. Even the clamp on ultrasonic flowmeter, which is less affected by corrosive media, often loses sensitivity due to the potholes on the inner wall of the metal pipe corroded by the media, which weakens the transmitting and receiving signals

d. if the corrosive medium leakage is not found and handled in time, it is easy to lead to safety and personal accidents

2. Measures for fluid corrosion in flow measurement

1) regularly replace the instrument

2) avoid the important and take the light

avoid the important and take the light is to reasonably select the measurement scheme on the basis of in-depth understanding of the process flow and the characteristics of relevant media, so as to achieve the purpose of measurement or control of the production process, avoid the strong corrosive parts, and select the less corrosive parts, and even change the type of adjusted parameters. For example, if feasible, replace the flow constant value regulation system with liquid level uniform regulation or other appropriate variable regulation, so as to avoid the problem of corrosion resistance of flow measuring instruments

3) select instruments with corrosion resistance

① instrument selection for general acid medium. Vortex flow sensor and turbine flow sensor, the part in contact with the fluid is acid resistant steel, which can be used for acid liquid and gas in general. The oval gear flowmeter made of acid resistant steel can meet the needs of accurate measurement of general acid liquid. As for whether a specific product of a company is suitable for a specific medium of a user, in addition to consulting relevant samples and data, it is also necessary to consult the manufacturer in detail, and it is better to make a commitment. People will also improve the endurance and sports performance of the car

② instrument selection of conductive liquid. There are many kinds of lining materials for the measuring tube of electromagnetic flowmeter, among which polytetrafluoroethylene has the best corrosion resistance. There are several kinds of electrode materials, which can meet the needs of most corrosive media

③ instrument selection for non-conductive liquid. The clamped ultrasonic flowmeter is suitable for various corrosive fluids because the fluid does not directly contact with the instrument during operation

4) corrosive gas instrument selection

a. ultrasonic flowmeter

as long as the inner wall of the measuring pipe is treated with anti-corrosion treatment. However, the specific application examples have not been reported yet

the ultrasonic flowmeter developed in recent years with clip on transducer (which converts non electric energy into electric energy without external power supply, called transducer, also known as active sensor transducer, which is the core component of ultrasonic equipment) does not need to consider the corrosion resistance of the instrument if the pipeline itself is corrosion-resistant. For example, the pipeline uses a corrosion-resistant lining, but if there is an air gap between the lining and the metal pipe, it will also cause trouble for the clamp on ultrasonic flowmeter. For metal pipes without corrosion-resistant lining, their inner walls often become uneven after long-term corrosion, which often results in inconsistent acoustic emission of transducers installed in "V" shape and "W" shape, so the signal strength becomes weak, and even cannot be measured normally in serious cases. These should be paid attention to when using ultrasonic flowmeter

b. throttling differential pressure flowmeter

at present, there is no report on the finalized and commercialized throttling differential pressure flowmeter suitable for corrosive media, but such instruments developed by users have been reported decades ago, including the successful chlorine flow measurement

the technology of dealing with corrosive gases in process equipment has been mature for decades

in a word, corrosion resistance of flow measuring instruments is a long-term topic. New materials, new methods and new experiences are reported every year, which means that there should be some changes in the detection of materials. For some unpopular media, you can consult relevant literature, such as corrosion data and material selection manual

flow measurement of multiphase fluids

the first commercial multiphase flowmeter appeared about ten years ago, which was the result of the multiphase measurement research project in the early 1980s. The research centers and oil companies that have been committed to and are studying the development of multiphase flow measurement include Tulsa, SINTEF, Imperial University, National Engineering Laboratory, CMR, BP, Texaco, elf, shell, Agip and Petrobras

1. Basic principle

the main data measured by multiphase flowmeter is the mass flow of water and gas in the fluid. The current technology can not directly measure the mass flow of two phases in the fluid. At present, the indirect measurement method is adopted, that is, measuring the instantaneous rate of each component and their respective cross-section holdup

through phase separation, there is no need to measure the cross-section holdup, and the three volume flows can be measured by the traditional single-phase measurement technology. However, phase separation is expensive and difficult to achieve in many cases. If through the host 1) peak power of power supply: ≤ 10 kW (including peripheral installation such as cooling and high-temperature installation); Homogenizing the mixture to equalize the velocity can also reduce the measurement requirements to three. This is a more economical option and is at the heart of some commercial flow meters. However, the range of homogenization is always limited

therefore, both measurement methods have essential defects. It is for this reason that no completely satisfactory measurement method has been obtained so far

2. Measurement method of multiphase fluid

compact separation method - the most widely used, reliable and bulky

phase fraction and velocity measurement - the use conditions are limited

multiphase measurement by measuring the total flow and phase fraction - the practice of various commercial flow meters is expensive

using tracers - for calibration and moisture measurement

flow pattern identification - hardware combined with software, cheap? 6. Each phase

is measured separately - it is complex and difficult to calibrate

3. Classification of multiphase flowmeter

(1) separated multiphase flowmeter - separated total flow and sampling separation

(2) homogenized multiphase flowmeter

homogenized multiphase flowmeter consists of static mixer, venturi flowmeter (measuring total flow) γ Composition of X-ray analyzer (measuring moisture content)

the main difficulty of this multiphase flowmeter is that it is difficult to obtain a homogeneous mixture, especially when the void fraction is greater than 30%, the gas-liquid distribution will be uneven. For the mixing efficiency of the mixer and the possible blocking effect caused thereby

(3) heterogeneous treatment multiphase flowmeter

the homogenized multiphase flow measurement system and the heterogeneous multiphase flow measurement system do not need to separate the fluid before metering, Direct measurement

(4) using neural network technology

an adaptive nonlinear dynamic system composed of a large number of simple basic components neurons connected with each other. The structure and function of each neuron are relatively simple, but the system behavior produced by a large number of neuron combinations is very complex

the sensors monitoring multiphase flow get complex signals containing rich information. In order to extract the information of single-phase flow rate, higher-level mathematical processing methods are required

caltec and EDS SCICON, with the support of the British Ministry of health and safety, which are rarely compared by petroleum consortia and even the whole country, use artificial neural network technology to predict multiphase flow without complex traditional data processing system

the artificial neural network system develops its own methods to solve problems by analyzing examples, so the artificial neural network system is comparison rather than calculation

the difficulty of multiphase flowmeter lies in the need to measure the phase separation and flow rate of oil, gas and water. When caltec designed its artificial neural network system, it carried out large-scale experiments on multiphase fluid measurement by using capacitance test box, g-ray densimeter, acoustics and pressure sensor

these experiments produce a large number of complex data containing rich information, which contains the characteristics of natural fluids. Neural network system is to extract useful information from these data and compare it with the data of the fluid to be measured

ability to analyze through examples. Although the theoretical study of multiphase fluid is limited, there are abundant information data that can be developed by network technology

ability to deal with non-linear problems. Multiphase flow, especially the multiphase flow in the transition of flow pattern, shows a high degree of non-linear, and the neural network system can handle it well

the ability to extract information from trunk signals. Non intrusive sensor

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