Flow measurement involves a wide range of applications. The demand for flow measurement in high energy-consuming fields such as process measurement, energy metering, environmental protection, and transportation has increased rapidly, and new requirements have been put forward for flow measurement technology. It is not only required that the flow measuring instrument be resistant to high temperature and high pressure, but also can automatically compensate for the influence of parameter changes on the measurement accuracy. From the consideration of special requirements in terms of energy conservation, cost accounting, trade communication, and medicine and health, it is required that the flow measurement accuracy is high and the pressure loss is small. High reliability.
The development and application of new technologies, new devices, new materials and new processes, and new software have made the measurement accuracy of flow meters higher and higher, and the flow measurement range has become wider. At the same time, the requirement of measuring medium for flowmeters is decreasing, the scope of application is also becoming wider and wider, and the degree of intelligence and reliability have been greatly improved.
In the previous article, we introduced measurement techniques for small-flow and large-flow fluids. Today, we next describe flow measurement techniques for corrosive media and multiphase fluids.
Flow measurement of corrosive media
Corrosion is a phenomenon in which metals are damaged due to chemical action in their environment. All metals and alloys can be resistant to corrosion in certain environments, but they are sensitive to corrosion in other environments. In general, industrial metal materials that are resistant to corrosion in all environments do not exist.
Corrosion can be classified as uniform corrosion or general corrosion and localized corrosion. Corrosion rates for general corrosion can be expressed in units such as mm/a (millions of corrosion per year). Materials with a corrosion rate of 0.1 mm/a or less are generally used as corrosion-resistant materials. The corrosion rate is one order of magnitude greater than this, that is, the corrosion rate is 1 mm/a, and materials that can be used as appropriate for general equipment are sometimes discretionary. For flow meter measuring elements, it is not allowed. According to the corrosion rate, the life of the metal can be predicted.
1. Damage of corrosive media to flow measuring instruments
The corrosiveness of the medium is a serious threat to the flow measurement instrument. Only individual types of flowmeters such as clip-on ultrasonic flowmeters are less affected by corrosion.
a. Corrosive media Corrosion of key components that are in direct contact with the flow measurement instrument and the media causes damage and loss of functionality. For example, corrosion caused the diaphragm of the differential pressure transmitter to be damaged, and the silicone oil leaked completely and failed. Electromagnetic flowmeter electrode leakage due to corrosion caused by the media, leading to the outer ring of the excitation line burned.
b. Reduce the life of the meter. For example, parts such as tapered tubes in metal tube rotameters were worn after being used for several years.
c. The key components of the flow measurement instrument are subject to corrosion by corrosive media for a long time to change the geometric dimensions, resulting in reduced accuracy of the instrument.
For example, when the rotor in a rotameter is corroded by a fluid, the external dimensions are reduced, resulting in a low flow indication. In another example, the vortex generating body in a vortex flowmeter is corroded by a fluid and the width dimension is reduced, and the surface of the inflow surface becomes rough, thereby causing a change in the flow coefficient. Even clamped ultrasonic flowmeters, which are less affected by corrosive media, are often pitted by corroded media in the inner wall of the metal tube, weakening the transmitted and received signals, and severely losing sensitivity.
d. Leakage of corrosive media. If it is not discovered and treated in time, it can easily lead to safety and personal accidents.
2. Measures for fluid corrosion in flow measurement
1) Change the instrument regularly
2) Avoiding light
Avoiding light is based on deep understanding of the process flow and the characteristics of the relevant media, and a reasonable selection of the measurement program, also to achieve the purpose of measurement or control of the production process, to avoid the corrosive sites, and selected in the corrosive Parts, and even change the type of parameters being adjusted. For example, if possible, the flow setpoint adjustment system is replaced with a uniform level adjustment or other suitable variable adjustment to avoid the corrosion resistance problem of the flowmeter.
3) Select instrument with corrosion resistance
1 general acid medium instrument selection. The vortex flow sensor and the turbine flow sensor, which are in contact with the fluid, are acid-resistant steels, and generally can use acidic liquids and gases. The elliptic gear flowmeter made of acid-resistant gauge can meet the needs of accurate measurement of general acid liquids. As to whether a certain company's specific product is applicable to a certain user's specific media, in addition to consulting relevant samples and materials, it is necessary to consult the manufacturer in detail and make a better commitment.
2 Conductive liquid meter selection. Electromagnetic flowmeters have a variety of measuring tube lining materials, of which the best corrosion resistance is PTFE. There are several electrode materials that can meet the needs of most corrosive media.
3 non-conductive liquid meter selection. The clamped ultrasonic flowmeter does not directly contact the instrument when it is working, so it is suitable for all kinds of corrosive fluids.
4) Corrosive gas meter selection
a. Ultrasonic flowmeter.
As long as the measuring tube inner wall for anti-corrosion treatment. However, no specific application examples have been reported yet.
In recent years, ultrasonic flow has been developed with a built-in transducer (conversion of non-electric energy into electrical energy, no external power supply, transducer, also called active sensor transducer, which is the core device of ultrasonic equipment). If the pipe itself is resistant to corrosion, there is no need to consider the corrosion resistance of the instrument. For example, pipes use corrosion-resistant linings, but if there is an air gap between the lining and the metal pipes, it will also cause trouble for the clamped ultrasonic flowmeter. For metal pipes with non-corrosion-resistant linings, the corrosion of the inner walls of the metal pipes is prone to prolonged corrosion, often resulting in inconsistent acoustic emission of transducers of “V†shape and “W†shape, so the signal strength is weakened. Can't even measure normally. These should be noted when using ultrasonic flow meters.
b. Throttle differential pressure flowmeter.
There is no report on the stereotyped commercial throttling differential pressure flowmeters for corrosive media. However, such meters developed by users themselves were reported several decades ago. Among them, there have been successful chlorine flow measurements.
The technology of process equipment specialized in dealing with corrosive gases has matured decades ago.
In short, corrosion resistance of flow measuring instruments is a long-term topic. New materials, new methods, and new experiences have been reported year after year. For some unpopular media, consult the relevant literature, such as "Corrosion data and selection manual"
Multiphase fluid flow measurement
The first commercial multiphase flowmeter appeared about a decade ago and was the result of a multiphase metrology research project in the early 1980s. Research centers and oil companies that have worked on and are studying the development of multiphase flow measurement include: Tulsa, SINTEF, Imperial University, National Engineering Laboratory, CMR, BP, Texaco, Elf Oil, Shell Oil companies, Agip oil companies and Petrobras.
1, the basic principle
The main data measured by the multiphase flowmeter is the mass flow of water and gas in the fluid. Current technology cannot yet directly test the mass flow of two phases in a fluid. The current method of indirect measurement is to measure the instantaneous rate of each component and its respective section
With phase separation, there is no need to measure the cross-sectional holding ratio, and the three volumetric flow rates can be determined by conventional single-phase metering techniques. However, phase separation is expensive and difficult to achieve in many situations. If the velocity is equalized by homogenizing the mixture, the measurement requirement can also be reduced to three. This is a more economical option and is at the heart of some commercial flow meters. However, the scope for achieving homogenization is always limited.
As a result, both metrology methods are inherently flawed and it is for this reason that no fully satisfactory metrology method has been achieved so far.
2. Multiphase fluid measurement method
Compact separation method - the most widely used, reliable and bulky
Phase ratio and speed measurement - limited use conditions
Multiphase metering by measuring the total flow and phase fraction - a variety of commercial flow meter practices, expensive
Use tracers - for calibration and moisture measurements
Flow pattern recognition - hardware combined with software, cheaper price
Separate measurements - complex and difficult to calibrate
3, multi-phase flow meter classification
(1) Separate multi-phase flowmeter - separation of total flow and sampling separation
(2) homogeneous treatment of multi-phase flow meter
The homogeneous multiphase flowmeter consists of a static mixer, a venturi flowmeter (total flow measurement), and a gamma ray analyzer (measurement of moisture content).
The main difficulty of this type of multi-phase flow meter is that it is difficult to obtain a homogenous mixture. In particular, when the gas content is more than 30%, the gas-liquid distribution will be non-uniform, and the mixing efficiency for the mixer and the consequent jamming may be caused. effect
(3) Heterogeneous treatment of multiphase flowmeters
The homogeneous multiphase flow measurement system and the heterogeneous multiphase flow measurement system do not need to separate fluids before measurement, and are directly measured online.
(4) Using Neural Network Technology
A large number of simple basic elements, neurons, are interconnected to form an adaptive nonlinear dynamic system. The structure and function of each neuron is relatively simple, but the systematic behavior of a large number of neuron combinations is very complicated.
Sensors that monitor multi-phase flow receive complex signals that contain a wealth of information. In order to extract information on single-phase flow rates, higher-level mathematical processing methods are needed.
CALtec and EDS-Scicon, using the support of the oil consortium and the Health and Safety Department of the United Kingdom, use artificial neural network technology to predict multiphase flow without the need for complex traditional data processing systems.
The artificial neural network system develops its own solution to problems by analyzing examples. Therefore, the artificial neural network system is contrasted rather than calculated.
The difficulty of multiphase flowmeters is the need to measure the phase fraction and flow rate of the three-phase oil-gas water. In designing its artificial neural network system, CALTec conducted large-scale experiments on multiphase fluid measurements using capacitive test boxes, g-ray densitometers, acoustics, and pressure sensors.
These experiments produce a large amount of complex data that contains a wealth of information. The data contains the characteristics of natural fluids. The neural network system extracts useful information from these data and compares it with the data of the fluid to be measured.
The ability to analyze by example. Although the multiphase fluids for the theoretical study are limited, there is data with rich information that can be developed using network technology.
Ability to deal with non-linear problems. Multiphase flow, especially in the multiphase flow with a flow pattern transition, shows a high degree of non-linearity, and the neural network system can handle it well.
The ability to extract information from the trunk signal. Non-intrusive sensors are characterized by signal interference. The neural network system can not only extract information from signal interference but also understand the characteristics of the sensor.
Summarize the capabilities from the examples. Neural networks can be interpolated from a limited number of examples and perform some degree of extrapolation.
Integrate the ability to source data from three sources. This breaks through the deficiencies of a single sensor.
The ability to quickly establish effective solutions. Data comparisons, not program calculations.
The network system is composed of many contrasts. The comparison group is again composed of known inputs and expected response values. The input and output enter the neutron input layer of the network. The activated neutron signal is fed back in the network. For the multiphase flow measurement input signal (inter-short observation) is the sensor input value, and the output value includes the known gas and liquid phase velocity. According to the difference between the expected output value of the current output and all the examples, a satisfactory output is finally obtained through correction and comparison in the network system.
The experimental verification of the neural network technology predicted by the gas, liquid flow and actual measurement values ​​are more consistent, the average error of gas, liquid flow is less than ± 10%.
4, foreign major multiphase flowmeter
Daniel's MEGRA Multiphase Flowmeter
Using the Dual Gama Ray technology developed by SHELL Oil Company to measure the oil-gas-water content in the homogeneous flow and the built-in venturi cone flow measurement technology, the measurement accuracy is ±7%. On-line measurement parameters include: total flow of the mixture, each phase Flow, cumulative flow, moisture content, gas content, mixture viscosity, process pressure, temperature.
Agar Online Multiphase Flow Meter
Including a turbine flow meter and two venturi tubes, secondary instrumentation (used to indicate, record, or accumulate measurement results from a meter) Calculate the volumetric flow of gas and liquid based on the output of the three sensors; moisture content microwave monitoring Instrument to measure. It cannot be used for the measurement of high gas well flow.
Roxor RFM and Fluenta 1900 VI Flow Meters
The flow rate was measured using a combination of several different sensors, the total density was measured using a Cs-137 gamma densitometer, and the phase separation rate was determined in conjunction with capacitive and inductive sensors. A venturi tube has also been added to measure single-phase liquids or gases, thereby expanding the scope of application of the flowmeter. Mainly installed in offshore oil fields.
Framo online multiphase flow meter
The flowmeter makes the gas-liquid mixture even. The mixer consists of a large plenum and a reticular tube. A venturi tube and a Ba-133 dual-energy gamma sensor were installed downstream of the mixer to measure the total flow rate and the phase fraction. The flow meter is suitable for three-phase metering of offshore oil fields.
ESMER multiphase flow meter
The core of ESMER technology is based on an intelligent software system using simple sensors. Its basic principle is that any multiphase flow has a unique flow state; the only flow state can be quantified and characterized by a set of turbulent stochastic features; random features can Extracted from sensor signals that are sensitive to the flow state; there is a one-to-one correspondence between random features and multiphase flows.
Solartron's DualStream Condensate Natural Gas Flowmeter
Condensed natural gas generally refers to gas wells that have a gas phase volume fraction of greater than 90% and liquid and other component volume fractions of less than 10% under operating conditions.
The flow meter uses a mixer and a double venturi method to measure the total flow of condensate gas, and compensates for the temperature and pressure of the gas-liquid flow. When the confidence probability is 90%, the measurement accuracy is ±5%.
McCrometer's V-CONE Flow Meter
The structure of the flowmeter throttle is special. The flowmeter is the highest among the measurement indexes reported so far, and the measurement accuracy of the gas-liquid phase can reach 4% or less. The single-phase measurement accuracy is higher and can reach 0.2%. For single-phase, two-phase, three-phase flow measurement. No report was found on the application of this product.
5, foreign multiphase flowmeters generally have the following problems
The measurement range is narrow, and the measurement accuracy is greatly affected by the ratio of water to gas.
Some use microwave, gamma ray and other test methods, which are expensive and difficult to use on a large scale;
Some require special installations, inconvenient on-site applications or complicated processes.
Field Application Reliability Poor
6. Challenges
The use of nuclear techniques to determine the gas content, or the multiphase flowmeters that measure the gas content and the water content in the liquid, is obviously one of the technical difficulties in the future.
Dual-energy systems can measure the percentage of gas and water in the full scale range in a non-intrusive manner, but there are several issues that require special attention. First, the wall is the largest attenuator, especially low photon energy, and therefore requires high energy. Second, the mass absorption coefficient is actually very difficult to determine, which will have a series of effects on the uncertainty. Therefore, any uncertainty in the mass absorption system can affect the moisture measurement.
7. Future Trends of Multiphase Flowmeters
Better accuracy, reproducibility and reliability
The system has three sensors, each measuring a phase flow (solid, gas, water), and the measurement of each sensor is not affected by the presence of the other two phases.
Multipurpose multiphase flowmeter
Moisture measurement
Applying soft-sensing techniques such as neural networks (mainly consisting of the selection of auxiliary variables, data acquisition and processing, and soft-sensing models). The basic idea is to combine the automatic control theory with the knowledge of the production process, and to use computer technology to make it difficult to measure or For variables that can't be measured temporarily, choose some other variables that are easy to measure, infer or estimate by constructing a mathematical relationship, and replace hardware functions with software.)
Although the development of flow measurement technology is becoming more and more mature, it is still unsatisfactory in terms of measurement and application. There are many kinds of instruments and different types of instruments are selected for different occasions. To date, there is no reliable and accurate instrument that can meet various requirements. Therefore, in the selection of flow measurement instruments, on the basis of meeting the actual operational measurement requirements, the economics of the instrument are also required.
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