Abstract: In industrial production processes, flow measurement is relatively complex and it is not easy to obtain accurate parameters. With the continuous development of industrial technology, especially the demand for stable control in the chemical industry, the role of flow meters in the industrial field is becoming more and more important. Because the mass flow meter has obvious advantages in the flow meter, it is widely used in the chemical, steel, electric power and other industries as a field application instrument with high measurement accuracy and many measurement parameters. Due to the complexity of the on-site working conditions, and the asymmetry of the technical parameters provided by the user, the mass flowmeter has various problems in the use of the site. At present, the cost of the mass flow meter is relatively high, or involves trade settlement, etc., which requires manufacturers to be more cautious in the selection stage. This article summarizes the many aspects of the work encountered in this area, providing some experience and methods for users to get a better product experience.
The Coriolis mass flowmeter has high measurement accuracy, and the measurement is not affected by the physical properties of the medium, and there is no requirement for the length of the straight pipe section on the upstream and downstream. Moreover, the Coriolis mass flowmeter is a highly intelligent instrument. In addition to its ability to display and output mass flow, the transmitter can also output volumetric flow, density, temperature, and conversion to 4 mA to 20 mA signals, pulse signals or bus signals. Process parameters such as viscosity and concentration. Therefore, Coriolis mass flow meters are widely used in various industrial sectors such as chemical, pharmaceutical, energy, rubber, paper, food, etc., and are suitable for use in rationing, loading, and custody transfer.
The Coriolis effect means that if an object is stationary or moves at a constant speed relative to a fixed point, there is no problem with motion on the object. If you want to go from point A at one end of the object along a straight line to point B at the other end, you will not feel any difficulty during the walk. However, if different parts of an object move at different speeds, the situation is quite different, assuming a rotating game table or any platform that rotates around its center. The whole platform is rotating, but draws a small circle at a point near the center and thus moves slowly, while a point near the outer edge draws a large circle and thus moves quickly. As shown in Figure 1.
Coriolis force is also known in some places as the Coriolis force, referred to as Coriolis force, which is the deviation of the mass of a linear motion in a rotating system due to the linear motion of inertia relative to the rotating system. A description of the shift. Coriolis force comes from the inertia of the motion of the object. A Coriolis mass flow meter (referred to as a Coriolis flowmeter) is a meter that directly measures mass flow by utilizing the principle that a fluid flows in a vibrating tube to produce a Coriolis force proportional to mass flow.
According to the principle of Coriolis effect, if you want to generate Coriolis force, you must have two conditions: 1 rotating coordinate system; 2 for linear motion medium. According to these two conditions, the mass flowmeter must artificially establish a rotating system. Taking the double "U" type measuring tube sensor as an example, the electromagnetically driven method is used to make the back portion of the "U" type measuring tube periodically small. vibration. This is equivalent to rotating the "U"-shaped tube around a fixed axis (OO axis) at a periodic time, and its rotation direction changes periodically, like a pendulum.
The inlet and outlet sections of the "U" tube are fixed so that a rotating system with a "U" shaped tube inlet and outlet section as a fixed axis is created. The two open ends of the U-shaped tube are fixed, and the fluid flows in and out therefrom. The top of the U-shaped tube is equipped with an electromagnetic excitation device for driving the U-shaped tube so that its lead vibrates at a natural frequency with the O-O as the axis perpendicular to the plane of the U-shaped tube (Fig. 2A). The vibration of the U-shaped tube forces the fluid in the tube to move vertically along the pipe while flowing along the pipe, at which time the fluid will be subjected to Coriolis force while the fluid acts on the U-tube with a reaction force (Fig. 2B). Since the flow direction of the fluid on both sides of the U-shaped tube is opposite, the Coriolis forces acting on both sides of the U-shaped tube are equal in opposite directions, so that the U-shaped tube is subjected to a moment, and the tube end is twisted around the R-R axis. Torsional deformation (Fig. 2C) is produced, the magnitude of this deformation having a defined relationship with the mass flow through the flow meter. Therefore, by measuring the amount of deformation, the mass flow rate of the fluid in the tube can be measured. The measuring tube continuously vibrates at a fixed vibration frequency, and the vibration frequency changes as the density of the fluid changes. Thus, the resonant frequency is a function of the density of the fluid, from which a corresponding density output can be obtained.
The Coriolis mass flow meter consists of two parts, the sensor and the transmitter. The sensor is used for detecting the flow signal, and is mainly composed of a flange, a shunt, a flow tube, a drive coil, a detection coil and a drive, a detection magnetic steel, and a casing; the transmitter is used for the drive of the sensor and the flow detection signal. And flow display, signal output, mainly consists of power, drive, detection, display, output and other parts of the circuit.
The temperature of the fluid is measured by the Pt100 temperature electrode in the flow tube. The measured temperature is used to compensate the temperature of the measuring tube during online flow measurement, and can also be used as an independent temperature signal output. U-shaped flow tube and shunt, vibration isolation fixed plate, vacuum welded, as shown in Figure 3.
There are many types of Coriolis mass flow meters, which can be divided into single tube type and double tube type according to the number of measuring tubes; according to the shape of the measuring tube, it can be divided into curved shape and straight shape, and the curved measuring tube is also divided into U shape, Ω shape, Triangle, straight pipe, etc. (as shown in Figure 4); according to the measuring tube material, there are 316L, 904L, Hastelloy C-22 alloy, titanium, tantalum, etc.; according to the flowmeter installation method, there are flanges, threads, cards, hoop, Swagelok, etc.
According to the actual situation and technical parameters of the project, comprehensively consider the user's needs and cost savings, select the appropriate mass flowmeter of shape, material, nominal diameter, nominal pressure and installation mode, mainly pay attention to the following aspects:
5.1 Media aspects
The mass flow meter measures the difference in the medium, which mainly affects the diameter, accuracy and material of the flow meter.
1) Media status
Mainly divided into gas, liquid (in special cases, there will be two phases of gas and liquid). It will involve the accuracy requirements of the instrument and the caliber and flow tube material.
The gas is generally 0.5%, while the liquid is generally 0.2% and the highest is 0.1%.
5.2 Caliber aspect
Each manufacturer's flowmeter specific caliber will correspond to the corresponding flow range. While satisfying the user flow measurement, the viscosity of the medium should also be considered. This indicator relates to the pressure loss mentioned later. Sometimes a larger diameter sensor is selected to meet the user's pressure loss requirements.
5.3 Material aspects
Mainly considering the corrosiveness of the medium, it is often necessary to inquire the corrosion manual, and select the corresponding corrosion-resistant material according to the characteristics of the medium to prolong the service life of the flowmeter. It should be noted that the guidelines provided are for reference only. Minor changes in fluid properties (eg, temperature, concentration, impurity mass, etc.) are more likely to affect the compatibility of the wetted parts. The material compatibility option is the sole responsibility of the end user.
5.4 Operating temperature and pressure aspects
These two technical parameters relate to the normal application of the flowmeter and the safety issues.
1) Operating temperature
Under normal circumstances, more than 180 ° C, the selection is split type, and the sensor-related components are selected for high temperature type, in other cases, you can choose to be integrated; if the temperature is selected as an integrated type, the electronic components of the flow meter will Damage due to high temperature, causing malfunction of the flowmeter.
2) Operating pressure
The flow tube and flange of the mass flow meter will have a certain pressure. For example, the flange has 1.6MPa, 2.5MPa, 4MPa, 6.3MPa, etc., the flow tube has 10MPa, 35MPa, etc., the flow tube pressure of the flow meter during the selection and The nominal pressure of the flange must be greater than the user's operating pressure, leaving a portion of the redundancy.
5.5 Security aspects
According to the user's requirements, combined with the user's site conditions, choose both explosion-proof and non-explosion-proof.
5.6 Flow range
Under normal circumstances, there are several calibers or series that meet the user flow range. It is necessary to consider the factors such as cost, pressure loss, and diameter reduction to select the most suitable caliber.
5.7 Precision requirements
Aspects related to accuracy, including the state of the media, the maximum and minimum flows that need to be met, and so on.
5.8 Pressure loss requirements
Coriolis mass flowmeters are mainly used to measure the mass flow rate of the medium or the volume flow calculated by the output. The selection parameters provided by the process specialist only involve the medium mass flow rate or the volume flow rate, so the influence of the medium flow rate is ignored. The Coriolis mass flowmeter's measuring tube has a small amplitude and can be regarded as no moving parts and without obstructions. Its measured value is not affected by the flow field inside the pipe, so it can measure various non-Newtonian fluids as well as viscous and particulate-containing ones. Slurry, and the flow range ratio can be large. However, the maximum measured value of a Coriolis mass flowmeter with a nominal diameter is actually limited, and the medium flow rate is a factor. Pay attention to the problem of measuring the flow rate of the medium, in fact, pay attention to the problem of the pressure loss generated by the flow meter. Moreover, many Coriolis mass flowmeters now use a measuring tube of a curved shape such as a triangle to easily form a "turbulent flow". Therefore, the Coriolis mass flow meter may generate a large pressure loss, and the pressure loss can be even unacceptably high (0.1 MPa to 0.2 MPa). In addition, a large pressure loss is also likely to cause "cavitation" phenomenon, damaging the measuring tube of the flow meter.
In summary, when selecting a Coriolis mass flowmeter, it is necessary to consider whether the pressure loss is allowed by the entire process flow. Restriction of pressure loss is required, which limits the upper limit of the flow rate of the medium at a nominal diameter; or, under the requirement of ensuring the medium mass flow or the upper limit of the volume flow, the nominal diameter of the first stage is selected to expand the flow area. In addition, the flowmeter has a small nominal diameter, high measurement accuracy, and relatively low price; the nominal diameter is large, although the flowmeter pressure loss is reduced, but the measurement accuracy is reduced, and the procurement cost is increased. Therefore, it is necessary to strike a balance between accuracy and pressure loss to calculate the most appropriate nominal diameter.
At the same time of selection, we must pay attention to the size of the user's initial pipeline. When selecting the corresponding caliber according to other technical parameters, taking into account factors such as pressure loss and on-site construction, we must also consider the problem of reduction in diameter. Under normal circumstances, the pipe diameter can be reduced by one step, and the maximum diameter is half of the pipe diameter. For example, if the diameter of the user pipe is DN80, the minimum diameter of the flow meter can be DN40. Sometimes it will encounter special circumstances, that is, if the accuracy requirement cannot be met from the flow rate to ensure the diameter reduction requirement, it is necessary to negotiate with the user to confirm the situation and requirements of the site.
5.10 Special needs
In some cases, such as the measurement of water-containing crude oil, users often need to know the water content and other data, which requires the selected mass flow meter to contain the function of concentration measurement. Other requirements such as dual current output with RS485, HARTProfibus DP/PA communication, etc.
How to better use the mass flow meter to make it work better in production, still need to accumulate more experience and carry out better analysis and improvement. Huaheng Instrument organizes the above content and hopes to give relevant units and personnel reference.