Abstract: The working principle of vortex flowmeter, precautions for selection and installation requirements are analyzed. Common problems and corresponding countermeasures of vortex flowmeter in the process of alumina production steam metering are analyzed.
In the process of producing alumina by the Bayer process, steam is an important energy source in the operation of alumina companies. It is used to heat caustic soda solution and bauxite ore, and it is also used to heat and concentrate the decomposed mother liquor. According to statistics, steam consumption accounts for 40% to 60% of alumina energy costs. For alumina companies, steam consumption has become an important part of the company's cost and plays a decisive role in the development and survival of alumina companies.
1 Distribution of steam consumption in alumina production and current status In the Bayer process alumina production, the overall energy consumption of the process is mainly composed of steam, electricity, water, compressed air and fuel, of which steam is the main energy consumption indicator for alumina production. First, the level of steam consumption directly reflects the company's production management and ability to control production. Taking a certain alumina as an example, the overall energy consumption of the process is about 15.72 GJ/t-Al2O3 and the steam is about 42%. The steam is composed of high-pressure steam and low-pressure steam. Table 1 shows the high and low pressure steam consumption points of an alumina company.
Taking Table 1 as an example, alumina companies need to have a measuring point of 8 to 10 steams and a steam consumption of 3.5 to 206 t/h. The temperature and pressure are not the same.
At present, there are two types of steam metering instruments used in alumina companies. One is based on the principle of the Bernoulli equation, such as differential pressure orifices, sonic nozzles, annubars, deltaba, balanced type, etc. In addition, differential pressure orifices, Annubar, Deltaba account for more; the other is based on the principle of linear continuous equations, such as vortex flowmeters, mass flowmeters, because of the high cost of mass flowmeters, and the high temperature state Under the loss of precision, it is mainly measured by vortex flowmeters.
Based on the principle of the Bernoulli equation, the flow metering equation is (1):
Equation (1) shows that on the one hand, because of the density Ïf of the medium vapor in the equation, the calibration of metering equipment is very troublesome and often inaccurate in alumina companies. Most of the production plants use water calibration. The calibrated accuracy does not mean that the accuracy of the steam is measured. On the other hand, because of the expansion coefficient Ï„ of a gas in equation (1), for different steams, different pressures, and different temperature conditions, the expansion coefficients are not the same. Therefore, the compressed air is used for calibration. Accuracy does not mean that the accuracy of the steam is measured.
2, vortex flowmeter working principle
3 steam metering design selection in alumina production
Taking Table 1 as an example, alumina companies need to measure about 8 to 10 points of steam, which are generally superheated steam. The mathematical model is a binary function. Therefore, pressure and temperature compensation must be considered when designing.
3.1 Selection of process parameters
Factors to consider when designing a model include: flow parameter range, pressure parameter range, temperature parameter range, and flow path selection. At the same time follow the following principles:
(1) Under the maximum pressure, consider whether the minimum flow is within the working range of the path;
(2) Within the working range where the maximum flow is considered to exceed the path at the lowest pressure;
In addition, when the maximum temperature corresponds to the pulse output, the expansion coefficient of the body width should be considered, and the flow coefficient Kt under the working condition temperature condition:
among them:
T is the operating temperature; K is the standard flow coefficient (usually on the instrument nameplate).
3.2 Path Selection
When designing and selecting the type, taking into account the reduction of pressure loss, in the steam metering, there is a certain requirement for the lower limit of the flow. The flow path should be selected according to the size of the steam flow, and the flowmeter path cannot be selected according to the original pipe inner diameter. .
Steam flow meter path selection can use table look-up method and calculation verification.
The table look-up method is to check the flow path on the product sample under the working pressure range under the operating pressure range, and then calculate whether the verification flow rate, Reynolds number is within a certain range.
(1) Calculation of flow rate: The range of steam velocity is generally 2m/s~60m/s, and the maximum should be less than 80m/s.
V=4×Q/(3600×π×D×D) (6)
Where: V is the flow rate; Q is the volume flow: D is the flowmeter path.
(2) Reynolds number calculation method: 1 The Reynolds number at the lowest flow rate shall not be less than 5000; 2 The Reynolds number at the minimum flow rate to ensure accuracy shall not be less than 20,000.
Re"[(Ï×V×D) /μ] (7)
Where: V is the flow rate; Ï is the density; μ medium viscosity; D is the flowmeter path.
Of course, some products can provide computer selection software, directly input the relevant process parameters, you can get the desired results.
3.3 Precautions
(1) The pulse (frequency) output method is recommended when selecting the signal output. If the output current signal is required, the conversion process will lose ±0.2%~0.4% accuracy.
(2) In alumina enterprises, the temperature of steam is generally greater than 150°C. It is recommended to use a separate vortex flowmeter. In addition, temperature and pressure compensation must be considered.
4 Installation requirements
In the steam measurement process of alumina enterprises, the steady flow state of the measured steam is a prerequisite for accurate measurement of the vortex flowmeter. To make the measured value of the vortex flowmeter more stable and accurate, the following points must be noted during installation.
(1) Installation Ensure that the arrow on the sensor nameplate points in the same direction as the fluid flow.
(2) The installation proposal follows the requirements of the straight pipe section. See Table 2. When the site cannot meet the requirements of the straight pipe section, it is recommended to use the flow ballast matched with the vortex flowmeter.
(3) Mount the sensor away from the vibration source or in the place where the vibration is less, away from the high-power equipment.
(4) When installing the tape, pay attention to the position of the ground iron to prevent the damage of the piezoelectric wafer. Generally, the left part of the welding part is left and the right part is welded.
5 Frequently Asked Questions and Precautions
5.1 No traffic signal
Firstly, determine if there is vibration. If there is slight vibration, the treatment method is external machinery vibration elimination measures, such as rubber soft joints, rubber cushions, etc., and use brackets to fix the pipeline; or adjust the parameter settings according to the product manual to eliminate vibration. . If severe vibration occurs, change the installation position of the flowmeter, reinforce the pipeline, and add soft connections before and after the flowmeter.
Second detection to determine whether there is 50Hz frequency interference usually use frequency meter measurement or current value calculation method. Here is how to calculate the current value, and calculate the formula (8):
If the calculated result is at or close to 50Hz, ensure that the housing ground screw of the converter is reliably grounded or that the entire converter floats.
5.2 No traffic signal
First, check whether the wiring is correct, whether the voltage level of the power supply meets the requirements, whether the polarity of the power supply is connected correctly, and whether the special type cable is connected with the correct type is checked, and then the check occurs.
Body, converter is damaged. Secondly, it is judged whether the actual process steam flow is too small. The judgment method is to continuously hit the pipe near the flowmeter in a regular manner. No matter which type of flowmeter, flow rate display will occur. The solution is to replace the smaller diameter flow meter or increase the flow rate as the process conditions allow.
5.3 Unstable Traffic
Observe whether the spacers are concentric or reach the pipeline when the flowmeter is installed, and whether the process pipeline meets the requirements of Table 2 alumina enterprise installation or the maximum requirements of the product specification. The treatment method is to correctly select the matching gasket and correctly assemble, change the installation position or add flow of the flowmeter.
Ballasts.
6 Summary
The vortex flowmeter is a new type of flowmeter that is based on the principle of linear continuous equations. It is an ideal alternative to throttling flowmeters due to its good media adaptability. It can measure high and low pressure steam in alumina companies and is widely used in alumina industry. However, accurate and stable measurement can only be ensured under the conditions of correct design, correct type selection and correct installation, and it should play its due role in reducing the energy consumption of alumina.
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