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Application analysis of vortex flowmeter in steam metering of alumina production

2016-05-23

During the production of alumina by the Bayer process, steam is an important energy source in the operation of alumina enterprises. It is used to heat caustic soda solutions and bauxite ore, and it is also used to heat the mother liquor after concentration and decomposition. According to statistics, steam consumption accounts for 40% to 60% of the alumina energy cost. For alumina enterprises, steam consumption has become an important part of enterprise costs and plays a decisive factor in the development and survival of alumina enterprises.

1 Steam consumption distribution and current status in alumina production

In Bayer alumina production, the comprehensive process energy consumption is mainly composed of steam, electricity, water, compressed air and fuel. Among them, steam is one of the main energy consumption indicators for alumina production. The level of steam consumption directly reflects the production of this enterprise. The ability to manage and control production.

Taking a certain alumina as an example, the comprehensive energy consumption of the process is about 15.72 GJ / t-Al 2 O 3 and the steam accounts for 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, the steam metering points of alumina enterprises are generally 8 to 10, the steam consumption ranges from 3.5 to 206 t / h, and the temperature and pressure are also different.

Table 1 Steam point of steam for an alumina company

At present, there are two main types of steam measurement instruments in alumina enterprises. One is based on the principle of Bernoulli's equation, such as differential pressure orifice plates, sonic nozzles, Annubar, and Delta Taba. The principle of linear continuous equations, such as vortex flowmeters and mass flowmeters, is mainly based on vortex flowmeters because mass flowmeters are expensive and accuracy is lost at high temperatures.

The steam metering instrument based on the principle of Bernoulli's equation, the flow metering equation is (1):

,因此,计量仪表的标定在氧化铝企业中是个很麻烦而且往往不准确,一般生产厂大都采用水标定,这种方法标定出来的精度,不代表测量蒸汽的精度。 Formula (1) shows that, on the one hand, because of the density of the medium vapor ρƒ in the equation , the calibration of measuring instruments is very troublesome and often inaccurate in alumina enterprises. Most manufacturers use water calibration. This method is used for calibration. The accuracy given does not represent the accuracy of the steam measurement. τ ,对于不同蒸汽,不同的压力,不同的温度条件下的膨胀系数都不尽相同,因此,即使用压缩空气进行标定出来的精度也不代表测量蒸汽的精度。 On the other hand, since there is also a gas expansion coefficient τ in equation (1), the expansion coefficients are different for different steam, different pressures, and different temperature conditions. Therefore, the accuracy of calibration using compressed air It does not represent the accuracy of measuring steam.

2 Working principle of vortex flowmeter

The vortex flowmeter is based on the principle of a linear continuous equation:

或者f=St×d/v (2) St = f × d / v or f = St × d / v (2)

among them:

St Strauhal number, dimensionless;

v medium flow rate;

f frequency Hz / S:

d Upstream width.

,另一个是斯特劳哈尔数 St 是雷诺数Re的函数,当雷诺数Re到达20000后,斯特劳哈尔数 St 就是一个常数。 In this formula (2), there are two functions, one is the fluid flow velocity v , and the other is the Strouhal number St is a function of the Reynolds number Re. When the Reynolds number Re reaches 20000, the Strouhal number St Is a constant. When the shape of the vortex flowmeter measurement body is fixed, the width of the front surface is constant. In this way, the frequency depends only on the fluid flow rate and is a linear continuous equation. The flowmeter measurement equation is derived by deriving the measurement model of the vortex flowmeter. (3):

Figure 1 Schematic diagram of measurement model of vortex flowmeter

(3)

斯特劳哈尔数,无量纲; f频率Hz/S;D流量计通径; d 迎流面宽度。 Among them: St Strouhal number, dimensionless; f frequency Hz / S; D flowmeter diameter; d front surface width.

The vortex flowmeter is based on the principle of a linear continuous equation. As long as the Reynolds number is greater than 20,000, the measurement has nothing to do with the nature of the fluid, and has nothing to do with density, viscosity, etc. The flow coefficient calibrated with water is also suitable for steam measurement, and the flow coefficient calibrated at normal temperature is also suitable for low temperature and high temperature steam. In this way, the vortex flowmeter is more able to meet the needs of high and low pressure, different temperature steam measurement in alumina enterprises.

make

K is the flow coefficient, which means that each liter of flow is represented by several pulses, that is, P / L. After formula conversion, the actual vortex flowmeter calculation formula (4):

(4)

In general, the flow coefficient of the vortex flowmeter is actually set at the time of design, but each factory must calibrate each flowmeter to check manufacturing errors. But no matter what this number is, the calibrated flow coefficient is a constant that is correctly measured, and is suitable for steam in different working conditions.

In addition, it should be noted that from the formula (4), the vortex flowmeter can only measure the working volume of the fluid, and the mass and standard volume are converted based on density or pressure and temperature.

3 Steam metering design and selection in alumina production

Taking Table 1 as an example, the alumina company's steam needs to measure about 8 to 10 points, and it is generally superheated steam. The mathematical model is a binary function. Therefore, pressure and temperature double compensation should be considered during design.

3.1 Selection of process parameters

Factors to be considered in design and selection include: flow parameter range, pressure parameter range, temperature parameter range, and flow path selection.

At the same time follow the following principles:

(1) Consider whether the minimum flow rate is within the working range of the diameter under the condition of maximum pressure;

(2) Consider whether the maximum flow rate exceeds the working range of the diameter under the condition of minimum pressure;

In addition, the maximum temperature corresponding to the pulse output should consider the expansion coefficient of the body width and the flow coefficient Kt under the working temperature conditions:

(5)

among them:

T is the working temperature; K is the standard flow coefficient (usually on the instrument nameplate).

3.2 Path selection

When designing and selecting, taking into account the reduction of pressure loss, there are certain requirements for the lower limit of the flow rate when measuring steam. The diameter of the flowmeter should be selected according to the size of the steam flow rate, and the diameter of the flowmeter cannot be selected based on the original pipe diameter .

The selection of steam flowmeter diameter can be verified by table lookup and calculation.

The table lookup method is to find the flow path on the product sample according to the steam measurable range table under working pressure; then calculate and verify whether the flow rate and Reynolds number are within a certain range.

(1) Velocity calculation: The range of steam velocity is generally 2m / s ~ 60m / s, and the highest should be <80m / s.

(6)

Among them: V is the flow rate; Q is the volume flow; D is the flow path diameter.

(2) Reynolds number calculation method: ① Reynolds number at the minimum flow rate shall not be less than 5000; ② Reynolds number at the minimum flow rate to ensure accuracy shall not be less than 20,000.

(7)

Among them: V is the flow rate; ρ is the density; μ medium viscosity; D is the flowmeter diameter.

Of course, some products can provide computer selection software, and directly input related process parameters to get the desired results.

3.3 Notes

(1) It is recommended to use the pulse (frequency) output mode when selecting the signal output. If a current signal is required to be output, the conversion process will lose ± 0.2% to 0.4% accuracy.

(2) In alumina enterprises, the temperature of steam is generally greater than 150 ° C. It is recommended to use a split type vortex flowmeter. In addition, temperature and pressure compensation must be considered.

4 Installation requirements

In the steam measurement process of alumina enterprises, the stable flow state of the measured steam is a prerequisite to ensure accurate measurement of the vortex flowmeter. To make the measured value of the vortex flowmeter more temperature and accurate, the following points should be paid attention to during installation.

(1) Installation Make sure that the arrow on the sensor nameplate points to the direction of the fluid flow.

(2) It is recommended to follow the requirements of the straight pipe section for installation, see Table 2. When the site cannot meet the requirements of the straight pipe section, it is recommended to use a flow ballast matched with the vortex flowmeter.

Table 2 Straight pipe length reference before and after special conditions

(3) Install the sensor to avoid the vibration source or place where the vibration is small, and keep it away from high-power electrical equipment. (4) Pay attention to the position of the ground during welding to prevent damage to the piezoelectric chip. Generally, the left and right welding parts are required.

5 FAQs and notes

5.1 No flow signal

First check to determine if there is vibration. If there is slight vibration, the treatment method is external mechanical vibration reduction measures, such as using rubber soft joints, rubber cushions, etc., and fixing the pipe with a bracket; . If there is severe vibration, change the installation position of the flowmeter, strengthen the pipeline, and add soft connections to the front and rear of the flowmeter.

Secondly, to detect and judge whether there is 50HZ power frequency interference, a frequency meter measurement or a current value calculation method is usually used. Here is the current value calculation method, calculation formula (8):

or (8)

among them:

Q f volume square m 3 / h; Q m mass square Kg / h; ρ operating density Kg / m 3 ; K t meter flow coefficient.

If the calculated mail is or is close to 50HZ, you can take to ensure that the grounding screw of the converter shell is reliably grounded or the entire converter is floating.

5.2 No flow signal

First check whether the wiring is correct, whether the voltage level of the power supply meets the requirements, whether the power polarity is connected correctly, and whether the special cable is connected correctly in the split type, and then check whether the generator and the converter are damaged.

Secondly, determine whether the actual process steam flow is too small. The judgment method is to regularly and continuously tap next to the pipeline near the flowmeter. No matter what type of flowmeter, the flow rate display will appear. The treatment method is to replace the flowmeter with a smaller diameter or increase the flow if the process conditions allow.

5.3 Unstable traffic

Observe on site whether the gasket is concentric or extends into the pipeline when the flowmeter is installed, and whether the process pipeline meets the maximum requirements of Table 2 alumina enterprise installation requirements or product specifications. The solution is to choose the correct gasket and assemble it correctly, change the installation position of the flow meter or increase the flow ballast.

6 summary

Vortex flowmeter is a new type of flowmeter based on the principle of linear continuous equation. Because of its good medium adaptability, it is an ideal substitute for throttling flowmeters. It can measure the high and low pressure steam of alumina enterprises and is widely used in the alumina industry. However, only under the conditions of correct design, correct selection, and correct installation, can its measurement be accurate and stable, and play its due role in reducing the energy consumption of alumina.

Article excerpts: Gan Changneng, Li Liang. Application analysis of vortex flowmeter in steam metering of alumina production . China Instrumentation, 2016 (4) .

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