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1, on-site water level meter:
The local water level gauge is a direct-reading instrument installed on the boiler standard. It is a basic equipment required by the boiler plant. The large-capacity units are all monitored by industrial television and transmitted to the central control room. Generally, they are equipped with two sets, respectively installed on the steam. Both ends of the package.
The local water level gauge is divided into glass, mica and bull's eye. The working principle is the principle of connecting pipes. The principle of connecting pipes is: Under the condition of the same liquid density, the liquid level of each branch pipe in the connecting pipe is at the same height. The in-situ water level gauge is shown in Figure 1. 
In the formula:
h - the distance from the normal water level of the steam drum to the sampling on the water side, mm
△h——The difference between the water level in the water level gauge and the water level in the drum, mm
Ps - saturated steam density, kg/m3
Pw - saturated water density, kg/m3
Pa - the average density of water in the water gauge, kg/m3
Ps' - the density of steam in the water level gauge, kg/m3
For the in-place water level gauge, the water temperature in the steam drum is the saturated temperature under the corresponding pressure, and the saturated steam enters the water level gauge through the steam sampling hole. The ambient temperature of the water level gauge is much lower than the steam temperature, so that the steam condenses into water continuously. And forcing the excess water in the water level meter to flow back to the steam drum through the water side sampling pipe.
From the characteristics table of water and steam, it can be seen that under the normal temperature and pressure, the density of water in the steam drum and water level meter is equal. From the formula (1), the water level in the water level meter is the same as the water level in the steam drum. , and has nothing to do with the value of h; With the increase of the steam pressure, the water density in the steam drum becomes smaller and the steam density becomes larger; and the water density in the water level gauge becomes smaller due to the effect of heat dissipation on the in-situ water level gauge. However, the magnitude of change is not as great as that of the water inside the steam drum; although the steam density is also increased, the change amplitude is not large in the steam drum, that is, Ps should not be equal to Ps', but the effect is negligible as long as the insulation treatment is good. The following calculations are based on Ps=Ps; the difference between the water level in the water level gauge and the water level in the drum is also increased. This difference is always the water level in the water level gauge is lower than the steam drum. The main factor of the water level; and when the value of h changes, the water level difference also changes.
In order to provide reference for the power plant, some boiler plants provided reference data of the difference between the in-situ water level gauge and the normal water level of the steam drum. 
From the data listed in Table 1, for subcritical boilers, the water level of the in-situ water level gauge is 100-150mm lower than the water level in the steam drum under the rated steam pressure. In the following, our plant (Orient Boiler Factory) analyzes when the drum water level deviates from the normal water level at a drum pressure of 18.2 MPa. According to formula (1), when the drum water level is zero, h=400 mm, and the calculated water level changes. The water level meter shows the situation at 100 mm. Pw and Ps are fixed values. Assume that Pa is a constant value and take the average temperature at 300°C. h'=h-Δh is the water column height in the in-situ water level meter. The calculation results are shown in Table 2. 
From the calculation results in the table, when the drum level changes by ±100mm, the displayed value of the in-situ water level meter only changes by ±68mm, or it is assumed that the temperature of the water in the water level gauge is constant, that is, Pa is a fixed value. In fact, when the water level in the drum changes, the average temperature and density of the water in the water level gauge will change. When the water level of the drum rises, the average temperature will drop and the density will increase due to the increase of the water cooling surface. The indicator of the water level gauge is also lower than the one calculated in the table; when the water level of the drum decreases, the heat radiation surface of the water decreases, its average temperature increases, the density decreases, and the indication of the water level gauge should be higher than calculated in the table. . When the water level of the drum changes by ±100mm, the change of the local water level gauge does not reach ±68mm, but is only about ±50mm, and the error of the local water level gauge is not a constant value. It changes under different conditions. The same boiler, Under different operating conditions, the change in error is still quite significant in different seasons. Therefore, it is unsafe and inaccurate to rely on the local water level meter to monitor the water level of the steam drum. It is necessary to change the indication of the in-situ water level gauge during operation is accurate, and require that other water level gauges be consistent with their instructions. The in-situ water level gauge can be used as a reference for checking the normal water level value (zero) of other water level gauges under the rated pressure.
2. The working principle of the water level gauge of the electric contact level meter is the same as that of the in-place water level gauge. It belongs to the communicating tube type. The electric contact point on the measuring tube connected to the pressurized container is immersed in the water and exposed to the steam. The difference in conductivity is shown by the meter displaying the water level. Generally only one set is installed at one end of the steam drum, which is sent to the centralized control room for monitoring through the signal cable, and some also brings the contact signal into the shutdown protection system.
The working principle of the electric contact water level gauge is the same as that of the local water level gauge. Therefore, the existing problem of the local water level gauge also exists. That is, there is a deviation between the water level displayed by the electric contact water level gauge and the actual water level of the steam drum, and it is not fixed. When the water level fluctuates, its display cannot correspond to it. Electrical contact water level gauges and on-site water level meters due to different structure, material, shape, installation, heat dissipation, the display value between them will inevitably there is a deviation; electrical contact water level meter also exists in the electrical contacts due to hanging water and send signals by mistake problem. Therefore, it is unsafe and inaccurate to measure the water level by using an electric contact level gauge on a subcritical boiler, and it is less desirable as a signal for protection.
3, differential pressure level gauge Differential pressure level gauge working principle is to install the balance container on the drum level sensor, the use of liquid static principles to convert the water level into differential pressure, the differential pressure signal with a pressure tube to poor Pressure gauge, differential pressure gauge shows that the drum is not in place. After the development, the intelligent differential pressure transmitter is now used to measure the water level of the drum. Especially the introduction of computer control technology has greatly improved the technical performance, safety and reliability. Subcritical boilers now use differential pressure. The water level meter is used as the main measure of the water level of the drum, and it is used as the water level control and protection signal of the drum.
Balance container is also called condensation ball. According to different requirements for measurement accuracy, there are several kinds of balanced containers: single-chamber balance container, double-chamber balance container, and steam hood compensating type balance container. With the introduction of computer control technology, the use of smart transmitters, the results obtained in the operation of the sensor are far more accurate than those corrected by compensation. Therefore, the subcritical boilers use a balanced container with simple structure to measure the water level. Here's how to measure the water level in a single chamber balance container.
The principle of measuring the water level in a single chamber balance container is shown in Figure 2:
A pipe is drawn from the steam side sampling hole to the balance vessel, the saturated steam entering the balance vessel is continuously condensed into water, and the excess water flows back to the steam drum from the sampling pipe due to the overflow principle, so that the water level in the balance vessel is kept constant. Therefore, the positive pressure head of the differential pressure transmitter remains unchanged because the equilibrium vessel has a constant water column, and the negative pressure head changes with the change of the water level of the drum. In order to avoid changes in the water level of the drum, affecting the change of the water level in the balance container and affecting the accuracy of the measurement of the water level of the drum, the area of ​​the container should be large enough.
The relationship between the differential pressure transmitter differential pressure and the drum water level can be obtained from Figure 2 as follows: 
In the formula:
H - distance of the sampling hole on the soda side, mm
L - distance between steam sampling hole and steam drum zero water level, mm
h - the value of the water level deviation of the drum water level, mm
â–³P - differential pressure value corresponding to drum level, mmH2O
Ps - saturated steam density, kg/m3
Pw - saturated water density, kg/m3
Pa——Balance of reference water column density,kg/m3
In formula (2), H and L are constant values, and Ps and Pw are functions of the pressure of the steam drum. Pa is affected not only by the pressure of the steam drum, but also the heat dissipation condition of the balance vessel and the ambient temperature. Saturated steam enters the balance vessel and continuously condenses into water. The water temperature on the inner surface of the vessel is close to the saturation temperature in the steam drum. The equilibrium vessel and its lower sampling pipe are cooled by the environment, and the temperature keeps decreasing. With the decrease of the height, the temperature in the sampling pipe decreases. Will be close to the ambient temperature. The water temperature of the reference water column is higher than the ambient temperature, but it is much lower than the saturation temperature in the steam drum. The water temperature of the reference water column generally adopts the method of taking the average value. Considering the constant, generally take 50°C or 60°C; now some power plants also adopt the method of directly measuring the temperature of the reference water column to correct.
Since the soda density changes with pressure, the pressure difference generated by the same drum level under different pressure conditions is different. Taking the natural circulation drum furnace of our factory as an example, it is known that the inner diameter of the drum is 1792mm, the zero water level is 50mm below the drum centerline, the water side sampling hole is 400mm below the zero water level, and the steam side distance is above zero water level 360mm, H=400 +360 = 760 mm, the average temperature of the reference water column is 60 °C, and the results shown in Table 3 are calculated. 
The results in Table 3 show that at atmospheric pressure, when the water level of the drum reaches the steam sampling hole, the pressure difference is the minimum, which is equal to zero; when it is dropped to the sampling hole on the water side, the pressure difference is the greatest, equal to 760 mmH2O. Therefore, the transmitter range for measuring the water level of the drum is 760 ~ 0mmH2O, which is the distance between the sampling holes on the steam side. As the steam pressure rises, the same amount of change in the pressure level of the drum water level decreases. Drum water level change soil 250mm, atmospheric pressure pressure change 500mmH2O, pressure rise to 9Mpa when the pressure difference is 329mmH2O, rose to 18Mpa, the pressure change is only 205mmH2O, and the higher the water level, the greater the impact of pressure, The lower the water level, the less affected. Therefore, the pressure change will bring considerable error to the water level measurement. However, this error is only caused by the pressure change. Therefore, adding pressure correction to the measurement circuit of the differential pressure level gauge can cause pressure-induced Measurement error eliminated.
The principle of pressure correction is as follows:
Available from (2) 
According to formula (3), the correction circuit shown in Fig. 3 can be obtained to correct the error caused by the influence of the pressure of the steam drum pressure on the water level measurement of the drum. Correcting the two functions F1(x) and F2(x) in the loop can be easily implemented and completed by the computer control system. The correction circuit is shown in Figure 3. 
Equilibrium vessel reference water column due to the influence of the environment, uneven temperature distribution, equilibrium container upper temperature close to the saturation temperature, down gradually until close to the ambient temperature, calculated according to the average temperature, there must be an error; and the reference water column The height is affected by the steam pressure, operating conditions, installation, etc., as well as errors.
When the reference injection column average density changes Δpa, the error of the drum water level measurement will be: 
When the average temperature of the reference water column changes, the error generated at 20°C is shown in Table 4. 
From Table 4, it can be seen that the influence of the average temperature of the reference water column on the measurement error of the drum level increases with the increase of the drum pressure and increases with the increase of the average temperature, 50°C and below. The impact is relatively small; therefore, the average temperature of the reference water column should be as small as possible and the distribution should be uniform.
When the height of the reference water column changes, set the height error to ΔH. The error caused by the water level measurement of the drum is: 
4. Reasons Affecting Drum Water Level Measurement According to the analysis of several water level measurement methods, the main reasons affecting the water level measurement are as follows:
4.1 Drum level meter installation conditions, location, environmental impact, water level meter positioning deviation is generally 10 ~ 50mm, the water level meter in the environment there is a difference, affecting heat dissipation;
4.2 The influence of drum installation conditions, the degree of steam drum installation requirements should be ≤ 5mm, but after a few years of boiler operation, will change, reaching 15 ~ 20mm, water level meter installation is based on the center line of the drum as the standard , resulting in error when installing the water level gauge;
4.3 Influence of the water entering the steam drum from the water supply and water wall, the temperature of the feed water is not constant due to the influence of various heating environments, and the water temperature is lower than the saturation temperature at the corresponding pressure; the water entering the water wall contains a large amount of Bubbles, and continue to evaporate, the density of which will be less than the density of water at the corresponding temperature and pressure;
4.4 Impact of the descending pipe In boiler operation, the water in the steam drum continuously enters the descending pipe at a high speed, making the water level in the steam drum not an ideal horizontal plane, and different levels may be generated with different positions of the descending pipe. Up to 40 ~ 60mm;
4.5 The inherent error of the measuring instrument itself. Although the accuracy of the instrument is already high, there are still measurement and installation errors. 5. Methods and measures to reduce the measurement error of the water level of the drum 5.1 Reasonable sampling position should be higher than the water level protection setting The height, and there is a certain margin;
5.2 suitable sampling pipe diameter, to reduce the flow resistance, to prevent water level display lag;
5.3 Minimize the length of the connecting pipe, reduce the flow resistance, increase the temperature of the medium in the connecting pipe, and the horizontal section before balancing the container should have sufficient length to facilitate the condensation of steam;
5.4 add a connecting pipe between the sampling pipe of the soft drink as damping to buffer the influence of the water level measurement when the water level of the steam drum fluctuates greatly;
5.5 Each water level meter should use independent sampling holes, sampling lines, and balance vessels to avoid mutual interference;
5.6 The steam side sampling pipe is inclined to the steam drum to facilitate the return of condensed water to ensure the water level in the balance container is constant;
5.7 Reasonable heat preservation of the pipeline can not only ensure the temperature of the medium, but also can fully dissipate heat.
The water level of the drum is one of the main monitoring parameters of the power plant. Correctly measuring the water level of the drum is a guarantee for the safe operation of the boiler. The traditional measurement methods are: local two-color water level gauge, electrical contact level gauge, differential pressure level gauge (single chamber or double chamber balanced container compensation type). The measurement errors of the in-situ water level gauge and the electrical contact water level gauge are affected by the boiler pressure, heat dissipation, installation form, and actual water level, and it is difficult to accurately calculate. Therefore, high-parameter, large-capacity units are mostly compensated differential pressure level gauges as the main meter for the measurement of the water level of the boiler. However, this measurement error of the water level meter is also affected by many factors. This article analyzes the measurement method of the water level gauge and the cause of the water level measurement error, and quantitatively analyzes the measurement of the drum level under specific conditions, and proposes methods and measures to reduce the error of water level measurement.