At present, the main water level measuring devices of Hanchuan Power Plant's 300 MW generating units are the local mica level gauge, in-situ magnetic flap level gauge, electrical contact level gauge, and buoyancy level switch. Differential pressure water level measuring device based on 1151 transmitter. In terms of its nature, mica level gauges and magnetic flap level gauges are local display instruments with the "seeing is believing" feature; electrical contact level gauges are remote display instruments; buoyancy level switches are used for remote interlock switches Quantity control; 1151 differential pressure level measurement device for remote analog display and automatic control. In terms of its measurement accuracy, the 1151 differential pressure type water level measuring device is much more accurate than the previous ones. The accuracy of the 1151 transmitter currently used in Hanchuan Power Plant is 0.25, and the error when measuring 1000 mm water level is only 2.5 mm. .
1151 One of the outstanding advantages of the differential pressure water level measurement device is that it converts the water level real-time signal into a 4-20 mA analog signal for transmission to distant places, and the signal processing is convenient. Especially with the wide application and development of the DCS system, the 1151 differential pressure water level measuring device and the DCS system are highly efficient, making the device not only used for display and analog quantity automatic control, but also gradually replacing the float type liquid level switch. Take the protection of the trend. The Drum Water Level MFT Protection Signal of 4 300MW Units in Hanchuan Power Plant . Deaerator water level signal and high water level signal are all from 1151 differential pressure water level measurement device. Therefore, a more in-depth study of the 1151 pressurizing water level measuring device has important practical significance for the safe and economical stable operation of large-scale thermal power plants.

First, the working principle of 1151 differential pressure level measuring device

1. 1151 Composition of differential pressure level measuring device

Taking Hanchuan Power Plant boiler drum water level measurement as an example, the 1151 differential pressure measuring device composition diagram is shown in Figure 1.

As can be seen from Figure 1, the 1151 differential pressure type water level measuring device is mainly composed of a communicating tube and an equilibrium vessel. Pressure tube, 1151 transmitter composition. When measuring different objects, the structure is slightly different.
2.1151 Operation principle of differential pressure water level measuring device Take the boiler water level measuring device in Figure 1 as an example. The saturated steam in the steam drum is in the condensing ball (equalizing container, the inside of the container is constantly condensing, and the liquid level in the balancing container is always kept constant. Therefore, the height of the water column inside the positive pressure tube is constant, and the height of the water column of the negative pressure tube changes with the water level H. Therefore, the differential pressure signal obtained from the positive and negative pressure introduction ports is:


Where H - container water level;
Ρ1 - balance the density of water in the container;
Ρ`——the density of saturated water at the pressure of the drum;
ρ" - density of saturated steam at drum pressure;
It can be seen from this formula that when the balance vessel's installation structure is certain (that is, L is determined), the pressure of the steam drum is fixed (p', p' is determined) and ρ1 is constant, the differential pressure output of the positive and negative pressure pipes is Δp and the steam drum The water level H has a reverse linear relationship, that is, the lower the water level, the greater the differential pressure.
The pressure signal of the positive and negative pressure tubes changes the distance between the diaphragms by squeezing the diaphragm of the capacitance membrane chamber in the 1151 transmitter, causing a change in the positive and negative membrane capacitances, that is, the following linear relationship:
(C1-C2)/(C1+C2)=Kâ–³P
Where C1, C2 - positive and negative membrane chamber capacitance;
K - proportional coefficient.
The measurement circuit in the 1151 transmitter converts the differential capacitance change into a 4-20 mA DC current signal and sends it to the central control room via the control cable. At this time, the 4l20mA DC current signal output by the 1151 transmitter has a reverse linear relationship with the drum water level H. That is, the lower the water level, the greater the differential pressure, and the larger the 4-20mA DC current signal.
The above signal flow is: The water level height signal H→ the differential pressure output signal of positive and negative pressure tubes ΔP→l151 differential capacitance signal→4-20 mA DC signal→DCS system.
It should be noted that, after the 1151 transmitter is inspected and repaired, in order to ensure the constant level of the equilibrium container, during the initial operation of the unit, the water level of the deaerator, the high water level, and the water level of the condenser must all pass through the manual irrigation gate. Balanced container irrigation, the water level signal can be measured correctly; for the drum level transmitter, the transmitter signal only returns to normal when the balance vessel is full of condensed water after the unit has been running for a period of time.

II. Field Installation of 1151 Differential Pressure Type 7k Position Measurement Device

1151 Installation of differential pressure water level measuring device involves sampling, balancing vessels, connecting pipes, closing doors, transmitter selection, materials, installation dimensions and many other aspects. For different measuring objects and requirements, the installation methods are different and can be completely carried out according to the design requirements. Now only from positive and negative pressure tube and 1151 transmitter connection analysis.
1.1151 Transmitter Connections Normally, gate transmitters are marked with H (high) and L (low). The former indicates the high side and the latter indicates the low side. After the three-valve bank is connected to the transmitter, the person faces the three-valve bank. "If the left side of the transmitter is H (high) and the right side is L (low), it is called positive mounting; otherwise, it is called reverse mounting.
2. Positive and negative pressure tube connection method Under normal circumstances, with the balance container (or steam side, connected pressure tube is called positive pressure tube C or high pressure side), and the water side of the pressure tube is called negative Pressure tube (or low pressure side). The positive pressure tube is connected to the high voltage side of the 1151 transmitter. The negative pressure tube is connected to the low voltage side of the 1151 transmitter. This is called positive mounting; otherwise it is called reverse mounting.
When the transmitter's differential pressure calibration output signal is 4mA:
(1) If the transmitter and the positive and negative pressure tubes are installed or the transmitter and the positive pressure tube are unmounted, the higher the water level, the lower the differential pressure, and the smaller the current signal output by the transmitter, 4mA corresponds to full water;
(2) If the transmitter and the positive and negative pressure tubes are installed and the other is reversed, the higher the water level, the lower the differential pressure. The smaller the current signal output by the transmitter, the 20mA corresponds to full water.
When the transmitter zero differential pressure calibration output signal is 20mA, the above is exactly the opposite.
From the actual situation, the installation of the transmitter and positive and negative pressure tube installation are both present. In the specific installation, "different conditions of the device for processing the differential pressure signal should be selected.

Three, 1151 differential pressure type water level measurement circuit de parameter setting

1151 The differential pressure level measuring device, the signal transmission cable and the DCS system combination are typical components of the 1151 differential pressure level measurement circuit. Its control loop parameter settings include the 1151 transmitter and the DCS system in two parts.
1.1151 Transmitter Parameter Settings At present, the intelligent 1151 transmitter has a small size and easy installation and calibration. Features such as low maintenance have been widely used. According to different needs, the 1151 transmitter can be easily calibrated in zero span. Zero migration, local status settings, etc.
2. DCS system parameter settings
The DCS system is mainly used for processing the 4-20 mA DC current signal sent by the differential pressure type water level measuring device, and is displayed on the CRT in accordance with the operating custom requirements. Different DCS systems have different parameter settings.
Take the high-water level measurement of #3 unit #3 of Hanchuan Power Plant (Figure 2) as an example. In the figure, the equilibrium vessel O is at a high normal level, ie, the CRT shows a zero water level (0 mm); A is a high full water level, the CRT shows +300 mm; B is the high and low water level, and the CRT shows - 300mm point.


Measurement loop:
(1) The pressure tube and transmitter are being installed. The calibration range of the transmitter is 0→600mm, the corresponding output current is 20→4mA, the corresponding differential pressure is -600mmH2O→0mmH2O, and the CRT display is -300mm (anhydrous) → +300mm (full water). Transmitter calibration, zero differential pressure output 4mA, negative pressure end pressure. (or positive pressure side pressure) to 600mmH2O. Adjust the transmitter output to 20mA.
The DCS system used by Hanchuan #3 unit is a WDPF-II type system, and its parameters show that the conversion coefficients C1 and C2 are calculated as follows:
+300=C1×0.004+C2
-300=C1×0.020+C2
(2) If the pressure tube and the transmitter are being installed, the zero differential pressure output of the transmitter will be 20mA when the transmitter is calibrated, and the transmitter output will be adjusted when the negative pressure terminal is pumped (or the positive pressure terminal is pressurized) to 600mmH2O. 4mA. The corresponding relationship at this time is: the current is 4→20mA, the corresponding differential pressure is -600mmH2O→0mmH2O, and the CRT shows -300mm (anhydrous)→+300mm (full of water).
at this time. DCS system parameters display conversion factor C1. C2 should be calculated according to the following formula:
-300=C1X0.004+C2
+300=C1X0.020+C2
Therefore, the installation of the pressure tube and the transmitter, and the verification of the transmitter should correspond to the settings of the DCS system parameters. Otherwise, the water level measurement will show an error. If the wrong measurement results enter the regulation and protection system, serious consequences will result.

IV. Problems in Practical Application of Differential Pressure Water Level Measurement Device
1. Problems in high-level water level measurement Figure 3 shows the high-level water level measurement of #4 unit of Hanchuan #4.


During initial installation, the transmitter has a range of 0 to 400mm, a differential pressure range of -400mmH2O to 0mmH2O, a CRT display of -200mm (anhydrous) to +200 (full), and a corresponding current value of 20mA to 4mA. For example, the zero point of the differential pressure transmitter should be point A in Figure 3. Then, the measuring range of the transmitter should be from point A to point C. Because the point O in the figure is high and the actual normal water level, ie, zero water level, the actual water level measured by the transmitter should be +300→-100 mm. When the transmitter outputs 12mA signal, 0mm is displayed on the CRT. When the actual water level is high and the normal water level is 0, the transmitter outputs 16mA and the CRT displays -100. Therefore, in the transmitter's measurement range (point A to point C), the CRT display value is lower than the actual value, and cannot be displayed when the actual water level is below the C point.
If the transmitter range is +100mm → +500, differential pressure range -400 → 0mmH2O, CRT display -200 (anhydrous) → +200mm (full of water), the corresponding current value of 20mA → 4mA. As can be seen from the above analysis, the midpoint of the transmitter's test should be +300mm, that is, the point A in the figure is down 300mm. This point is the same plus constant water point. Therefore, when the transmitter's measurement midpoint coincides with the actual zero water level, the transmitter displays the correct value within the normal measurement range. When the actual water level is +300mm → 200mm and -300mm → -200mm, it cannot be displayed.
Therefore, the transmitter range should be changed to 0mm → +600mm or -600mm → 0mm. In this way, the correct display is guaranteed and the measurement range is expanded.
2. Problems in the deaerator water level measurement The deaerator water level measuring devices of Units #1 and #2 of Hanchuan Power Plant have electrical contacts, local magnetic flap water level gauge, water level alarm and protection level switch. DAS water level transmitter, CCS water level transmitter, their display zero point is not uniform, the range does not correspond to the actual display value, and does not meet the design requirements. Among them, the DAS water level transmitter, CCS water level transmitter differential pressure is 0mm → 1500mm, the display value of 3400mm → 900mm, and the actual should be 3640mm → 2140mm, the indication is low 240mm. Deaerator water level relies on this transmitter to adjust, it makes the deaerator water level is high, the water level high alarm and protection of the liquid level switch is easy to misoperation. In addition, the trip signal of the feed pump is also given by the transmitter, so the water level is low and protection is easy to refuse.
In order to make the deaerator water level measurement accurate, according to the design requirements, on the electrical contacts, in situ magnetic flap water level meter. Water level alarm and protection level switch, DAS water level transmitter. The zero point of the CCS water level transmitter is unified. The zero point for all meters is 1900mm below the geometric centerline of the deaerator tank (deaerator design zero). CCS system transmitter range is changed to: differential pressure 0mmH2O → 1600mmH2O, the display value is 3640mm → 2040mm, current 20mA → 4mA; cancel the water level is high. High-level liquid level switch, the signal is given by the DCS system. such. It ensures the reliability of the deaerator water level adjustment and protection.
3. Drum water level measurement application problem The original installation status of differential pressure water level gauges for boilers A and B in #2 boiler of Hanchuan Power Plant is shown in Figure 4.

The calculation and analysis show that when the water level of the drum is -381mm. Drum water level low MFT protection action, when the differential pressure detected by the transmission is 670.96mmH2O, is close to the maximum differential pressure that the transmitter can detect to 664.5mmH20, the margin is only 6.46mmH2O. If considering the impact of installation, ambient temperature and other errors, the differential pressure required for the low level and low MFT protection of BDP will exceed the maximum differential pressure that can be detected by the transmitter, which will directly lead to low protection of the water level.
Therefore, the position of the sampling hole on the water side of the differential pressure level gauge shall not be moved by 60mm, so that the distance from the water sampling point to the normal level of the steam drum shall be -460mm; meanwhile, the installation position of the sampling tube on the steam side and the condensation ball shall be unchanged. In this way, there is a margin of 26.9232 mmH2O between the differential pressure required for the low water level protection action and the maximum differential pressure that the transmitter can detect. Compared with the original installation of the water level measurement, the margin is increased by 20.46mmH2O and the low water level protection can operate correctly.

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