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When the installation position of the transmitter is fixed, the heights measured by steam and water become fixed values, and the parameters for change are the steam and water density of each section and the actual water level in the steam drum. In the environment where the external environment is relatively stable, the density of the liquid in the steam and water side outlet pipe to the transmitter is also relatively stable, and the parameters of the change are just the steam steam, water density and the actual water level of the steam drum. Through the correction of the pressure of the steam drum, the influence of changes in steam density and water density of the steam drum on the measurement of the transmitter can be reduced, so that the differential pressure measured by the transmitter can directly reflect the change of the water level of the steam drum. In order to prevent the transmitter indication from affecting the drum water level adjustment and protection control, three redundant transmitters are generally installed on both sides of the steam drum. 
Sanhe Power Company boiler steam drums are equipped with 6 sets of water level transmitters, and 3 sets of steam drums are installed on both sides (Figure 2). The water level transmitters on one side share a pair of soda water extraction pipes, which are divided into 3 channels to connect 3 sets of water level transmitters respectively. The transmitter measures the output value and sends them to DCS. After the pressure and temperature of the steam drum are corrected, it is used for water level adjustment control.
Second, the impact of changes in the external environment on the water level measurement In the normal operation of the unit, the drum level is relatively stable, due to changes in the external environment may cause individual water level transmitter measurement values ​​deviate from the actual water level, affecting the water level regulation and control, and even cause Water level protection malfunctions.
(1) At 22:50 on July 25th, 2005, the measurement values ​​of the transmitters of the No. 1 and No. 2 drums gradually increased, the flow of feedwater gradually decreased, and the measurement indications of other water level measuring points gradually decreased. The indication of the television two-color water level gauge also gradually decreased. Upon inspection, it was found that the water level transmitter closed the rain at the top of the small room to the connection line of the transmitter.
(2) In order to prevent the freezing of winter due to the poor insulation of the water level transmitter pipeline, a heating cable is laid in the pipeline insulation. When the temperature is lower than the set value, the heating power supply is automatically input. At 17:20 in November 2002, the measurement indicators of the water level transmitters No. 1 and No. 3 gradually increased and stabilized, and the measured value deviation of one of the three transmitters increased. When the water level transmitter was After the electric heat tracing input, the transmitter measurement indicator is normal.
(3) At 7 o'clock on December 4, 2005, the measurement values ​​of the 6 drum water level transmitters of Unit 2 gradually decreased to a minimum of -145 mm. Upon examination, it was found that the steam side sampling tube was frozen.
(4) In February 2005, due to cooling in winter, the measured values ​​of the 2 drum water level transmitters of Unit 2 gradually increased to a minimum of 150 mm. The two transmitter measurement output values ​​are used for water level regulation control, causing the feedwater conditioning control to issue a rapid reduction of the feedwater flow command.
It can be known from the above phenomenon that when the water level of the drum is normal, some of the water level transmitters will deviate from the normal value due to the influence of the external environment due to the connection pipeline. However, the same causes may not be the same. For example, because the connection line is frozen, the water level transmitter's measured value rises and falls.
From formula (2), it can be concluded that when the water level and pressure of the drum are constant, the density of steam and water in the steam drum is also constant. When the external environment changes, the density of the water in the pipeline changes due to the poor insulation of the water level transmitter connecting pipe, which will affect the measured output value of the water level transmitter (Table 1). 
Third, to reduce the influence of the external environment on the water level measurement measures (1) The drum steam, water side sampling tube, sampling valve and connecting pipe should be well insulated, and the two pipes leading to the differential pressure transmitter should be laid in parallel. Keep warm together, and take anti-freezing and rain-proof measures as needed.
(2) The average temperature of the reference water column selected for temperature correction of the differential pressure water level measuring device shall be determined according to the external environment temperature, and the correction circuit shall be set periodically according to the change of the external environment temperature.
(3) periodically compare the zero level of the differential pressure type water level measurement device under rated steam pressure with the zero level of the local water level gauge. If the deviation is too large, it should be based on the zero water level of the in-situ water level table under the rated steam pressure. Correct the zero water level of the differential pressure level gauge.
IV. Conclusion For the abnormal phenomenon of the water level transmitter of the drum, take the heat preservation and heat treatment to the sampling pipeline of the water level transmitter of the drum. At the same time, the drum water level transmitter cabinet was closed, effectively preventing the influence of rain, snow, and windy weather on the transmitter measurement. By analyzing the influence of external environmental changes on the side of the steam-side connection pipeline to the measurement output of the transmitter, the operating personnel can determine the cause of the fault based on the trend of the water level indicator change, take timely measures, shorten the fault time, and ensure the safe operation of the unit.
One, drum level transmitter measurement principle Power plant steam drum water level measurement using differential pressure level transmitter, the measurement principle shown in Figure 1, the steam pressure P1, water pressure P2, by the following equation Steam, water side differential pressure: