Abstract: This paper introduces the composition and working principle of the liquid level measuring instrument system of the steam condensate balance tank of a nuclear power plant, the installation precautions and migration calculation methods of the differential pressure level transmitter, and the specific problems arising in the installation and commissioning of a nuclear power plant. The installation and debugging of the differential pressure level transmitter was analyzed and improved. In actual operation, the differential pressure level transmitter is stable in measurement and convenient in commissioning and maintenance.
In the nuclear power plant, the evaporation device and the cooling device are important components such as a boron recovery system and a waste liquid collection and treatment system. The steam condensate balance tank is used to collect the steam condensate, and the liquid level control is used to keep the cooler filled with the cooled. The liquid level control of liquid and steam condensate tank plays an important role in the normal and stable operation of the nuclear power plant auxiliary process system. Since the gas and liquid generated by the boron recovery system and the waste liquid collection and treatment system of the nuclear power plant are all radioactive, the steam condensation tank is placed in an area where the personnel are prohibited from approaching or inaccessible, which places requirements on the installation position of the meter, and the measuring instrument should be installed. For areas that are easy to maintain and have low radioactivity. The differential pressure liquid level transmitter is widely used in nuclear power station liquid level measurement due to its stable performance, high precision and separate installation. Taking the liquid level measurement of the steam condensate balance tank of a nuclear power plant as an example, the design and application of the differential pressure level transmitter in the steam condensate balance tank is described in detail, and combined with the actual installation and debugging of a nuclear power plant. Analysis and improvement.
1. 1 Differential pressure level transmitter measurement principle
The differential pressure level transmitter mainly consists of a differential pressure sensor and a transmitter.
The differential pressure sensor measurement principle is: the pressure from the double-sided pressure guiding tank acts on the isolating diaphragm on both sides of the sensor, and is transmitted to the measuring component through the sealing liquid in the diaphragm, and the measuring component converts the measured differential pressure value. The corresponding electrical signal (4 ~ 20mA signal) is output. Working principle of differential pressure level transmitter: Through the specific system arrangement, according to the liquid pressure calculation formula (P = ), the liquid level measurement of the measured container is converted into differential pressure measurement, and then calculated by the corresponding formula can be calculated Measure the real-time level value of the container. Differential pressure level transmitter applications mainly include two levels of atmospheric pressure vessel level measurement and pressure vessel level measurement.
1. 2 steam condensate balance tank liquid level measuring system
The steam condensate balance tank liquid level measuring system adopts the differential pressure level measurement principle, and its system setting is shown in the flow chart of Figure 1. The liquid level measuring system mainly consists of storage tank 013BA (with pressure vessel), balance vessel, balance vessel water supply pipeline and connecting pipe, instrument root valve, differential pressure transmitter 405MN and pressure piping. The liquid level measuring device is specially provided with a balance container to avoid the error caused by steam condensation of the gas measuring end of the differential pressure transmitter and the damage of the measuring instrument caused by the high temperature steam. The workflow and principles are as follows:
The balance tank is installed above the container to be tested, and the middle of the balance tank is connected to the upper intake/liquid line of the condensate tank through a pipeline. Before the system is commissioned, firstly, the balance container is replenished by the top water supply line of the balance tank. When the liquid in the balance tank exceeds the connection of the communication pipe, the excess liquid flows into the waste liquid storage tank, stops the water supply, and maintains the pressure and waste of the upper part of the balance tank. The upper pressure of the liquid storage tank 013BA is the same. One end of the differential pressure transmitter is connected to the bottom pressure port of the waste liquid storage tank (405VML root valve), and one end is connected to the bottom pressure port of the balance tank (405VML root valve). According to the principle of differential pressure level measurement, the differential pressure transmitter 405MN actually measures After the differential pressure value is converted by the formula, the real-time liquid level of the waste liquid storage tank 013BA can be obtained.
2. 1 level measuring instrument installation
The correct specification installation of the differential pressure level transmitter and the calculation and setting of the corresponding range, upper limit and lower limit of the differential pressure level determine the correctness of the liquid level measurement of the container. When using differential pressure level transmitter in the measurement of steam condensate balance tank level, the following should be noted:
(1) The installation height of the sensor (differential pressure transmitter) should be lower than 0% of the container level;
(2) The installation height of the balance tank should be higher than the upper source of the container. The container and the balance tank should be kept at least 2% slope to ensure that the excess liquid replenished or condensed in the balance tank is returned to the container to maintain the sensor. The balance tank measures the pressure constant;
(3) The upper source of the container should be higher than the 100% container level;
(4) The pressure piping from the instrument root valve to the sensor should be kept at least 2% down, so that no gas is collected in the pressure tank;
(5) The sensor should be installed in the area with low radiation dose (yellow, green or white) for on-site maintenance.
2. 2 liquid level measurement instrument migration calculation
The steam condensate tank level measuring instrument converts the measured value of the differential pressure transmitter into the actual liquid level value through the migration calculation during the debugging process, which is the main work of debugging. The following takes a condensate tank level measuring instrument as an example to explain the migration calculation method of the meter (see Figure 2). HP sensor high side pressure, LP is low side pressure. Ρ1 is the density of the measured container, ρ2 is the concentration of the medium in the balance tank, L1 is the liquid level of the measured container, and L2 is the distance from the level of the balance tank to the level 0 of the container under test. P is the sum of the fixed pressure value generated by the sensor below the normal water level and the internal pressure value of the container.
When the container is at 0% level, HP and LP are calculated as follows:
When the container is at 100% liquid level, HP and LP are calculated as follows:
The corresponding values of the sensor values are shown in Table 1.
Sensor value correspondence
Since the sensor output mA value and the differential pressure display value change linearly, according to the above formula, the actual liquid level value corresponding to the sensor ΔP value can be obtained.
The liquid level measuring instrument of the steam condensate balance tank of a nuclear power plant does not meet the measurement requirements during the debugging process.
3. 1 Problem Analysis
Check the relevant drawings and materials and the on-site mapping, the liquid in the condensing tank and the balance container are all water, the density ρ1, ρ2 are both 998. 35Kg/m 3 , the measuring range of the liquid level of the container is 0 ~ 500mm, the L2 is 1. 1m, the scene is The maximum range of the cargo meter is 6 kPa. The liquid level measurement flow chart is shown in Figure 1, and the migration calculation is shown in Figure 2.
As can be seen from the second chapter, when the normal water level is 0, the absolute value of the differential pressure across the differential pressure transmitter reaches a maximum value. According to the actual measurement data, the maximum differential pressure is about 11 kPa, which is beyond the range of the existing meter. The reasons for this situation are as follows: the instrument control professional selects the instrument according to the process input conditions, and the instrument control professional receives the professional input condition of the system, which needs to measure the liquid level of 013BA, the measurement range is 0 ~ 500mm, the designer The installation method and measurement principle of the differential pressure level measuring instrument are unclear. The selection of the measuring range of the meter is not related to the position of the meter balancing tank. It is only designed according to the process input conditions, which makes the meter not suitable for the scene. This requires the designer to master the measurement principle and installation method of the liquid level measuring instrument before designing to avoid the occurrence of on-site problems.
3. 2 Problem Processing
Through the above analysis, we can see that the problem of the selected differential pressure transmitter is too small, and the maximum pressure difference existing in the process is related to the height of the balance tank (that is, the length of L2 in the migration calculation book). As the length of L2 becomes smaller, the corresponding maximum differential pressure becomes smaller. Then, there are two options to solve the problem:
(1) Replace the meter range and select the meter with the maximum range greater than 11kPa;
(2) Reduce the height of the balance tank by about 500mm, so that the existing instruments can meet the requirements of on-site measurement.
Option 1 mainly involves the procurement cycle. It takes at least 3 months from instrument procurement to instrument arrival. The nuclear power plant construction cycle and time node have strict control. It only takes more than one month from the problem feedback to the next important node. The time is too late, so the plan will not be considered.
Option 2 Consider the possibility of balancing tanks to reduce height. Based on the principle that the liquid in the balance tank is returned to 013BA, the line from the balance tank to the upper outlet pipe of the 013BA should be laid down slightly horizontally. Therefore, the balance line should be lowered at the same time as the balance tank is lowered. After consulting the equipment drawings, the distance from the bottom of the 013BA to the top outlet of the equipment is about 550mm, so this scheme is feasible. Although the on-site pipeline and insulation have been installed, changing the height of the balance tank will involve the SED water supply pipeline, the balance tank connection pipeline, and the insulation layer. The workload is large, but in order to meet the charging time node, the scheme is finally adopted.
The differential pressure liquid level transmitter has been widely used in the liquid level measurement of the nuclear power plant steam condensate balance tank. In the operation feedback of each power station, the liquid level measurement mode is stable in operation, high in measurement accuracy, easy to maintain and economical. high. However, during the installation and commissioning process, various problems still occur, which requires the design, installation, and commissioning personnel of the nuclear power plant to have a clear understanding of the measurement principle, installation requirements, and migration calculation methods of the liquid level measurement transmitter. In order to ensure the correct and accurate measurement of the condensate balance tank level.