核電廠中蒸發裝置、冷卻裝置是硼回收系統、廢液收集處理系統等重要組成部分，蒸汽冷凝水平衡槽用于收集蒸汽冷凝水，并通過其液位控制來保持冷卻器內充滿被冷卻的液體，蒸汽冷凝槽液位控制對核電站輔助工藝系統的正常穩定運行起著重要作用。由于核電站硼回收系統、廢液收集處理系統產生的氣體、液體均帶有放射性，蒸汽冷凝槽放置于人員禁止靠近或不易靠近的區域，這就對儀表的安裝位置提出了要求，測量儀表應安裝于便于維護、放射性較低的區域。而差壓液位變送器由于其性能穩定、精度高、分離式安裝等特點，其廣泛應用于核電站液位測量中。本文以某核電廠的蒸汽冷凝水平衡槽液位測量為例，詳細闡述了差壓液位變送器在蒸汽冷凝水平衡槽中的設計應用，并結合某核電廠實際安裝調試問題對其進行了分析及改進。
差壓液位變送器測量原理
差壓傳感器測量原理為:來自雙側導壓罐中的壓力作用于傳感器兩側的隔離膜片上，通過膜片內的密封液傳導至測量元件上，測量元件將測得的壓差值轉換成對應的電信號(4 ～20mA 信號)輸出。差壓液位變送器工作原理:通過特定的系統布置，依據液體壓強計算公式(P = )，將被測容器的液位測量轉化成壓差測量，再通過相應的公式推算可計算出被測容器實時液位值。差壓液位變送器應用場合主要有常壓容器液位測量、帶壓容器液位測量兩種。 蒸汽冷凝水平衡槽液位測量系統
蒸汽冷凝水平衡槽液位測量系統采用差壓式液位測量原理，液位測量系統主要有儲槽 、平衡容器、平衡容器補水管線及連通管、儀表根閥、差壓變送器及引壓管路組成。該液位測量裝置專門設置平衡容器，以避免差壓變送器氣體測量端由于蒸汽冷凝帶來的誤差以及高溫蒸汽對測量儀表造成的損害。
其工作流程及原理如下: 平衡罐安裝于被測容器的上方，平衡罐中部通過管線與冷凝槽上部進氣/液管線連通。系統調試運行前，首先通過平衡罐頂部補水管線對平衡容器進行補水，當平衡罐中液體超過連通管接口時，多余的液體自流進廢液儲槽，停止補水，保持平衡罐上部氣壓與廢液儲槽上部氣壓一致。差壓變送器一端連接廢液儲槽底部壓力接口，一端連接平衡罐底部壓力接口，根據差壓式液位測量原理，差壓變送器實際測量的差壓值經過公式換算后，可得到廢液儲槽的實時液位。
差壓液位變送器安裝與調試方法
差壓液位變送器的正確規范安裝以及安裝方式對應量程、上限、下限的計算和設置決定了容器液位測量的正確性。差壓液位變送器在蒸汽冷凝水平衡槽液位測量中應用時，應注意以下事項:
1、傳感器(差壓變送器)的安裝高度應低于 0%容器液位;
2、平衡罐的安裝高度應高于容器上部取源口，容器與平衡罐的連同管線應保持至少 2% 的坡度，保證平衡罐中補充或冷凝的多余液體回流到容器中，以維持傳感器平衡罐測得壓力恒定;
3、容器上部取源口應高于 100%容器液位;
4、從儀表根閥至傳感器的引壓管線應保持往下至少 2%的坡度，使引壓罐中不會集氣;
5、傳感器應安裝于輻射劑量較低的區域(黃區、綠區或白區)，以便于現場維護。
目前，差壓液位變送器在核電站蒸汽冷凝水平衡槽的液位測量中已得到廣泛的應用，液位測量方式運行穩定、測量精度高、便于維護且經濟性高。但在安裝調試過程中，仍然出現各種各樣的問題，這就要求核電站設計、安裝、調試人員應對液位測量變送器的測量原理、安裝要求、遷移計算方法等有比較清楚的認知，這樣才能保證冷凝水平衡槽液位的正確、精確測量。
Evaporator and cooler are important parts of boron recovery system and waste liquid collection and treatment system in nuclear power plant. Steam condensation water balance trough is used to collect steam condensate and keep the cooler filled with cooled liquid through its liquid level control. Steam condensation trough level control is positive for auxiliary process system of nuclear power plant. Constant stable operation plays an important role. Because the gas and liquid produced by the boron recovery system and waste liquid collection and treatment system of nuclear power plant are radioactive, the steam condensation tank is placed in the area which is prohibited from being near or not easy to be near by the personnel. This requires the installation position of the instrument. The measuring instrument should be installed in the area which is easy to maintain and has low radioactivity. Differential pressure liquid level transmitter is widely used in nuclear power plant level measurement because of its stable performance, high precision and separation installation. Taking the level measurement of steam condensation water balance tank in a nuclear power plant as an example, this paper expounds the design and application of differential pressure level transmitter in steam condensation water balance tank in detail, and analyzes and improves it according to the actual installation and commissioning problems of a nuclear power plant.
Measuring principle of differential pressure level transmitter
The measuring principle of the differential pressure sensor is that the pressure from the two-side pressure tank acts on the isolation diaphragm on both sides of the sensor, and the sealing fluid in the diaphragm is transmitted to the measuring element, which converts the measured pressure difference into the corresponding electrical signal (4-20mA signal) to output. The working principle of differential pressure liquid level transmitter: According to the formula of liquid pressure calculation (P=), the liquid level measurement of the vessel under test is transformed into the pressure difference measurement through the specific system arrangement, and then the real-time liquid level value of the vessel under test can be calculated by the corresponding formula calculation. Differential pressure liquid level transmitter is mainly used in atmospheric pressure vessel level measurement and pressure vessel level measurement. Steam condensate balance tank level measuring system
The liquid level measurement system of steam condensation water balance tank adopts the principle of differential pressure liquid level measurement. The liquid level measurement system mainly consists of storage tank, balance vessel, balance vessel filling water pipeline and connecting pipe, instrument root valve, differential pressure transmitter and lead pressure pipeline. The liquid level measuring device is specially equipped with a balancing vessel to avoid errors caused by steam condensation at the gas measuring end of the differential pressure transmitter and damage to the measuring instrument caused by high temperature steam.
The working process and principle are as follows: the balance tank is installed above the vessel under test, and the middle part of the balance tank is connected with the upper air/liquid inlet pipeline of the condensation tank through a pipeline. Before debugging and running, the balance tank is supplied with water through the top water supply pipeline of the balance tank. When the liquid in the balance tank exceeds the connecting pipe interface, the excess liquid flows into the waste liquid storage tank by itself, and the water supply is stopped, so that the upper pressure of the balance tank is consistent with the upper pressure of the waste liquid storage tank. One end of the differential pressure transmitter is connected with the bottom pressure interface of the waste liquid storage tank, and the other end is connected with the bottom pressure interface of the balance tank.
Installation and debugging method of differential pressure level transmitter
The correct and standard installation of differential pressure liquid level transmitter and the calculation and setting of corresponding range, upper limit and lower limit of installation mode determine the accuracy of liquid level measurement of container. The application of differential pressure transmitter to the level measurement of steam condensate water balance tank should pay attention to the following matters:
1, the installation height of the sensor (differential pressure transmitter) should be lower than that of the 0% container.
2. The installation height of the balancing tank should be higher than the source inlet of the upper part of the vessel. The same pipeline of the vessel and the balancing tank should maintain at least 2% slope to ensure that the excess liquid added or condensed in the balancing tank can be refluxed into the vessel to maintain the constant pressure measured by the sensor balancing tank.
3, the upper part of the container should be higher than the 100% container level.
4. The pressure pipeline from the instrument root valve to the sensor should be kept at least 2% downward so that the pressure tank will not collect air.
5. Sensors should be installed in areas with low radiation dose (yellow, green or white) for on-site maintenance.
At present, differential pressure liquid level transmitter has been widely used in the level measurement of steam condensation water balance tank in nuclear power plant. The method of liquid level measurement is stable, accurate, easy to maintain and economical. However, in the process of installation and commissioning, there are still various problems, which require the design, installation and commissioning personnel of nuclear power plant to have a clear understanding of the measurement principle, installation requirements, migration calculation method of the level transducer, so as to ensure the correct and accurate measurement of the level of condensation water balance tank.
