I. Methods for Measuring Water Level A normal condenser water level is crucial for the safe and economical operation of the steam turbine. Over the long term, using the differential pressure formed by the water level measuring cylinder to measure the condenser hot well water level is a primary method. The principle of this method is as follows: the positive pressure water level formed in the water inlet funnel of the measuring cylinder is sent to the positive pressure side of the differential pressure transmitter through the positive pressure inlet pipe. The condensate water level in the hot well is sent to the negative pressure side of the differential pressure transmitter through the negative pressure inlet pipe of the measuring cylinder. The pressure difference between the two inlet pipes represents the water level of the condenser hot well. The differential pressure transmitter converts the pressure difference change in the measuring cylinder into a corresponding 4-20mA signal, which represents the change in the condenser water level. This method of measuring water level has a wide range of applications and has been successfully applied in many situations, such as measuring the water level in the boiler drum. However, problems arise when measuring the water level under vacuum or low-pressure conditions, as shown in Figure 1. Because the upper part of the condenser hot well is always under vacuum, the water in the positive pressure side water funnel will lose its positive pressure level due to the siphon effect of negative pressure, resulting in a large error in water level measurement. This affects the measurement of the condenser hot well water level. When the measured variable also participates in automatic regulation, it directly affects the control of the condenser water level. To ensure water on the positive pressure side, it is necessary to frequently replenish water to the positive pressure side through the water supply valve. Obviously, differential pressure water level measuring cylinders have limitations when used under vacuum and low pressure conditions. Using a float-type measuring cylinder is another method for measuring water level. Xinhai Power Generation Co., Ltd. uses float-type B-series pneumatic base-type regulating instruments on two 200MW units. It effectively solves the water level measurement problem under vacuum and low pressure conditions, as shown in Figure 2. Its working principle is as follows: When the water level in the condenser hot well changes, the depth of the float's immersion in the water changes, causing a change in the buoyancy felt by the float. This change, through a series of complex mechanical transmissions, causes the pneumatic regulating components to output corresponding pneumatic signals to control the pneumatic regulating valve, thereby achieving the purpose of displaying and controlling the condenser hot well water level. It can be seen that the float only senses the buoyancy change of the hot well water level; the vacuum pressure environment inside the float chamber does not affect the float. Therefore, the base-mounted pneumatic regulating instrument can effectively measure and control the water level of equipment operating under vacuum and low pressure conditions. However, the base-mounted pneumatic regulating instrument cannot transmit the condenser hot well water level to the control room, and operators cannot promptly grasp the water level changes, which is a drawback of this type of instrument. In the DCS upgrade of Unit #1 (200MW), Xinhai Power Generation Co., Ltd. incorporated the control of condenser hot well water level and high and low pressure heater water levels into the DCS. The original base-mounted pneumatic regulating instruments' measurement and adjustment components were removed, and an electro-pneumatic positioning controller was installed. Reliable pneumatic actuators were retained, transforming the original pneumatic regulating instruments into electro-signal pneumatic regulating instruments. This accurately controlled the water level and transmitted the water level display remotely to the control room. However, for equipment operating under vacuum or low pressure conditions, differential pressure water level measuring cylinders have limitations, and the float, which offers relatively ideal measurement results, was also eliminated. So, what equipment should be used to measure the water level under vacuum and low pressure conditions? After investigation, we selected four Rosemount 3051 transmitters with an additional 1199 remote diaphragm sealing system, used for measuring and controlling the condenser hot well water level, and the water levels of Units #1, #21, the low-pressure heater, and the shaft seal heater. Applications of the Rosemount 3051 Transmitter: The Rosemount 3051 transmitter is an intelligent transmitter manufactured by Rosemount. Besides its stable, reliable, accurate, and multi-output protocol features, it also includes a 1199 remote diaphragm sealing system for special applications such as high and low temperatures, corrosion resistance, hygiene, viscosity, and vacuum. This system consists of the transmitter, a remote diaphragm seal, filling fluid, and a capillary tube. The diaphragm is mounted on the main unit via a matching mounting flange, directly sensing pressure changes in the process medium. The elastic diaphragm deforms under the pressure of the process medium. This deformation is then transmitted through the capillary tube to the diaphragm of the 3051 transmitter. The transmitted pressure causes deformation of the sensing diaphragm on the pressure-sensitive element of the 3051 transmitter, which is converted into a 4-20 mA current signal or a digital signal conforming to the HART protocol. This signal is proportional to the pressure of the process medium. Obviously, the remote diaphragm system is not affected by the working environment, so it can solve the water level measurement problem under vacuum and micro-pressure conditions. When applying the remote diaphragm system, some factors affecting its performance must be considered. (1) Diaphragm elasticity: The better the diaphragm elasticity, the more sensitive it is to the pressure changes of the process medium, and the less the temperature influence can be reduced. The elasticity of the diaphragm is related to the surface diameter, structural material, thickness and corrugated structure of the diaphragm. The most important factor is the diameter of the diaphragm. The larger the diameter, the better the elasticity of the diaphragm. (2) Expansion characteristics and volume of the filling liquid: The smaller the expansion coefficient and volume of the filling liquid, the less it is affected by temperature. (3) Response time: Using the remote diaphragm system will increase the total response time of the measuring device. The response time varies with factors such as temperature, pressure, capillary length, inner diameter, filling liquid, and viscosity. In summary, when selecting a remote diaphragm sealing system, attention should be paid to selecting a diaphragm with a larger diameter to improve sensitivity; selecting a capillary with a larger inner diameter to speed up the response time; and selecting a filling liquid with the lowest viscosity and expansion coefficient to reduce the influence of temperature. It should also be noted that the installation position of the 3051 transmitter with the remote diaphragm sealing system must be at least 1 m lower than the water intake, as shown in Figure 3. If the selection is based solely on experience, it may result in a large error. This is because it is very difficult to quantitatively calculate the impact of comprehensive factors such as temperature, material, structure, filling fluid, capillary length, and inner diameter on the measurement accuracy. These factors are precisely those that cannot be ignored. Rosemount has specially developed a set of software tools, SOAP2000, for selecting remote diaphragm sealing systems. With the help of this software, Rosemount can quantitatively select each diaphragm sealing system according to the specific conditions of the site when ordering, thereby ensuring that the selected system is in the best condition: (1) diaphragm thickness, diameter, material and hardness; (2) filling fluid volume, mass and expansion coefficient; (3) capillary length and inner diameter; (4) ambient and process temperature; (5) working and static pressure; (6) application under vacuum conditions; (7) transmitter position and performance. III. Summary To ensure operators can remotely monitor water levels from the control room, we installed electrical contact level measuring devices for condenser hot well water levels, low-pressure heater water levels, and shaft seal heater water levels on both 200MW units. To meet the needs of the original DAS system, differential pressure transmitters were also installed at the condenser hot wells to measure the condenser hot well water levels. However, while the electrical contact level measuring devices accurately indicated the water levels, the differential pressure transmitters showed significant errors in measuring the condenser hot well water levels. To address these issues, during the thermal control system upgrade of Unit 11, Xinhai Power Generation Co., Ltd. selected four 305L transmitters manufactured by Rosemount Corporation of the United States, equipped with a 1199-type remote diaphragm seal system. After more than a year of use, their performance has been stable and reliable. The measured water levels of the low-pressure heater and shaft seal heater are consistent with the values ​​displayed by the electrical contact level measuring devices, especially solving the long-standing problem of differential pressure transmitters being unable to measure the condenser hot well water levels. The measured values ​​not only match the indications of the electric contact water level gauge, but also ensure the long-term stable and reliable operation of the condenser hot well water level automatic regulation system, reducing the operational burden. Practical application demonstrates that the 305.1 transmitter with an added remote diaphragm seal system is a highly effective device for water level measurement in equipment operating under vacuum and low pressure conditions, and its use should be vigorously promoted. Application of the remote diaphragm differential pressure transmitter in the DCS retrofit of a 200MW unit: PDF