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Application Analysis of Water Level Measurement Instruments in the Waste Heat Boiler at the Tail of Cement Kilns 1 Importance of Water Level Measurement in the Drum Maintaining the water level in the drum within a certain range is a necessary condition to ensure the safe operation of the boiler and turbine. An excessively high drum water level directly affects the efficiency of steam-water separation, resulting in increased humidity in the saturated steam and a higher salt content. When the water level rises to a certain level, the steam will contain water, and the salt concentration in the water is much higher than that in the steam, which deteriorates the quality of the steam. Salts will form scale on the walls of the superheater tubes, causing them to be damaged or even burst; in severe cases, this can lead to water entering the turbine. If the drum water level is too low, it disrupts the natural steam-water circulation in the boiler, resulting in damage to the water wall tubes; in cases of severe water shortage, accidents such as tube bursts can occur. Therefore, the use of water level measuring instruments in the drum is an important aspect in ensuring the stable and safe operation of waste heat boilers. 2 Introduction to the Applications of Several Drum Level Measurement Instruments At present, the main types of drum level measurement instruments used in waste heat boilers are as follows: dual-color level gauges, differential pressure level gauges, and electrode-type level gauges. 2.1 The dual-color water level gauge is constructed based on the principle of a communicating vessel; according to optical principles, all parts related to the boiler’s water and steam are colored – steam appears red while water appears green. This type of water gauge is an auxiliary device for boilers and is installed on-site. The water level is observed directly: red when there is steam and green when there is water; it turns completely red when the steam level is full, and completely green when the water level is full, changing automatically and continuously with the water level. It is used to monitor the drum water level during boiler startup and shutdown, and to periodically calibrate other types of water level gauges during normal operation. The observation with a two-color water level gauge is clearly intuitive, but in actual operation, due to corrosion caused by chemicals added to the boiler and erosion by steam, the inner wall of the quartz glass tube wears out significantly after some time, resulting in an indistinct steam-water interface. Especially these days, industrial television monitoring is commonly used; the cameras on site are affected by changes in light levels, which results in a blurrier display of the water level. Additionally, since the water level gauges are located on the steam drums and are exposed to high ambient temperatures, the workload required for maintaining their lighting systems increases significantly. 2.2 Differential pressure level gauge: Based on the principles of hydrostatics, this gauge measures the static pressure difference between the changing water level and the constant water level, converts this differential pressure value into a water level value. Then, through a differential pressure transmitter, it transforms the drum water level into an electrical signal that changes continuously with the water level, which serves as an important parameter in the automatic feedwater control system. In practical applications, the problem with differential pressure level gauges is that when measuring the water level in a boiler’s drum, changes in the drum pressure cause changes in the relationship between \"water level\" and \"differential pressure\", which leads to significant measurement errors. Currently, the balance vessel measurement method with drum pressure compensation is widely used, but its accuracy remains greatly limited. This is because the balance vessel compensation device designed through calculations is based on a water level at zero, and when the boiler’s water level deviates from this zero level during operation, measurement errors occur. When the steam pressure drops suddenly, the condensate water inside the pressurized vessel evaporating can also cause abnormal readings on the instruments. All these pose significant difficulties in the operation of boilers; in particular, the use of automatic water supply control can lead to erroneous operations, resulting in boiler accidents. Differential pressure level gauges are more suitable for measuring the water level when the boiler is operating stably; however, the error increases significantly when the operating parameters change greatly. Therefore, in actual operation, try to avoid working on the differential pressure measurement system (such as during sewage discharge or calibration). If work must be carried out, it is necessary to coordinate with the boiler operator in order to minimize the impact on differential pressure measurements. For example, at the beginning of operation of the waste heat boiler in Kiln No. 2, an accident occurred due to incorrect indications of the boiler’s water level, which resulted from the removal of insulation from the balance vessel of the differential pressure gauge; this happened because no prior coordination was done with the boiler operator, and the automatic feedwater system was not deactivated. 2.3 Electrode-type level gauges utilize the difference in conductivity between saturated steam and saturated steam condensate to convert changes in boiler water level, which are non-electrical quantities, into electrical signals; secondary instruments then display the water level from a distance. Electrode-type level gauges basically overcome the influence of changes in drum pressure, and can be used during boiler startup, shutdown, and operation under variable parameters. The electrode-type level gauge is located very close to the steam drum, and electrical signals are used for the transmission of data from the electrodes to the secondary instruments. As a result, this type of gauge exhibits minimal delay and error; there is no need for error calculation or adjustment, which greatly simplifies its maintenance and calibration. In practical applications, the measurement of water level by electrode-type level gauges is intermittent, and it is necessary to choose the water level container connected to the steam drum appropriately in order to reduce measurement errors. In summary, the dual-color water level gauge serves as an on-site instrument for monitoring the drum water level during boiler startup and shutdown, as well as for periodically calibrating other types of water level gauges during normal operation ； Differential pressure water level gauges are used for indicating the water level under stable operating conditions, and are suitable for the automatic regulation of water supply ； Electrode-type level gauges are well-suited for the variable-parameter operation of boilers, offer high accuracy, and serve as a monitoring and supplementary tool to the measurements taken by differential pressure level gauges. Therefore, these 3 types of instruments are generally used simultaneously for measuring the water level in the boiler drum. 3 Precautions in practical applications: The water level in a boiler’s drum is determined by the amount of water stored in the drum and the volume of bubbles beneath the water surface. There are many factors that can cause changes in these quantities, such as the boiler’s steam load, feedwater flow rate, and furnace heat load. In practice, when the steam flow increases (i.e., when the load rises), even though the water supply to the boiler is less than the amount of water that evaporates, the water level not only does not drop but instead rises rapidly ； Conversely, when the load decreases, the water level drops first; this is the phenomenon of \"false water level\". The reason is that when the load increases (decreases), the volume of the bubbles under the water surface increases (decreases) very rapidly. On the other hand, dual-color level gauges, electrode-type level gauges, and differential pressure level gauges cannot overcome the effect of \"false levels\". Therefore, when monitoring the drum water level indicator during boiler operation, it is necessary to closely watch changes in parameters such as steam flow and feedwater volume as well, in order to promptly understand the actual condition of the drum water level and ensure the stable and safe operation of the boiler. 4 Application in our plant: The waste heat power generation system at the end of kiln No. 2 in our plant is equipped with one CPC afterburner and one SP waste heat boiler. In design, the CPC furnace uses dual-color level gauges, single-color level gauges, and differential pressure level gauges, while the SP furnace uses dual-color level gauges and differential pressure level gauges. During the initial trial operation, the operating conditions of the boiler changed significantly, and the frequent separation and connection of steam between SP boilers and CPC boilers caused large fluctuations in water level, resulting in several serious incidents of low water level and high water level. Therefore, another set of electrode-type level gauges was installed in the CPC furnace and SP furnace. During operation, feed water is automatically supplied using a differential pressure level gauge; the level is monitored and verified through a dual-color level gauge (industrial television monitoring) and an electrode-type level gauge, with the two methods verifying each other. In the event of any abnormalities, automatic feed water supply is switched to manual mode. After running it for a period of time, it has been proven that this method is very effective.