Preface

The saturated steam temperature of water

When water is heated under specific conditions, its temperature will gradually rise. When the temperature reaches a certain specific value, water begins to boil. At this point, if heating continues, the temperature will no longer rise, and the water will change from liquid to gas. This specific temperature is the boiling point of water and also the saturated vapor temperature of water. The steam produced at this temperature is called saturated steam. If the saturated steam is further heated, it will turn into superheated steam. And this specific condition depends on the pressure in that state, that is, the saturated vapor pressure of water. In daily life, under standard atmospheric pressure (101KPa), the boiling point of water is 100℃, and we can cook food thoroughly. However, if one is at a high altitude, the atmospheric pressure is only about 70KPa (absolute pressure), and at this time, the boiling point of water is less than 90℃, making it difficult to cook food thoroughly. In this case, a pressure cooker should be used. Its pressure can reach 180 to 200KPa (absolute pressure), raising the boiling point of water to approximately 120℃, thus easily cooking the food. Therefore, there is a one-to-one correspondence between the saturated steam temperature and pressure of water. This relationship can be obtained by downloading the parameter query in the "Digital Sulfuric Acid" APP or by looking up the "Thermodynamic Properties Table of Water and Steam". In addition, a simple formula can also be used for estimation: Take the kilogram number of saturated steam pressure twice and then multiply by 100 to approximately obtain the saturated steam temperature. For instance, the waste heat boiler of a sulfuric acid plant typically operates at a pressure of 4MPa. After taking 40 square roots twice and multiplying it by 100, the result is approximately 251℃. However, by referring to the table, it can be seen that the saturated steam temperature corresponding to 3.97MPa is 250℃. Low-pressure saturated steam is generally controlled at 0.8MPa. After taking 8 square roots twice and multiplying it by 100, the result is approximately 168℃. Looking up the table, the saturated steam temperature corresponding to 0.79MPa is 170℃, and the two are basically consistent.

After understanding the saturated steam temperature of water, let's analyze the reasons for the fluctuation of the boiler liquid level during the production process of the sulfuric acid plant:
When starting up the device, under normal circumstances, the boiler should maintain a stable water supply before the sulfur injection. The liquid level of the steam drum should be controlled through the boiler's fixed discharge system to prevent the economizer from burning out the heat exchange tubes due to overheating during the start-up process. After feeding materials, the liquid level in the boiler will fluctuate. However, due to different operation methods, the amplitude of the liquid level fluctuation also varies. At this time, the liquid level fluctuation is caused by several aspects:

1. Sulfur is injected into the sulfur-burning furnace for combustion, causing the drum to heat up. A large amount of water vaporizes, and the water in the drum turns into a mixed state of water and steam, expanding in volume and thus raising the liquid level. To prevent the liquid level from rising too rapidly, when driving, the amount of sulfur should be gradually increased as much as possible. It is strictly forbidden to increase or decrease it significantly. At the same time, before spraying sulfur, the liquid level of the steam drum should be controlled at a relatively low level to ensure there is sufficient buffer space when the water vapor expands.

2. After the sulfur injection operation, the boiler pressure gradually rises. If the pressure increases too rapidly, the temperature of the saturated steam will rise accordingly. The steam that had already vaporized will condense back into water, causing the liquid level to drop. On the contrary, if the pressure suddenly drops during the control process, the temperature of the saturated steam will decrease, a large amount of water will vaporize rapidly, and the liquid level will rise significantly. Therefore, during the start-up process, stably controlling the boiler pressure is crucial for maintaining the balance of the boiler liquid level. If there is a significant fluctuation in pressure, the liquid level will frequently fluctuate up and down. If the pressure remains stable, the liquid level can also be effectively controlled.

3. During the operation of the device, if one of the refined sulfur pumps suddenly stops, the heat of the sulfur-burning furnace will drop significantly, and the evaporation volume of the drum will also decrease greatly, resulting in a sharp drop in the liquid level of the boiler. Due to the automatic water replenishment of the boiler feed water system, if the steam production is not adjusted in time and remains at the original steam production level, the boiler pressure will drop rapidly, which in turn will cause the saturated steam temperature to decrease, and the evaporation volume of the drum will increase sharply, resulting in a rapid rise in the liquid level. Therefore, the shutdown of the refined sulfur pump will cause the boiler liquid level to first drop and then rise. If not dealt with in time, it may cause the boiler to be full of water.

4. During the operation of the device, the fluctuation of pipeline pressure has a significant impact on the liquid level of the boiler. If the pressure in the pipeline network suddenly rises, the temperature of saturated steam will increase accordingly. The steam that has already vaporized will condense back into water, causing a significant drop in the boiler liquid level. Due to the automatic operation of the boiler feed water system, a large amount of water is replenished to maintain the original liquid level. However, when the pressure recovers rapidly, the temperature of the saturated steam drops, and a large amount of water vaporizes again, causing the boiler liquid level to rise rapidly. Therefore, a sudden increase in the pressure of the pipeline network can cause the liquid level in the boiler to drop first and then rise. If not handled properly, it may lead to the boiler being full of water. If the pressure in the pipeline network suddenly drops, the temperature of saturated steam will decrease, a large amount of water will vaporize, and the liquid level will rise rapidly. Due to the automatic regulation of the boiler feed water system, the water supply volume will be significantly reduced or even completely shut down. When the pressure recovers rapidly, the temperature of the saturated steam rises, and a large amount of vaporized steam condenses, causing the liquid level to drop rapidly. Therefore, a sudden drop in the pressure of the pipeline network can cause the liquid level in the boiler to rise first and then fall. If not handled properly, it may lead to the boiler drying out.