If the liquid level inside a steam generator falls below the saturation temperature, it can lead to a phenomenon known as "dryout." Dryout occurs when the heat transfer surface of the steam generator is no longer in contact with liquid water, resulting in a decrease in heat transfer efficiency and potential damage to the equipment.
When the liquid level falls below the saturation temperature, the remaining liquid water in the steam generator will start to evaporate rapidly. As the liquid water is depleted, the steam generator's internal surfaces become exposed to high-temperature steam instead of the cooler liquid water. This steam can have a significantly lower heat transfer coefficient compared to liquid water, leading to reduced heat transfer rates.
Dryout can have several consequences, including:
Decreased heat transfer efficiency: The reduced heat transfer coefficient of steam compared to liquid water leads to lower heat transfer rates. This can result in inadequate steam generation and reduced overall efficiency of the system.
Increased metal temperatures: Without the cooling effect of liquid water, the metal surfaces in the steam generator can experience higher temperatures. This can potentially lead to overheating, distortion, or even structural damage to the equipment.
Formation of deposits: When water evaporates and leaves behind dissolved or suspended solids, they can deposit on the heat transfer surfaces. These deposits, known as scale or fouling, can further decrease the heat transfer efficiency and increase the likelihood of equipment damage.
Potential for tube failure: In steam generators with tubes, dryout can cause the tubes to overheat and potentially fail due to increased stresses. This can result in leaks, pressure drops, or even catastrophic failures.
To prevent dryout, maintaining the liquid level above the saturation temperature is crucial. This is typically achieved through various control mechanisms, such as level sensing devices and automatic feedwater systems, which continuously monitor and maintain the desired liquid level inside the steam generator.