What does the term "priming" refer to in boiler operation?

In the context of boiler operation, "priming" specifically refers to the phenomenon where water droplets are carried over with steam as it exits the boiler. This occurs when the steam rises through the water and some of the water gets entrained in the steam because of the boiling action.

This situation is crucial because the presence of water droplets in the steam can lead to operational issues, such as damaging steam traps, reducing the quality of the steam for heating applications, and affecting processes that rely on dry steam. When steam is saturated with water, it can also lead to erosion and corrosion of piping and equipment downstream, which is why it's essential in boiler operation to minimize or control priming to ensure efficient and safe operations.

The other choices refer to different aspects of boiler operation. The build-up of steam pressure relates to system operation but does not focus specifically on the phenomenon of priming. The process of bringing a boiler online involves several steps for safety and efficiency, rather than the specific behavior of steam and water. Gradual heating describes temperature management in the boiler, but it does not pertain to the carrying over of water droplets. Thus, the correct answer relates directly to the unique definition of priming in the context of boiler operation.