What can cause a sudden slug of water to be entrained with steam, known as priming?

Priming in a boiler context refers to the phenomenon where water is carried over with steam into the steam system, leading to moisture in the steam, which can cause issues in turbines and other downstream equipment. One of the primary causes of priming is related to the operating conditions of the boiler.

High firing rates of the boiler can indeed cause priming. When the boiler operates at high firing rates, it creates turbulent conditions within the water in the steam drum. This turbulence can disturb the water surface, allowing small water droplets to be carried along with the steam. The rapid production of steam increases the velocity and turbulence, leading to the entrainment of water in the steam flow. Consequently, maintaining a stable firing rate and avoiding excessive turbulence is critical to prevent this issue.

While rapid cooling of steam, low water levels in the drum, and excessive heating of the feedwater all have their impacts on boiler operation, they do not directly produce the same conditions leading to priming as high firing rates do. High firing rates provide a specific environment where increased turbulence and steam generation rates enhance the risk of water being pulled into the steam flow. Understanding this helps in managing boiler operations to minimize priming and its effects on system performance.