What can cause a bag to develop in an HRT boiler?

The development of a bag in an HRT (Horizontal Return Tubular) boiler is primarily caused by the deposit of scale or mud. In a boiler system, impurities in the water can lead to the formation of scale on the heat exchange surfaces. Scale reduces the effective heat transfer area, and as the temperature of the surfaces increases, steam can form within the deposits. This trapped steam accumulates, creating a bubble or "bag."

The presence of scale also encourages localized heating, which may further exacerbate the issue, leading to overheating and possibly damaging boiler components. Conversely, mud deposits can settle due to poor circulation, also contributing to the formation of bags by trapping steam. Thus, the accumulation of these contaminants is a critical factor in the operational integrity of the boiler, making proper water treatment and regular maintenance essential to avoid these problems.