High-water-level conditions. When a boiler operates with an excessive amount of water carryover and priming are likely to occur. As water rises above the horizontal center line of a boiler drum the water-surface or steam-release area decreases.
Water Handbook -Boiler Deposits: Occurrence And Control | SUEZ
Sludge is the accumulation of solids that precipitate in the bulk boiler water or enter the boiler as suspended solids. Sludge deposits can be hard dense and tenacious. When exposed to high heat levels (e.g. when a boiler is drained hot) deposits are often baked in place. deposits hardened in this way can be as troublesome as scale.
Boilers rely on the water to protect the steel boiler tubes from the temperatures in the furnace which greatly exceed the melting point of the tube material. A buildup of deposits inside the tubes will produce an insulating layer which inhibits the ability of the to remove the heat from the tube.