Hardness salts become less soluble as water heats, so they precipitate right on the hottest metal, forming calcium carbonate, sulfate or silicate layers. Scale insulates the metal from the water, so fuel use and stack temperature rise and the steel underneath runs hotter until it bulges or cracks. Softening makeup, controlling cycles and internal treatment chemicals prevent it; removing it takes acid cleaning or mechanical work.
Opened the handholes and the tubes are scaled up, eggshell thick on the bottom row, call the chemical company in.
If the boiler kettles, the stack temperature creeps up and the softener has been ignored, assume scale until you see clean steel.
Also heard as
- boiler scale
- lime scale
Related terms
Water softener
An ion exchange unit that removes calcium and magnesium hardness from makeup water by swapping them for sodium.
Blowdown
Draining some boiler water on purpose to remove concentrated dissolved solids and settled sludge.
Bag
A local bulge in a boiler tube, furnace or shell plate where overheated metal has been pushed out by pressure.
Stack temperature
The temperature of the flue gas as it leaves the boiler, a direct measure of how much heat is going up the chimney.
Kettling
A rumbling, crackling or whistling noise from a hot water boiler, like a kettle about to boil.