Feed Water Treatment and Feed Systems
Why treatment matters
Raw water contains scale-forming salts (calcium and magnesium), dissolved gases (oxygen and carbon dioxide), suspended solids and alkalinity. Scale insulates the heating surface, wastes fuel and can cause overheating and tube failure. Oxygen and carbon dioxide cause pitting and corrosion. Treatment aims to prevent both.
Internal and external treatment
External treatment removes hardness before the feed enters the boiler - softening, demineralisation, deaeration. Internal treatment adds chemicals inside the boiler: phosphate to precipitate hardness as sludge, alkali to control pH, oxygen scavengers (sulphite or hydrazine) to remove dissolved oxygen, and anti-foam agents. Blowdown removes dissolved solids and sludge; the TDS level governs how much.
Feed systems
Feed pumps (reciprocating, centrifugal) and injectors deliver water against boiler pressure. The injector uses steam to lift and force feedwater in; it needs a good steam supply and cool feedwater. Check valves prevent backflow. The feedwater temperature should be high (economiser or deaerator) to reduce thermal shock and oxygen.
Traps
Questions confuse scale (hardness salts, insulating) with sludge (precipitated, removable by blowdown), or corrosion with scaling. Remember that a deaerator removes oxygen by heating, not by chemicals.
Worked rule of thumb
If scale is found, hardness control has failed - check softener and phosphate dosing. If pitting is found, dissolved oxygen is the culprit - check deaeration and scavenger dosing.