Hardness

Ca(HCO₃)2,    Ca and CaCO₃, Mg salts) CaSO₄,CaCl₂ Mg(HCO₃l₂ MgCO₃,MgSO₄ MgCl₂)

Scale deposits in boiler; forms insoluble curds in washing proces­ses; MgCl₂is also corrosive

0-2 ppm* in high pres­­sure boilers, under 17-25 ppm* for low-pressure boilers with enough sodium alkalinity present

Softening plus internal treat­ment in boiler

Calcium salts are chief cause of scale in boilers, followed by magnesium. Mag­nesium scale is not as hard as calcium scale.

Sodium al­ka­-linity

NaHCO₃,

Na₂CO₃,

NaOH

Foaming; Car­bonates form CO₂ in steam to cause return line corrosion; may contribute to embrit­tlement

0-5ppm* for high pressure boilers, depends on individual conditions for low-pressure boilers

Hydrogen exchanger, deioni­zation, acid treatment with deaeration

Sodium salts are found in most waters, are very soluble and can't be removed by chemical precipitation.

Sulfates

SO₄

Hard scale if calcium is present

100-300 ppm* for general purposes

Deionization

Chlorides

Cl

Priming, foaming if present in large amounts

Depends on indivi­dual conditions

Deionization

Iron Mananese

Fe

Mn

Deposits in boiler if present in large amounts; harms process

Under 0.3 ppm for general purposes

Aeration plus filtration; ion exchange

Silica

SiO₂

Hard Scale in boilers, turbine-blade deposits

Under 10-15 ppm if concentrated salines of high-pressure boilers if alkalinity is low; to under 3-5 ppm to avoid turbine deposits

Deionization; lime­-soda process with addition of ferric sulfate (cold) or Mg reagent (cold or hot)

When combined with aluminium salts forms extremely hard scale in boilers.