Page 156 - Basic Principles of Textile Coloration

BOILER WATER 145

sequestrants are less helpful since they tend to hydrolyse rapidly to simple
phosphates in boiling water. Treatment with simple phosphates also ensures
absorption of colloidal silica. This is significant, since silica scale on pipes and
walls is very difficult to remove.

The precipitation of calcium and magnesium in a non-crusting form in the
boiler produces suspended material, the rate of accumulation being greater the
higher the pressure and the capacity of the boiler. It is quite common to add
dispersants such as polyacrylates to the feed water as these keep the precipitates
well dispersed to prevent scaling. Discharge of the sludge is necessary from time to
time to avoid excessive accumulation and to keep the salinity of the water within
reasonable limits. In addition, dispersants and anti-foam chemicals prevent carry-
over of sediment and foam with the generated steam.

It is vital that there is minimum corrosion of the boiler and piping. This can be
caused by acids, and by dissolved carbon dioxide and oxygen. Iron fittings corrode
rapidly if boiler feed water is too acidic, so it is usual to condition the water to
about pH 8–9 by addition of NaOH. This also ensures that all carbon dioxide is
converted into bicarbonate.

Too high a pH causes caustic embrittlement of non-ferrous metal fittings such
as rivets. On the other hand, at pH values below 6, carbon dioxide can attack iron
to form ferrous bicarbonate. This can occur in the boiler and in the piping for
steam distribution and condensate return. Sometimes, volatile amines, such as
ammonia or cyclohexylamine, are added to the water to neutralise any acidity and
prevent this.

Dissolved oxygen is a major source of corrosion. Preliminary heating removes
most oxygen, since this gas is much less soluble in hot water. Alternatively, it
reacts with reducing agents such as sodium sulphite or hydrazine added to the feed
water. Sulphite is oxidised to sulphate but hydrazine has the advantage that it does
not produce any ionic products (Scheme 8.15).

2Na2SO3 + O2 2Na2SO4
N2H4 + O2 N2 + 2H2O

Scheme 8.15