2.7.1 GLOSSARY OF WATER SUPPLY TERMS
Dissolved Oxygen: Oxygen that is dissolved in
the feedwater will cause the steel in the boiler and
the feedwater system to be attacked by the water
in a manner described as “pitting”. The pits that
are
produced can vary from tiny depressions to holes
large enough to penetrate the boiler metal and are
usually covered with tubercles of iron oxide. Once
pitting starts, it may be extremely hard to arrest.
Pitting can proceed at a surprisingly rapid rate and
can occur
not only in the boiler proper, but also in pre-boiler
equipment such as economizers, feedwater tanks, and
feedwater lines.
Suspended Solids: Sus
pended solids are the
undissolved matter in water, including dirt, silt,
vegetation, iron oxides, and any other insoluble
matter.
Normally suspended solids are expressed in terms
of turbidity. Suspended solids may also deposit in
low velocity areas and create fouling. In line filters, or
various types of pre-treatment can be used to lower
the suspended solids level. Periodic blowdowns will
eliminate suspended solids.
Alkalinity: Alkalinity is the capacity of a water to
neutralize acids. Common water alkalinities consist of
bicarbonate, carbonates, hydroxide, phosphate, and
silicate. These alkalinities, especially bicarbonates
and carbonates, break down to form carbon dioxide
in steam, which is a major factor in the corrosion on
condensate lines. High alkalinity also causes foaming
and carry over in boilers. Both foaming and carry over
cause erratic boiler operation. The reason for the high
alkalinity should be determined. It may result from lack
of sufficient blow off. The source of alkalinity may be
due to an overdose of alkaline internal water treatment
chemical.
pH: pH is a measure of the degree of acid or base of
solution. A pH range of 8.5-10.5 will have little influence
on the corrosion rate of carbon steel. A low pH can
result in corrosion of metals, while a high pH can result
in scale formation or caustic embrittlement. In order
to control boilers and equipment used for the external
treatment of make-up water, it is essential that reliable
pH measurements be made. RO/DI water will have a
pH of 6.0 - 6.5 and will require neutralisation if used in
a carbon steel vessel. It is critical that the boiler pH be
alkaline (8.5-10.5) whenever water is in the boiler.
To Next Page
Loading...
