Page 316 - Corrosion Engineering Principles and Practice

286 C h a p t e r 8 C o r r o s i o n b y W a t e r 287

8.4 Cooling Water Systems
Most of the water used industrially removes heat from production
processes. It is one of the major applications for water and is a major
factor in siting plants and processes. Cooling systems suffer many
forms of corrosion and failure. The diversity of attack stems from
wide differences in cooling water system design, temperature, flow,
water chemistry, alloy composition, and operation. An almost endless
variation of process stream chemistries may be involved in cooling
water systems. Refinery and chemical process industries can employ
hundreds of heat exchangers at a single plant, each with a different
process stream chemistry [14].
The three basic types of cooling water systems are once-through,
closed recirculating (nonevaporative), and open recirculating
(evaporative). True closed systems neither lose nor gain water during
service. Open systems, however, must have water added to make up
for losses. Open recirculating systems employing cooling towers and
spray ponds allow the dissipation of enormous heat loads while
limiting the amount of water consumed.
In closed systems (nonevaporative) where water loss is low, the
total waterborne material entering the system is limited. Thus,
deposited minerals accumulate at a much slower rate than in systems
in which large amounts of makeup water are added. Open recirculat-
ing (evaporative) and once-through systems are exposed to large
quantities of solutes, suspended solids, and biological materials. As a
consequence, fouling and associated corrosion are generally more
significant in open systems than in true closed systems.

8.4.1 Once-Through Systems
Traditionally, industry has tended to develop in areas with an
adequate supply of cooling water. Originally, it was sufficient to pipe
water through the plant and discharge it back to its natural source.
Only nominal attention was paid to control of water chemistry, and it
is in fact economically ridiculous to attempt the chemical treatment of
large volumes of once-through water.
However, even such minor additions as the “threshold” treatment
with 1 to 2 ppm sodium hexametaphosphate, for example, would be
unacceptable to modern environmental standards. Many countries,
states, or provinces now forbid the return to source even when the
concentrations of natural constituents were higher in the intake water
than in the return water. In some areas, thermal pollution is forbidden
since the discharge of the same water at a higher temperature than
the inlet temperature may be harmful to certain marine species (e.g.,
oyster beds).
The consensus today for once-through cooling systems seems to
be that, where permitted, the materials of construction must be chosen
to be resistant to the water, be it fresh water or seawater. All natural