Page 327 - Corrosion Engineering Principles and Practice

298 C h a p t e r 8 C o r r o s i o n b y W a t e r 299

Almost all of the components in a supercritical steam plant are
made of austenitic stainless steels of the 18-8 variety, for example,
S30403 or 31603. These materials are employed to minimize corrosion
products and their transport through the system. The steam
temperature is no higher than in an ordinary superheater, but the
pressures are such that many chemicals and corrosion products may
show appreciable solubility in the steam.
Many chemicals exhibit an inverse solubility when the temperature
of the steam is above the critical temperature and will therefore
deposit out at these higher temperatures. In one supercritical plant,
about 140 ppm caustic was accidentally introduced into the plant.
Within 30 minutes, caustic stress corrosion cracking (SCC) occurred
in that part of the plant where the temperature was about 425°C. This
temperature corresponds to the minimum in the caustic solubility-
temperature curve. The supercritical steam also undergoes a marked
density increase above this temperature range which could accelerate
the deposition of chemicals.
For such reasons, the dissolved solids content of the water must
be kept as close as possible to zero. A large fraction of the water is
continuously cleaned up in a bypass circuit containing ion exchange
(demineralizer) beds. At start-up, 100 percent of the water may be
passed through the cleanup beds. Chlorides and caustic are the most
undesirable salts because they are known to cause SCC of the
austenitic stainless steels. Dissolved oxygen may be reduced by the
methods described previously to a few parts per billion.
Copper deposition on turbine blades was an early problem with
supercritical units. It was found that trace amounts of copper were
dissolved from the condenser tubes and recirculated into the boiler
section. Because of its excellent solvent properties, the superheated steam
carried the dissolved copper into the turbine where, at the lower pressure,
copper was deposited upon the turbine blades. This not only affected the
efficiency, but threatened to destroy the turbine by mechanical imbalance
due to uneven deposition. The problem has been largely eliminated by
using stainless steel or titanium in the condenser.

8.5.4 Waste Heat Boilers
In many chemical or petrochemical processes, economy dictates that
superfluous exotherms be utilized to generate steam as an energy
conservation measure. Examples are the cooling of a butane oxidation
reaction or the condensation of hot sulfur vapors, steam pressure being
generated on the shell side of specially designed heat exchangers.
Corrosion problems in waste heat boilers usually arise either from
unusual materials of construction or from inattention to the required
details of water treatment. Austenitic stainless steels may be required
from the process-side corrosion aspects, yet be highly susceptible to
SCC from boiler feedwater. The most frequent problem, however, is
that the operating department personnel, whose primary concern is