Page 325 - Corrosion Engineering Principles and Practice

296 C h a p t e r 8 C o r r o s i o n b y W a t e r 297

FIGURE 8.16 Type 321 stainless steel expansion joint in 2.8 MPa (400 psi)
steam service cracked from caustic carryover in the steam. (Corrosion
Basics: An Introduction, 2nd edn., NACE International, by permission)

8.5.2 Fossil Fuel Steam Plants
Conventional steam generation plants or more modern cogeneration
steam plants (Fig. 8.17) consist of many sections operating at different
temperatures and pressures:
• Feedwater heaters with water inside and steam outside the
tubes
• A boiler (water inside the tubes, hot combustion product
gases outside), where water is heated to high temperatures
under pressure and is sometimes flashed to steam
• A steam drum, wherein steam is formed from water and
water is separated from the steam (note: the steam drum is
omitted in certain once-through systems)
• A superheater, where the steam is further heated to even
higher temperatures
• A turbine, where the steam expands against the vanes of a
wheel to drive the turbine which generates electricity
• A condenser, where the low-pressure steam is condensed to
water and returned to the feedwater heaters

In such plants, there are special corrosion problems in each of
these sections. There are also special problems associated with
exposure to hot combustion gases. The materials used in high-
temperature steam and water include steels, stainless steels, and