4.1.2 Other copper-alloy tube failure mechanisms - part 1
Other types of copper-alloy tube failures can occur and these are briefly reviewed under each of the following headings:
(a) Deposit attack.
(b) Hot spot corrosion.
(c) Stress corrosion cracking.
(d) Corrosion fatigue.
(e) Steamside ammonia corrosion.
(a) Deposit attack
Deposit attack on condenser tubes occurs under conditions of stagnant or low water velocities, generally less than 1 m/s. Deposition of inert materials, such as sand and silt particles, causes oxygen depletion at the particle-to-tube material interface, leading to differential aeration and anodic dissolution of the tube material. The presence of decomposing biological materials and polluted water, particularly sulphide-containing water, accelerates this type of attack. Deposit attack can be experienced in both inland and coastal power stations, particularly in locations where polluted waters are used.
By maintaining the cooling water velocity in the tube above 1 m/s and avoiding stagnant conditions in aggressive media by flushing and periodically refilling the condenser with town water during shut down, major deposit attack may be prevented. However, a permanent solution can be achieved only by the selection of a more resistant material. Such materials are the 70/30 and 90/10 cupro-nickels (of which the 90/10 is claimed to have the better resistance) and titanium. Titanium offers the best solution, as it is immune to this type of attack under normal condenser operating conditions (Fig 4.10).
(b) Hot spot corrosion
This form of localised pitting corrosion occurs at 'hot spots' on the condenser tube wall which can occur because of low water velocity and/or high heat fluxes. These conditions can be caused by poor steam distribution or the absence of water on the cooling water side of the tube. The cupro-nickel alloys are less resistant than aluminium brass to this type of attack. However, this form of attack is rarely experienced in main steam condensers as the steam side temperatures are not high enough (Fig 4.11 (a)).
Problems can be experienced in the dump steam sections of the condenser or in separate dump steam condensers with high superheat, where direct steam impingement on the tubes occurs. Retubing in titanium tubing can provide a satisfactory solution or, alternatively, an engineering solution to avoid direct steam impingement such as redirection of steam flow can be found; the introduction of effective steam desuperheating is also considered to be a satisfactory option.