4.1.1 Corrosion prevention, part 2
(b) Inlet end impingement attack
Inlet end attack is a localised form of impingement attack associated with high inlet water velocities and air release. The attack has the same characteristics as general impingement attack and is aggravated by a high suspended-solid burden. It has been suggested that annealing of tube ends may lower the resistance of the material to impingement attack.
(c) Impingement attack at lodged debris
General debris causes partial blockage of the condenser tubes and gives rise to high local water velocities of the order of 6 m/s. This exceeds the threshold value for conventional copper alloys and causes severe highly localised impingement attack. Tube failures caused by general debris can be minimised by concentrating attention on removing the source of the problem.
(d) Iron oxide scale
This is produced from the corrosion of carbon steel waterbox and CW pipe work components and is prevented by applying cathodic protection or by protective coatings (see Section 6.2.3 of this chapter). The oxide scale which forms as easily dislodged 'plate-like' sheets, becomes lodged in the tubes and causes failures due to highly localised impingement attack.
(e) General waterborne debris
This is produced from ineffective screening plant allowing shells, wood and marine debris to pass, in addition to debris from the culverts in the form of concrete spoil.
(f) Mussel fouling
This is caused by ineffective use of low level CW chlorination or bio-fouling equipment. If impingement attack cannot be controlled by preventive maintenance, consideration can be given to the use of secondary screening in the form of reduced throat inserts (Fig 4.9). These are nylon inserts with an internal profile resembling a venturi. When fitted, the insert initially absorbs water and effectively locks itself into the tube.
There is a 0.24 mm feather edge on the upstream edge of the insert which fits closely to the tube wall to prevent significant disturbance to the waterflow. Tests confirm that erosion problems from debris lodged in these inserts are contained and cause no damage to the tubes.