4.1 Cooling water quality
The cooling water quality can have a marked effect on the design and shape of the waterside components, such as waterboxes, which can influence both the corrosive and erosive effects of the cooling water. These aspects are discussed in more detail in the subsections which follow.
The demand for substantial quantities of cooling water leads to an increasing tendency for large power stations to be located on the coast and on the estuaries of major rivers. Currently, over 40% of operational stations are placed in such locations and are therefore subjected to a wide variety of cooling water conditions.
A typical water analysis can contain abrasive sand particles and marine life. Debris and cyclic tidal variations affect tube velocity and promote two-phase flow under syphonic conditions: this is particularly damaging to condenser tubes and CW system materials. It is against this background of operating experience that a broad knowledge of materials has been accumulated.
Fortunately, the position is less severe with the operation of inland power stations, where cooling water quality is generally satisfactory. There are, however, instances of condenser tube corrosion due to products of biological decomposition and operation in polluted waters.
It is important to secure information concerning CW quality as early on in the planning stage of a power station as possible, since it forms the basis upon which material selection will be made later. A survey of other industries using the same water supply, along with the local water authorities, is of tremendous value in identifying the history of CW quality. Meanwhile, systematic monitoring and discussions with local authorities yield information on the current situation, and on possible future developments which may influence water quality.
Lack of attention to this type of detail often leads to the wrong choice of materials. In condenser tubes, for example, tube perforations and condensate contamination levels exceeding the boiler water chemical requirements can have serious economic consequences. If leakage of CW into the condensate persists, costs can be incurred from tube replacement, loss of availability and consequential damage to other sections of the plant, such as boiler tube failure from feedwater chloride contamination.
These problems can be avoided and the associated costs minimised by combining the detailed knowledge accumulated on CW quality with that associated with the known corrosion performance of condenser tube materials, which forms the basis of material selection strategy discussed later.