The primary reasons for chemical cleaning of boilers/ heat exchangers are to prevent tube failures & improve unit availability. Tube failures in low pressure boilers/heat exchangers are normally the results of creep which occurs when internal deposits produce excessive metal temperature. A relatively smaller quantity of deposit creates difficulties in high pressure boilers. Caustic corrosion & hydrogen damage, which occur only in the presence of deposits, may cause tube failures at temperatures well below the creep limit. Deposits originating both from fabrication & during operation should be considered potential problems.
All the pressure parts of steam generator may be subjected to heat treatment of some sort during fabrication or erection - during forming operations, stress relief, welding or bending. Whenever carbon low alloy steels are subjected to high temperatures in the presence of air, oxidation occurs, the oxide produced is known as mill scale. Mill scale on boiler/ heat exchanger tubing is normally very thin with the exception of areas near welds & bends. Even where mill scale is initially uniform its brittleness upon cooling may produce flaking. The resulting non-uniform surface is undesirable from the standpoint of corrosion susceptibility. During operation mill scale is rapidly eroded from the steam generating surfaces & may subsequently be redeposited in critical areas. Preoperational acid cleaning removes mill scale & serves to remove atmospheric rust which inevitably accumulates to some degree during erection.