Forensic Engineering

Short-Term Overheating

Short-Term Overheating

Background

Commissioning is likely the most critical stage in the life time of a power plant due to the transient conditions which boilers are subjected to.

From the first ramp-up to normal operation, the plant is exposed to a variety of transitory conditions, thermal expansions cycles, chemical excursion and equipment adjustments that potentially increase the chance of failure.

Description

Short-term overheating is the primary cause of tube damage encountered during commissioning. The nature of this problem is mainly due to blockage in the fluid path by the inclusion of foreign materials, surface detachments and/or deposition.

Obstructions can also happen as a result of chemical cleaning when residues are not effectively removed from the system.

Analysis of the Problem

The attached photo displays an example of overheating where metal scales were found clogging all over the internal surface in the lower section of a solar preherater.

Dissimilar polarity between the scale and the tube material in the point of contact did influence changes in the electrical potential of each material forming a corrosion cell and promoting the growth of deposition.

Consequently, deposits decreased the heat transfer leading to the formation of bulges, micro-cracks and finally boiler tube burst.

Forensic Engineering

Chloride Attack on CS

Chloride Attack on CS

Background

This photo illustrates the corrosion pattern of chloride attack on carbon steel and it was taken during an inspection carried out in a cooling water pump of a steam-water cycle.

Description

The pump was subject to severe degeneration due to the formation of corrosion cells in which the corrosion product forms complex structures known as tubercles. The presence of chlorides will destroy the passive film, promote the accumulation of deposits and reduce the oxygen content in those regions. This will create gradients of anaerobic zones which produce acidic conditions and differences in the electrochemical potential. As a consequence, the equipment is generally drastically affected and fated to failure.

The formation of tubercles is primary controlled by the water chemistry, conductivity, oxygen concentration, and temperature. This phenomenon is common in cooling systems when operating with deficient water treatment and chloride levels around 200-300 ppm.