Corrosion in HVDC valve cooling systems

Stainless steel couplings in the main cooling water pipes of HVDC thyristor valves have been in use since 1983, with overall satisfactory behavior. However, some water leakage due to corrosion below the sealing O-rings of the couplings was observed during 1992. An extensive investigation and follow-up worldwide showed a direct correlation between water quality and the corrosion rate of these stainless steel couplings. Recommendations are given as to actions to be taken in order to maintain a long lifetime for the fine water systems.