Failure analysis on unexpected leakage of electro-hydraulic servo valve in digital electric hydraulic control system of 300 MW thermal power plant

Electro hydraulic servo valves, which are used to transform weak electric signals into high power hydraulic energy, are the crucial part of digital electric hydraulic control system. In this paper, inspired by the safety operation of the digital electric hydraulic control system, an unexpected leakage failure of the electro-hydraulic servo valve in the digital electric hydraulic control system of 300 MW thermal power plant was studied in depth. Systematical analysis results finally revealed that this unexpected failure was caused by pitting corrosion induced by chloride ions, which are initiated from improper cleaning agent containing trichloroethane. It reacted with the phenolic compounds came from the degradation of the fire-resistant oil, giving rise to the production of the chloride ions. Passive film on the contacting surface was then destroyed by the chloride ions, leading to the formation of naked areas where the pitting corrosion occurred. After that, the coarse carbides with size exceeding 5 mu m in the naked areas were peeled off from the contacting surface under the effect of the pitting corrosion, resulting in the blockage of the servo valve since the maximum allowable clearance between valve spool and valve sleeve was 5 mu m. This eventually hindered the movement of the valve spool and caused the unexpected leakage failure. Considering that the failure of the electro-hydraulic servo-valves caused by corrosion is seldom reported before, the achievement of the paper could enhance the understanding of the failed mechanism of the electro hydraulic servo valves under actual service condition.