Failure analysis on abnormal blockage of electro-hydraulic servo valve in digital electric hydraulic control system of 125 MW thermal power plant

Abstract

Electro-hydraulic servo-valves, which are considered as the crucial core element of digital electric hydraulic control system, are widely applied in thermal power plants. In this paper, motivated by the long stable operation of thermal power plant, an abnormal blockage failure of the electro-hydraulic servo valve in digital electric hydraulic control system of 125 MW thermal power plant was investigated systematically. Three body abrasive wear was identified as the main failure mechanism. Comprehensive analysis results finally revealed that the unqualified material of the valve spool with abnormal coarse carbides caused by improper preparation process was ascribed to be the root cause of the failure. During the operation, the abnormal coarse carbides acted as stress concentration source and were peeled off from the valve spool under the effect of the friction between the feedback wire and the valve spool. After that, the exfoliated carbides served as third bodies, resulting in three body abrasive wear. In this process, some of the carbides with size exceeding 5 μm driven by fire-resistant oil entered the clearance between the valve spool and the valve sleeve, leading to the abnormal blockage failure since the maximum allowable clearance was 5 μm. Countermeasures were then proposed to address corresponding problems. Considering that practical failure of the electro-hydraulic servo-valves is seldom reported before, the finding of the paper could be helpful for better understanding the failure mechanisms of the electro-hydraulic servo-valves under actual service conditions.