[Weekly Accident Case Sharing] Accident Case 3: Misoperation of the valve position switch.

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This post was last edited by tiandiyi*an on 2017-8-22 at 10:01. I. Fault symptoms: At 11:56 on May 20, 2016, the main steam valve interlock caused Unit #1 of a certain company to shut down, which in turn led to the shutdown of the #1 rich gas compressor. The instrumentation and control personnel inspected the position switch for the high-pressure main steam valve and found water accumulation in the switch corresponding to the fully closed position of the #1 high-pressure main steam valve. II. Cause analysis: (1) Water accumulation in the full-close position switch of the #1 high-pressure main steam valve, which is part of the shutdown logic, caused the full-close travel switch of the #1 high-pressure main steam valve to send false signals, and this condition persisted. When the #1 high-pressure main steam valve is fully closed, a turbine shutdown signal is generated, which triggers the protection mechanism of the #1 rich gas compressor. (2) The steam leaking from the stem of #1 main steam valve condenses into water, which then seeps along the cables into the position switch, causing abnormal activation of the signal for fully closing the high-pressure main steam valve. (3) The signals used for logical protection in the control system and those used for display are from different sources, which prevents effective monitoring. (4) None of the various regulations require that ‘protection signals must be displayed on the operator station’; only the ‘reliability’ regulation stipulates that various fault conditions must be displayed on the operator station for operational analysis purposes. (It’s too obvious, which is why no one pays attention to it.) III. Problems identified: (1) Inadequate inspection and maintenance of the main steam valve position switch. During unit startup, steam leaked from the stem of the main steam valve; the waterproofing measures for the position switch of the main steam valve were inadequate, which caused the condensed steam to seep into the position switch along the cables. (2) There are deficiencies in protection combing, resulting in the signals used by the control system for logical interlock protection and the signals displayed on the screens having different sources. IV. Preventive measures: (1) Display important interlock protection signals on the operator panel to ensure that these signals are consistent with those shown on the operator’s screen, thereby facilitating monitoring. (2) Before conducting important online tests, instrument technicians should first carry out signal checks and resolve any issues found promptly. (3) In the event of steam or water leakage, the instrumentation team should promptly take protective measures and check whether there is water accumulation or a decrease in insulation in the relevant equipment.