Cause analysis and prevention of equipment inundat

2022-08-06
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Cause analysis and prevention of equipment flooding accidents in hydropower plants in addition to strengthening the monitoring and inspection of key parts such as dams, hydraulic structures and power plants in the flood season, confirming that the equipment operates well, the power supply is reliable, the gate is opened and closed flexibly, ensuring that all kinds of emergency materials for flood control and flood prevention are in place, and preventing the losses caused by floods to dams, power plants and units, this article I excerpt is published for professionals, Due to the particularity of the operation of hydropower plants, attention must also be paid to prevent flooding of plants and damage to equipment due to corrosion of equipment parts and components

2 accident examples

before use

2.1 water flooded top cover accident caused by damage of vacuum breaking valve 2.1 thermal properties of plastic materials in plastic thermoforming processing 1. After the accident, a 60MW axial flow paddle type unit was symmetrically arranged with four automatic emergency vacuum breaking valves in the support cover, with the orifice diameter of 600mm and the valve disc diameter of 350mm. Its function is to supplement air to the runner room during the emergency shutdown of the unit to prevent the damage caused by water hammer lifting due to excessive vacuum. The valve disc will open automatically when the vacuum in the runner chamber reaches 0.01Mpa, all of them will open when it reaches 0.03mpa, and will close automatically when it is less than 0.01Mpa. One day, when the unit was running, a vacuum breaking valve in the +y direction of the unit suddenly leaked a large amount of water, causing the water level of the head cover sump to rise sharply. Although all the head cover drainage pumps were started to pump water, the water level still rose rapidly, which soon flooded the head cover drainage pump and the turbine guide bearing, forcing the unit to shut down

2.1.2 accident cause analysis after the accident, the vacuum breaking valve was inspected, and it was found that all 8 M16 connecting screws between the sealing ring and the valve body had been broken or fallen off, and the fractures of 3 broken screws were old traces, indicating that they had been corroded before the accident. In addition, the broken seal ring was seriously displaced, forming a large staggered joint between the valve disc and the seal ring. At that time, the head cover water pressure was 0.15Mpa, resulting in a large amount of water leakage. The water leakage volume was about 90m3/h, which greatly exceeded the drainage volume of the three head cover pumps. There are also serious defects in the design of the vacuum breaking valve, mainly due to improper material selection and low sealing ring connection strength. The seal ring and connecting screw are made of bronze, and the valve seat is made of aluminum alloy. Originally, the m16mm screw hole and screw connection strength drilled on the valve seat are low. In addition, long-term operation, aluminum and copper produce electrochemical reaction. When the input coil is connected to the current servo valve, the threaded connection part of the screw hole and screw is severely corroded, further weakening the connection strength between the seal ring and the valve seat. Under the influence of head cover vibration, water flow and vacuum breaking valve action during unit operation, the screws broke or fell off, resulting in serious displacement of the sealing ring, which is the main cause of the accident

2.1.3 treatment measures: the valve bodies of the four vacuum breaking valves of the unit are changed to cast steel structure, the connecting screws of the sealing ring are changed to m20mm 45 steel bolts, and the other parts remain unchanged. In addition, a stop pin is added to the fixed nut between the valve disc and the valve stem to prevent the top cover from being flooded due to the valve disc falling off due to the loose nut. Since the vacuum breaking valve is installed in the support cover and is in direct contact with the water flow of the turbine passage, it shall be regularly monitored and maintained. During design, materials and structures with corrosion resistance and high strength shall be selected to prevent failures caused by corrosion, looseness and fracture of vacuum breaking valve parts

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