In 2013, with the operation of 1000MW steam turbine unit in a power plant, the exhaust steam temperature of intermediate pressure (IP) steam turbine exceeded the normal value. After overhaul of the IP steam turbine, a long crack was found through the drain hole of the IP inlet diffuser. The reheat steam leaked out from inside the diffuser to the exhaust cavity. It caused the exhaust temperature to exceed the design value, and reduced the efficiency of the units. Also, if the high temperature steam, which is close to 600°C, came into contact with the outer casing, a significant reduction in life would be obtained. In order to find the root cause of the crack, many efforts have been done within each department of the shanghai turbine plant (STP). Such as material detection, design, manufacture, assembling, etc.
In this paper, a three-dimensional finite element diffuser model was built and the stress state during startup and operation was obtain. It was found that the drain hole at the inside and the groove at the outside of the diffuser are dangerous regions because of the stress concentration. That’s the primary reason for causing the diffuser crack. Then the fracture mechanics software ZenCrack was used to analyze the diffuser crack route. This crack route consistently matched the actual diffuser crack.