PSI - Issue 13

Marko Katinić et al. / Procedia Structural Integrity 13 (2018) 2040 – 2047 Author name / Structural Integrity Procedia 00 (2018) 000–000

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2. Chronology of failure and fact-finding 2.1. Steam turbine technical data

The turbine cross-sectional drawing can be seen on Fig. 1. This turbine is a typical design of a single-cylinder condensing turbine for industrial application. It has seven steam expansion impulse stages. Each stage is a combination of a stationary nozzle cascade with nozzles fastened in nozzle chest or diaphragms and a moving blade cascade mounted on the next rotor disc. The first stage is usually called the governing (the control) stage. The turbine rotor is a flexible type (the 1 st critical speed is below running speed). It contains seven discs that are forged integral with the rotor shaft (there are not any shrink fit disk). The turbine moving blades are mounted in the grooves of the relevant rotor disk. These blades are tied at the ends into packs by means of strip banding (shroud). The turbine stator has a casing with conventional horizontal flanged joint and additionally one vertical joint that divide it into the front portion and exhaust end. The front end of the casing is cast while the exhaust end is of welded design. Inside the turbine casing are the housings of end sealing glands, diaphragms and their sealing glands. The housings of turbine bearings also belong to stationary parts of the turbine. The front housing contains the journal and thrust bearing and rear one, only the journal bearing. Type of both journal bearings is tilting pads. In addition, the thrust bearing is tilting pads type. The turbine has automatic governing system that contains four valves for controlling steam admission to the turbine, a distributing valve lever driven by a rack from a piston-type servomotor and speed governor. (Katinić (2009)). The main technical data of the steam turbine are shown in Table 1.

Fig. 1. Cross-sectional drawing of the steam turbine

2.2. Short chronology of blades failure Until 16 July 2009 the turbine was running rather smoothly with pretty low shaft vibration levels on both journal bearings of the turbine (about 15  m pp). On 16 July 2009 about 8 o'clock am, the turbine was suddenly tripped due to some process reason. While the rotor running speed was gradually decreasing the turbine operator was trying to return the turbine in normal operation in order to continue the production process. The rotor running speed had dropped at 1120 rpm when the operator had realized conditions for restarting the turbine. When the rotor speed reached 6335 rpm the vibration level on the exhaust end of the turbine sharply increased on approximately 100  m pp (Katinić (2009)). It can be seen on Fig. 2.