抄録
In the event of a steam turbine shutdown, pressure in the steam path of the turbine suddenly decreases to a vacuum. This causes pooled drain in the heater to immediately boil and turn into Flash-back steam coming back to the turbine, the so-called Flash-back phenomenon. This phenomenon can cause serious damage to long blades.
To verify the structural reliability of the blades, model steam turbine tests were conducted. The size of the model turbine was 1/4.2 of the 1500rpm full scale actual steam turbine, and rotated at 6300rpm to keep the velocity triangles the same as that in the full scale turbine. In terms of mechanical features, the blade stress of the model turbine was the same as that of the full scale turbine. The test facility contained two sets of a hot water tank and a connecting line to the turbine. Quick-move gate valves in the lines were opened during low load and full speed steam turbine operation. This caused preheated water in the tank to boil and come back to the steam turbine, which simulated the Flash-back phenomena. The Flash-back mass flow rate and wetness of Flash-back flow were varied as test parameters to investigate the influence of Flash-back on blade stress with various Flash-back flow conditions. The test parameters were decided considering the situation that the turbine is shutdown at 100% load condition which is the most severe condition in actual steam turbine operations and was not verified in previously reported tests.
The vibration stress was obtained from strain gauges mounted on the suction side of each blade. The relationship between Flash-back conditions and vibration stress was investigated. As a result, it was confirmed that the vibration stress on the long blades caused by the Flash-back phenomena is sufficiently low compared to the strength of the blade material.
