An Experimental Investigation of the Influence of Flash-Back Flow on Last Three Stages of Low Pressure Steam Turbines

A shutdown operation of a large size steam turbine could possibly cause flashing phenomena of the pooled drain water in low-pressure heaters. The boiled steam is sometimes in the same amount as the main flow in the case where shutdown is executed during low load conditions, and returns to the steam flow path through the extraction lines. A series of experimental work with a subscale model turbine facility has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressures under the flashing back conditions. It was observed that the blades of the two stages before the last stage (L-2) and a stage before the last stage (L-1) presented their peak vibration stresses immediately after the flash-back flow reached the turbine. In the meantime, the vibration stresses of the last stage (L-0) blades were reduced for a few tens of seconds. It can be thought that the flash-back flow pushed out the reverse flow region around the L-0 blades and allow the blades to be more stable. A detailed examination with measured data of the L-2 blade explained that, as long as the flash-back flow has small wetness, the blade is excited in its specific vibration modes in larger than 8^th harmonic of rotational speed, but once the flash back flow carries water droplets, the fluid force in random frequencies remarkably increases and excites the blade in less than 7^th harmonic range.