Press Release Back Number(2006)

It is known that random vibration occurs, especially in the final stage (stage 14), and that it can also happen at the stage before that (stage 13).

The steam flow around the vanes is shown in the figure below, and a large reverse flow (vortex flow*) occurs at no load and low load. The results of this analysis indicated that in Reactor No. 5, as the turbine got larger and the vanes longer, the impact extended even to the 12th stage.

*Eddies that are generated move in radial and other directions over time.

Random vibration analysis evaluation results for Hamaoka Reactors No. 4 and 5

	Hamaoka Reactor No. 4 • Rated output: 1137 MW • Speed: 1800 rpm • Low pressure turbine stages: 7 - 14	Hamaoka Reactor No. 5 • Rated output: 1380 MW • Speed: 1800 rpm • Low pressure turbine stages: 8 - 14
At 100% load
At 25% load
At 5% load

During cases such as load cutoff of the generator, the steam supplied to the turbine is suddenly reduced*, causing the pressure in the turbine to drop. As a result, the pressure in the water supply heater is dropped, causing the hot water inside to boil, and the steam creates a fast reverse flow to the turbine. This is called the flashback phenomenon. This reverse flow steam hits the turbine's vanes and causes vibration.

*In the event that an automatic stop signal is sent to protect the turbine, for example from excess turbine shaft vibration, the steam intake valve is quickly closed and the supply of steam to the turbine is stopped. There is a similar arrangement in cases where generator load suddenly disappears because of power line failure (this is called load cutoff). (Reactor No. 4 is an exception.)

Moreover, in cases of load cutoff when the generator output is low, the turbine is not stopped, but a very small amount of steam is supplied to maintain the rated speed.

The mechanism of flashback

(1)	During normal operation, steam flows through the turbine and is condensed in a condenser. Also, some of the steam is supplied to a water supply heater (a system for heating water supplied to the reactor) midway through the stages of the turbine.
(2)	The load cutoff, etc. causes a combined intermediate valve to close quickly, and the inflow steam to the turbine is suddenly reduced.
(3)	While the inflow steam is cut off, steam within the turbine is pulled to the condenser, which has a high vacuum, so the pressure within the turbine drops suddenly.
(4)	The pressure within the turbine and the pressure within the water supply heater reverse themselves, creating a reverse flow of steam from the water supply heater to the turbine.
(5)	The reverse flow of steam causes the pressure within the water supply heater to drop, after which the hot water in the water supply heater is depressurized and boils, creating steam that causes a fast reverse flow to the turbine.

The No. 5 bleed air pipe in the Hamaoka No. 4 reactor emerges just before the 11th stage and there is little flashback effect on the 12th stage vane. No. 5 bleed air pipe emerging position Hamaoka No. 4: Before 11th stage Hamaoka No. 5: Before 12th stage No. 6 bleed air pipe emerging position Hamaoka No. 4: Before 13th stage Hamaoka No. 5: Before 13th stage