Steam turbines have a protection of reverse power to avoid heating of turbine blades in case prime mover is not sufficient to produce that much power to achieve synch speed or for the loss of steam supply turbine blades heat up. My question is how can steam cool the turbine blades? Because we always apply steam of temperature more than 1000 degree F and pressure more than 1500 PSI. This is so much of heat and if we remove that why blades heat up?

 

Rathore... you can find several threads on this topic in Control.Com Archives.

But, the one that I think best addresses your question regarding "Steam as a coolant" is JFB's reply in Control.Com thread:

http://www.control.com/thread/1387628531

Regards,
Phil Corso

 

The steam conditions you mention are at the high pressure inlet to the steam turbine. The high pressure (and intermediate pressure or reheat) sections are designed with bucket metallurgy that can withstand these temperatures. The concern for overheating when steam flow is shutoff is generally with the low pressure section, mainly with the last stage buckets, which are designed for the steam conditions they normally see. The admission to the LP section is at much lower pressure and temperature than the HP and IP sections.
When the steam flow is shut off, there is only some air and water vapor in the turbine. If it is still spinning at rated speed, the last stage buckets will heat up due to windage, and will exceed their design temperature limits.

On a single-shaft combined cycle application, "cooling steam" is admitted to the LP section of the steam turbine until sufficient steam is available from the HRSG to admit to the HP and IP sections of the steam turbine, which will, in turn, provide steam at the proper temperature and pressure to the LP section.

The reverse power breaker trip is usually there to insure that the steam flow has been shut off before opening the generator breaker to prevent overspeeding the turbine.

Hope this helps.

 

> ... The admission to the LP section is at much
> lower pressure and temperature than the HP and IP sections....

the most concern is at the exhaust of the LP section where the steam pressure is approaching condenser vacuum. even several stages back towards the admission, the pressure will be below atmospheric and if one recalls the saturation temp of steam will be 100C and going towards last stage it could even be down to 35C. as in you can easily hold your hand on an LP hood

after a few minutes of motoring, the paint will blister on the LP hood.