I want to know why main steam temperature drops in steam turbine during expansion.

Why turbine should be trip at low and high steam temp.

mukesh
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Hi,

As I understand it, and I’m sure I will be corrected if I am wrong...

The trip of protection on an ST unit is there to protect against Long & Short Rotor expansion (force cooling protection).

Long Rotor expansion will happen when the unit is heating up and is not temp controlled, the rotor will expand quicker than the casing (because it has less mass). If the rotor expands quicker than the casing, then the rotor moving blades could touch the fixed blades of the casing... Bang, Crash, Wallop... and we don’t want that do we...?

Short rotor expansion happens when the unit is cooling down, again if it is not temp controlled the same will happen.

So given that the gap between the moving blades and the fixed blades is only millimetres, the force cooling protection is in the logic to protect against uncontrolled temp adjustment.

Hope this helps.

 

PV = nRT I believe is the explanation for the temperature drop as steam is expanded and work is extracted as steam flows through the nozzles and blades of a steam turbine. As any gas is expanded as it flows through a valve which is acting as a restriction, it cools.

If steam temperature is too low the steam can condense as it expands through the steam turbine, and water damage is very catastrophic. The steam should not condense until it reaches the condenser. That's why it's so important to properly warm the steam lines and inlet valves before admitting steam to the turbine.

 

If steam at low temperature enters into turbine it becomes condensate after certain stages. The rpm of steam turbine is usually above 3000. If there is condensate into turbine the blades of the turbine eroded as like as pitting with these high velocity condensates. In consequence rotating mass balance of the turbine damage which in turn damage all the bearings & seal strips before tripping at "high vibration".