Calculate the Theoretical Minimum Steam Flow to a Condensing Steam Turbine

[ManTicora]

Could one share his approach in "Calculating the Theoretical Minimum Steam Flow to a Condensing Steam Turbine"?
The web is full of "artistic" comments of this minimum flow to be about 25% rating of full-load limits.
But one help elaborate on a methodical way of computing (Mathematical Formulation) the theoretical minimum steam flow required to guarantee a stable operation of a given Condensing Steam Turbine?
Certainly Diaphragm size, Nozzles geometry, Number of Stages, Synchronous speed should affect this minimum flow.
Thank you all for your contribution.

 

[racookpe1978]

To "guarantee" a stable operation?

Depends too much on cooling water temeprature, flow rate, dew point and atmospheric pressure (effectiveness of the cooling system) and the specific efficiencies and problems of the last two turbine blade rows: Erosion, wetness, particle size, etc.

Better to run a low power test series for winter, spring, fall, summer conditions.

 

[snoopnoon]

The typical issue with running min condensing steam is the temperature is much higher in the later stages. Depending of your design, but if your bearing is integral to the exhaust casing inside of the exhaust cone, the increase in temperature will cause the LP end of the rotor to lift and cause misalignment. For argument purposes a safe way of knowing how little flow is OK is by looking at the exhaust temperature. If it is between 150 and 175F, you should be OK>

 

[ManTicora]

@racookpe1978 "to guarantee stable operation" free of vibration and wet steam. The goal of the post if to have a "systematic" method based on turbine geometry.
@snoopnoon Exhaust temperatures are below 45C (113F) and wet. Bearing temperature are stable... and vibration occurs.
Could one share his approach in "Calculating the Theoretical Minimum Steam Flow to a Condensing Steam Turbine"?

 

[FredRosse]

The minimum steam flow is highly variable depending on stage design. At low flows the velocity ratios between blades and nozzles can change enough such that the later turbine stages actually heat the flowing steam with churning action, and the resulting high temperatures present problems. Sorry to provide the "loose" answer you specifically said you did not want to hear, but that is indeed the answer.

 

[crshears]

FredRosse's post reminds me of references to 'cooling' or 'ventilating' steam in a few of the technical steam engineeering volumes in my possession. Too bad for the OP, or as waross wrote in response to one of my posts, "sorry for the tough love," but the 'theoretical minimum steam flow to a condensing steam turbine' is not something that can just be looked up in a table, or extrapolated using a simple nomograph, or even derived by plugging the right numbers into a formula.

It seems someone with years of operating and troubleshooting experience is needed to visit the site and figure out what's happenieng and how to fix it, as in a consultant...and finding a knowledgeable one can be a tall order.

Any reader recommendations for the OP?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]