Estimate minimum load and pressure drop for a turbine? 1

[LaSalle1940]

Here's the issue: let's say we have a turbine using steam at some known inlet condition that has a known nameplate horsepower rating. Assuming the shaft coupling fails (i.e., the turbine is disconnected from its load so that it spins freely) but the steam supply remains intact, is there some rule of thumb to estimate what the minimum power is under those free-wheeling conditions? Or is it safe to assume that this minimum free-wheeling power is equal to the difference between the total power supplied by flowing steam and the net power delivered to the load?

As a follow-up: I'm assuming that in this free-wheeling state, the pressure drop across the turbine would be significantly less than it would if everything were intact. If so, it seems to me that the pressure drop across the turbine itself could be estimated from a mechanical energy balance; from that balance, the enthalpy change could be calculated, and the pressure drop could possibly be ESTIMATED by approximating the turbine as an isentropic expansion. Does that seem reasonable as a first pass?

 

[blanjp]

If the turbine is dissconnected from the load it was carring the steam admission valves should close at a predetermined turbine speed value to prevent an destructive overspeed condition

 

[MJCronin]

I agree with the above remark....

Overspeed trip is the first order of business.....there is no substitute.

In the long deveopment history of steam (and gas) turbines, one of the first safety features added was an inertial governor/trip valve that protected the turbine from catastrophe...

-MJC

 

[abeltio]

the amount of steam required to spin the unit at Full Speed without load (not synchronized) is a value given by the manufacturer - the difference between this value and the full load value is very big... usually the unit will run at rated speed with about 10~20% (depending on size) of the steam at full load.
the full load steam applied to a free-wheeling turbine will accelerate her to complete destruction.

what you are describing is very similar to a "Load rejection test", a typical test performed during commissioning where the generator breaker is open at full load...
at that moment the governor should not allow the unit to overshoot above 106%~107% of the rated speed...
and bring her back to 100% speed (idle, ready to synch again) at typically 110% speed the overspeed trip should actuate and trip all the valves.

saludos.
a.