Steam Turbine with 40 " LSB

[swapanburo]

I am now working for a project where a GE steam turbine (250 MW, 60 Hz) , originally designed for watercooled condenser application, will now be used with air cooled condenser. So the backpressure will now increase from 2" Hg to about 4.5 - 5.0" Hg. The turbine has 40 " last stage blade (LSB). Can anyone provide any tip about the maximum allowable exhaust pressure for a GE turbine with 40" LSB ?

 

[byrdj]

There are now LSB designed to allow high back pressure operation. I don't know all the available sizes we offer (and office is closed due to weather today)

Thus you need to know for what condition does buckest were made for. If an original bucketed older unit (5 to 10 years or older) it probly will require reduced load as backpressure increased

 

[rmw]

Do you have the curves that GE supplied in the turbine O&M manual?

rmw

 

[swapanburo]

Thanks byrdj and rmw
I have a curve for "Exhaust pressure Vs. Change in Output", which is drawn upto 5" HgA. Somewhere in the manual it is also noted that the alarm and trip setting are at 5" and 7.5" Hg. So may be it is safe upto below 5" , say around 4.5". What do you think ?

 

[rmw]

If your alarm is 5" then you should be safe right up to and until you reach 5". Frankly, 7.5" sounds real liberal to me. Most large machines I have had experience with had the 'drop dead' cut off point at 5.4".

There is a curve in the manual that shows a curve for the exhaust losses at both ends of the spectrum, high back pressure and very low back pressures. Either can be equally detrimental. The sweet spot is right in between.

There should also be a formula in the manual with which you can do some calculations in order to answer your questions.

rmw

 

[josh1990]

We have a Westinghouse 27 megawatt machine that was installed in 1952. About 3 years ago we started having vacuum problems. We went from 28'' to about 26 '' and going from one air set to two air sets. We are still using two sides to the air ejector and our vacuum has dropped to 25''.
we have cleaned the tubesheet too many times with no increase in vac.
We are presently checking the horizonitial joint to check for tightness.
Air leaks to the condenser are a gimmy but I suspect the air sets and the loopseal back to the condenser.
Does anyone have a gauge glass on their loopseal?

In the old days with one air set the air leakage was 2.0
cu.ft. of air, with two sides of the air set I can't get an accurate reading the manometer is off the range.
Can the loopseal get so plugged up that the steam and condensate overwork the aftercondenser?
I would appreciate any help in the matter.
Also we have had to increase our steam seal pressure from 40 psi to about 100 psi,that should be a tipoff to seaking seals.
Leo