Effects of low temp steam to a turbine generator 1

[crawfishdaddi]

Due to recently increased production rates at the cogen plant where i work, we have started to send low superheated steam to one of our GE turbine generators. The design parameters for the inlet steam to this turbine are 540 PSIG at 750F. We have send steam as low as 680F and i wanted to know if this has any negative internal implications (i.e. blade corrosion/pitting or possible failure).

Thanks,
CFD

 

[orenda1168]

CFD:

I would think GE would be in a better position to answer your question.

Orenda

 

[rmw]

Can your read a mollier diagram? If so, plot your original conditions, which will show you where you cross the saturation line into the moisture zone, and where the new expansion line will cross.

It will also make the turbine require more steam to produce a given output because there is less available energy in the steam.

This, too could change the flow rates through the turbine, and have unintended consequences.

The higher steam flow could also raise your condenser back pressure, (assuming there is one) and further require the turbine flow rate to increase to meet the load requirements.

Katmar software (google'em or search this site-mentioned many times) has a turbine steam consumption program that you can download and play with for your conditions.

I took a simple case, based on 10MW, and found that at the same back pressue it took about 5% more flow, and the exhaust was about 5% wetter, assuming the same back pressure. Bump the BP up by 1/2" hg., and it gets worse.

I concur with orenda that you should get GE to provide you with a heat balance for this condition.

rmw

 

[crawfishdaddi]

rmw,
Maybe you misunderstood my question. Yes we have consulted a Mollier diagram and GE is already in the process of running an energy balance for us. The fact that it will take more steam to produce the same power output as the design inlet temp would produce is a given. Increased flow rates to the turbine raising back pressure to the surface condensor is also a given. My question was in regards to physical/mechanical damage to the nozzle, blades or shell of the turbine due to 90-95% quality "wet" steam at the back stages of the turbine- Water errosion and/or possibly pitting is my main concern. Since we are a cogen plant our primary objective is to optimize production rates of Sulfuric Acid (which is worth more to us than the waste heat steam). I just don't want to damage our tubrine in the process.

I did find the software you recomended useful and thank you for the suggestion.

Regards,
C.F.D.

 

[orenda1168]

CFD:

Maybe I missed something here, but I assume that if GE is doing an energy balance for you, they have also been asked about the issue of nozzle, blade or shell erosion potential of your wet steam.

Orenda

 

[crawfishdaddi]

Orenda:
They have not gotten back with me for 3 days (I'm not entirely impressed with there tech support right now)so I thought I would ask if anyine had previous experience with this since we have already dropped the temp to the inlet steam and may drop it some more (I kindda need to know very soon)...

C.F.D.

 

[rmw]

No, I didn't misunderstand your question at all. I just don't have enough information to answer it with more than generalities. Without being able to look at your heat balance, I can't know where, meaning in which stage, in your turbine steam path the steam crosses over into the moisture region.

I would also need your condenser design parameters to know how its operating point would change with the new inlet flows and enthalpies.

The last stage or states of large steam turbines that exhaust into a deep vacuum are normally built with specific features to minimize the damage from wet steam.

I don't have enough information to determine if your change has moved that cross over point into a stage of the turbine where it would be detrimental. Probably only GE can.

Since there is less available work in the steam entering the turbine, and since it has different physical characteristics, and it is a given that more steam is required, there is an effect on the turbine in every stage, from the inlet nozzles all the way to the bottom.

I suspect when you do finally see something from GE, the slope of your expansion line will shift to account for the changed efficiency. Interestingly enough, making some rough assumptions, and doing a layout on a mollier diagram, it appears that the possibility exists even with a small change in the slope of the expansion line, due to the slope of the saturation line on the mollier, that you might actually shift the moisture zone farther downstream.

I did not mean to insult you in questioning your ability to read a mollier diagram but these fora are full of folks that are asking questions way beyond their capability to understand the answers given. You have put that concern to rest.

It is a complicated question due to the change in not only the temperature of the steam, but in the volumetric flow rate through the turbine as well, so give GE some time.

Let us know what is the outcome.

rmw

And, PS, for whatever it is worth, I had some boiled crawfish last weekend, and they were great!!!