LP Condensation: Usable work? 1

[stinems]

Let's say steam starts to condense while still in the LP section of a turbine, exiting at a quality of 95%. Is the amount of latent enthalpy given off while still in the LP available as mechanical work to the turbine?

I question that a simple delta h times mass flow in this case gives a real world indicator of available work, since LP blades capture enthalpy dynamically as opposed to kinetically (impingement) as in the HP stages. Is a dynamically-designed turbine blade able to capture anything from water droplets flying by?

I know, I know, condensation hell on the turbine. Just asking hypothetically though...

Cheers,
~Sam

 

[rmw]

The enthalpy at that partucular stage of the LP reflects the reduced work capacity of moist steam. It can't do much work, but it still can do some. Almost all LP's that discharge into vacuum condensers operate wet in the last couple of stages. Do the calculations or look at a Mollier diagram and you will see that the last stage(s) contribute a significant portion of the overall load.

A turbine stage captures work from the heat passing by. All the steam does is carry the heat by.

rmw

 

[EdStainless]

Some turbines are desinged to work very wet. If you have cold enough cooling water available you can extract a bit more work. In the overall scheme it is very minor.

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[stinems]

So...you're saying the latent heat/enthalpy given off in the LP by the portion of steam that condenses at (theoretically) constant temperature and pressure IS mechanically capturable by the turbine?

Our situation includes three 50+ year old GE steam turbines (2x7.5MW, 1x9.375MW) retrofitted into a combined cycle behind two 70's vintage Westinghouse CT's (2x30MW). The condensers are in bad shape and the air ejectors can't keep up, pulling 5 inches of vacuum at best. What happens is the steam coming through the turbine may "see" a lower temperature from the circ water tubes, but isn't able to expand to that temperature's saturation pressure. To me, this would mean condensation in the LP.

I'm trying to figure out how much efficiency we are losing due to condensation in the LP. Or maybe we could be gaining some extra power (at the expense of turbine health). Hence my question.

An intimate knowledge of what exactly happens *real world* in terms of pressure/temperature/enthalpy between the LP and hotwell is something I lack at this point.

Any thoughts?

Thanks for both or your previosu replies!

~Sam

 

[byrdj]

Real world from turbine controls guy.
Operating with high back pressure would be a concern I would recomend being investaged and corrected. In a similar situation, I have contacted our thermodynamics engineer and latter repairs were made to condensor

from mechanical side, most LPs have a low vacumn alarm trip to prevent the mechanical force on the longer last stages buckets becoming excessive and causing damage. the steam in last stage bucke will be hotter than design, also causing stress problems, even though exhuast temp can be lowered.

I have some heat balance correction charts that shows as much as 8% increase in heat balance for only 2".

the flow through the unit will be reduced and thus maximun load.

I would think a consultant could perform a few calcs and show how the corrections to the condensor would pay for thier self. (unless some here will do it for you)

 

[EdStainless]

Why is your back pressure too high? Poor heat transfer? Fouled tubes? Low cooling water flow? High cooling water temp? Excessive air inleakage? Poor sizing?

I am just a condenser guy, but I know of a lot of cases where 4% per 1" is not uncommon.

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[stinems]

I should clarify something: by 5 inches of vacuum, I meant 5 inches of mercury absolute, or 25 inches vacuum. For the steam flow we put through the condensers (60-80 klb/hr each), the vacuum we *should* be pulling, as per the design performance curves, is about 2.5" mercury absolute. I think the condensers are a bit undersized since the turbines were designed for 2" backpressure.

The impetus for this entire study deciding on a design wet bulb temperature for our new cooling tower being installed in the spring. The current one (piece of junk) is designed around 75 deg WBT, but our consultant says 78 deg WBT design criteria fits our climate better. Comes out to about 16% increased capital cost. I've been trying to justify the extra cost with the increased capacity and the numbers look doubtful for the more capable tower...

Little bit about the condensers: The old wooden cooling tower is falling apart, wood chips and debris are clogging the condensers. Air ejectors are not performing to spec. Not sure when the last time a retube occurred, hoping it was since the last stone age. Can't imagine they have anything less than severe fouling since we don't do a whole lot for circ water treatment right now.

Hence the need for a new cooling tower whose circ water will have at least a baseline treamtent. Besides that, the current tower is a fire and safety hazard.

90MW peaker that runs 6 months out of the year, and probably 40% cap factor during that time. A ~150kW total increase in capacity (what my study has been showing) due to colder circ water is hard to justify alone when peaking capacity can be bought for pretty cheap these days.

Somewhat recent changes in management have resulted in a turnaround in maintenance philosopies (for the better) and we've been putting to right the "sins of our fathers" so to speak.

 

[rmw]

If you are operating at 5 in. back pressure, I doubt that you have to worry about moisture in your exhaust steam.

Good luck on your maintenance program turnaround. You have too many problematic areas to begin to try to help you.

A better performing cooling tower will help. Less cooling tower debris will help. Better chemical control will help. Fix the vacuum equipment. Find the air leaks and fix them.

Then live happily ever after.

rmw

PS: I do recommend that you get ahold of a good thermo book or something similar and try to get a better understanding of what goes on on the bottom end.

 

[EdStainless]

Are you making rated power and burning 10% too much fuel to do it?
or
Are you burning rated fuel and making 10% too little power?

Either way you need to look at temperatures and flows around you system and check the heat balances. Determine where your biggest defficencies are. Don't overlook the cooling wtaer flow rate. Old pumps might not be moving nearly as much water as you suppose.

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