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Steam turbine power or "work" estimating

Steam turbine power or "work" estimating

Steam turbine power or "work" estimating

I'm not very knowledgeable on steam and turbines, but have a question on steam turbine power estimating (if my "budget" number is attractive then I can call in an expert to detail engineer exact).

Here is problem:  We have a 650# boiler, saturated steam.  Since our process doesn't need all the 650# steam, we let down 20,000 #/hr to 135# steam across a pressure control valve.  If I were to put in a let down turbine in place of the control valve, how much HP or work could I estimate to gain?  Seems attractive to put in a turbine and generator set and generate some power off this let down process.

Is there a calculation that will give good ballpark numbers?  Is it worth look at further?  A 0.25MW generator would not be worth much to me (long payout), but a 0.75MW may be getting attractive, more even better.

Thanks for any help.


RE: Steam turbine power or "work" estimating

Thanks, that is exactly what I'm looking for.  Doesn't seeem to offer quite the power generation I was looking for but that is what I was looking for but gives me something to plug into the economics and let the numbers speak for themselves.

If anyone has the formulas handy, I'd love to see the behind the scenes of such a calculation and any detail on assumptions or generalizations it makes.

Thanks again Diborane.


RE: Steam turbine power or "work" estimating


I'm not a turbine designer but I find your topic interesting.

Ideal throttling is a constant enthalpy process.
Initial conditions:
P =    650    psia
T =    495.0245972    deg F
s =    1.43802464    BTU/lbm-R
h =    1202.905884    BTU/lbm

Process equipment actually gets superheated steam (assuming no heat loss):
For throttle process delta h=0        
P =    135    psia
T =    367.0059814    deg F
s =    1.590979457    BTU/lbm-R
h =    1202.906006    BTU/lbm
X =    1    nd

Ideal turbine work has a constant entropy (s):

Turbine = constant entropy        
P =    135    psia
T =    350.26474    deg F
s =    1.438024998    BTU/lbm-R
h =    1078.921997    BTU/lbm
X =    0.869081199    nd

Delta h    = 123.9838867    BTU/lbm
flow    = 20000    lbm/hr

Available energy is 726.75 kW

If you assume an efficiency 0.75 the work is 545 kW.
Note: the steam quality is no longer saturated; therefore, moisture separators would be needed to scrub out the water droplets. Thus, you would loose 2618.38 lbm/hr of steam flow.    Of course if you were to reboil the lost condensate your net available work is only 58 kW.

If your boiler has excess capacity or your process can use less flow (17,380 lbm/hr) at the lower temperature of 350 F then depending on turbine/generator efficiency you could get close to your .75 MW number.

Just out of curiosity, what did you get back from the dresser rand website?

Also, if your interested in running some numbers in an excel spreadsheet like I did, then you can downloaded the thermal fluids calculator from spreadsheetworld.


RE: Steam turbine power or "work" estimating

Mauner, thanks

DR website gave me about 0.34 to 0.4MW depending on RPM (I assumed 2200 to 3000).  Seems to match up close to your numbers when using a little lower of efficiency. My next step is to give them a call and discuss preliminaries.

The boilers primary heat source is a gas turbine (COGEN set Solar Taurus turning a generator)and we know we loose some energy up the stack so we might have room for superheater before going to letdown / steam turbine if needed.  We've outgrown the cogen's capacity so this possible additional "free" electricity is looking more and more attractive.  At the 0.5MW and up we could be looking at not much over a year payout at our current $0.10kW-hr.  

Thanks for help.

RE: Steam turbine power or "work" estimating

Mauner's reply is correct, through the statement "If you assume an efficiency 0.75 the work is 545 kW."

The statement "moisture separators would be needed to scrub out the water droplets" may not be correct, steam turbines typically run with wet exhaust conditions, and no separators are needed.

Also note that the energy taken out of the system in the form of steam turbine work results in less available energy for process use.  The sum of the two outputs (generation and process heat) remains virtually constant.

Your input of 2200-3000 RPM is lower than I would expect.  With respect to turbine efficiency, such a small machine would need much higher RPM to attain good efficiency, the last small turbine-generator I worked on was 10,000 KW, and turned 30,000 RPM, driving a 3600 RPM Generator through a gearbox. Older 10,000+/- HP marine turbines run in the 3000 - 7000 RPM Range.

Turbine efficiencies run all over the map, large utility turbines (500,000KW-900,000KW) manage in the vicinity of 90% efficiency, with generator efficiency about 99%. Small mechanical drive turbines often have efficiency well below 50% of the isentropic potential.  

RE: Steam turbine power or "work" estimating


My statement regarding the moisture separator was not for the steam turbine.  The moisture separator would be needed to condition the process steam.

You don't want wet steam traveling into the plant piping system with the potential for creating severe water hammer.

As a minimum, you would need to add low point condensate steam traps capable of removing about 5.5 gpm.

RE: Steam turbine power or "work" estimating

In such a situation with significant uncertainty about what output can be achieved I would not base any economic evaluation on figures determined in the manner shown above.

I would involve the salemen of a number of potential turbine suppliers and discuss what options are available in practice and what the potential costs and output will be.



RE: Steam turbine power or "work" estimating

EdStainless, that is a thought we are entertaining but could require more infrastructure (we currently have no steam turbines...all electric drivers, small plant, 20,000BPD).  The 650# steam was basically a side effect of having to put in a cogen unit because our electrical utility was not reliable. The best use we had for the excess steam at the time was to use it in process heating coils.  

The steam turbine on this letdown seems to roll it all into one neat package, but your point on lost efficiency is well taken.  Thanks.

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