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.

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.

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