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Steam Turbine Efficiency

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#1 daraj

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Posted 09 December 2021 - 02:33 AM

Hi,

 

does anyone have links/articles or can help with regard to steam turbines which are used to drive syngas or other turbines in the chemical industry and what is their accepted maximum overall  efficiency ? 

 

Lets us say I needto drive a 15-20 MW compressor using HP driven steam turbine. For conceptual design/heat balance purposes, can i assume overall efficiency is 80%, or is too high?



#2 latexman

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Posted 09 December 2021 - 08:41 AM

Too high.  At that size, for condensing, I'd use 72%, and for non-condensing, 76%.

 

But the devil is in the details.  It depends on what you mean by HP steam, superheat, the speed, and pressure ratio.

 

Steam Turbine Efficiency



#3 Bobby Strain

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Posted 09 December 2021 - 10:10 AM

Check the manufacturer's site. Like GE.

 

Bobby



#4 breizh

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Posted 10 December 2021 - 12:42 AM

Hi ,

You may find pointers in the link underneath

https://www.energy.g...eam Turbine.pdf

 

Good luck

Breizh



#5 daraj

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Posted 10 December 2021 - 12:44 AM

Thanks



#6 daraj

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Posted 10 December 2021 - 12:52 AM

breizh, iam using 100bar superheated(5-7C) steam as inlet and expanding down to 50bar where i extract some steam and then remaining steam is condensed down all the way. I think 70-75% should be reasonable for this system. Doing a high level conceptual study.

 

also from CAPEX point of view do you think condensing is higher than backpressure because it requires surface condensers etc to completely condense the steam?



#7 breizh

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Posted 10 December 2021 - 05:24 AM

Hi,

Your overall efficiency within the range 70 to 75 % seems reasonable to me .

Underneath you will find a calculator with examples to simulate your steam turbine and other system .

https://www4.eere.en...amo_steam_tool/

 

For Capex , consider the link underneath 

https://www.cheresou...e-for-turbines/

 

Note : about the cost don't forget to add for the condenser the cost associated for cooling tower and pump .

 

Good luck

Breizh 



#8 latexman

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Posted 10 December 2021 - 12:21 PM

The old, quick method I linked to above suggests about 70%.  You have to extrapolate some curves, interpolate others, and we don't know the RPM yet, but that's what I'm seeing.  OP's low superheat and high pressure ratio reduce the efficiency of the base method's assumptions.

 

Take a look, it's easy.  Another person eyeballing those charts and crunching the numbers, would be welcome.

 

Here's how I saw it.  I looked at both condensing and non-condensing, because OP's application does not sound exactly like the traditional condensing turbine.:

 

Condensing Turbine Efficiency = A x C x E = 0.71 x 0.975  x 1? = 0.69

 

Non-condensing Turbine Efficiency = B x C x D x E = 0.76 x 0.975 x 0.955 x 1? = 0.71

 

That might be a max. of 70%.  The speed correction has more downside, than upside!



#9 breizh

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Posted 10 December 2021 - 11:29 PM

HI ,

To add to Latexman's  figures a graph  from "Rules of thumb for chemical engineers" by Carl Branan .

 

Note :

More information available in GPSA (Gas processors supplier association) engineering data book , chapter 15 

 

Good also to take a look at Perry's chemical engineers' handbook.

 

Hope this is helping you.

Breizh



#10 daraj

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Posted 11 December 2021 - 08:01 AM

Breizh and Latexman, thanks. is a final pressure after expansion of about 1.2 bar(a) fine for a turbine like this? or is it usually sub-atmospheric?



#11 Bobby Strain

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Posted 12 December 2021 - 10:16 AM

If you don't have use for the exhaust steam, a condensing turbine at about 2 psia is economical. However, the inlet steam conditions must be appropriate for the exhaust condition. Seems like your interest is broader than you initial query.

 

Bobby



#12 daraj

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Posted 12 December 2021 - 11:30 PM

Thanks Bobby. In a simulator when iam doing a quick design, it warns me after expansion saying that dew point is reached or that there is

detection of liquid phase etc. I am not sure if this is issue with simulation. when you expand superheated HP steam down to 0.2 bara do we expect any liquid phase or water formation anywhere in the intermediate steps? i wanted to know if a KO drum is needed to knock out any water. I am extracting some steam at a intermediate pressure for process use. So it is a condensing cum extraction turbine 



#13 PingPong

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Posted 13 December 2021 - 04:38 AM

 i am using 100bar superheated(5-7C) steam as inlet .......................

Is that (5-7C) a typo, or does that really mean that the inlet steam has only 5 to 7 oC superheat?

 

You can't operate a steam turbine with 100 bar steam having only 7 degrees superheat as that would ruin the turbine due to enormous condensation inside the turbine. You will need more than 100 degrees superheat at the inlet, not a miserly 5 - 7 degrees.

 

Typically condensation shall not exceed 10 wt% at turbine exhaust. Check with vendor for maximum allowable condensation in your particular turbine.

 

Use a steam enthalpy-entropy diagram (h-s diagram, Mollier chart) instead of software so that you better understand what is going on in your turbine.



#14 daraj

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Posted 13 December 2021 - 11:38 PM

Pingpong, whne you say it does not exceed 10 wt% at turbine exhaust, the remaining 90% is condensed where? in the surface condensers?



#15 PingPong

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Posted 14 December 2021 - 02:45 AM

Yes.

 

The condenser turns all steam into condensate, thereby creating a very low turbine outlet pressure, e.g. the 2 psia mentioned by Bobby. The exact pressure achievable depends on the temperature of the cold utility available for use in the condenser.

The lower the turbine outlet pressure the higher the power produced per ton of steam. Check with h-s diagram.

 

In my experience HP steam, like your 100 bar steam, is typically superheated to 500 oC for driving a turbine. The higher the superheat the higher the efficiency of the Rankine cycle of which the turbine is part.



#16 daraj

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Posted 14 December 2021 - 11:11 PM

Thanks Pingpong



#17 PingPong

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Posted 16 December 2021 - 11:52 AM

Pingpong, for energy/ower balance, often we need to know efficiency values for steam turbine(that drive compressors) aswell as efficiency of compressors themselves(such as syngas compressor). do you know if 70% for steam turbine and 75% for compressor overall is conservative enough? vendors claim higher efficiencies(like 80% for compressors)but i have heard that those numbers are not real. My duty values are within 20MW for these equipment.

Efficiency of centrifugal compressors and steam turbines depend very much on the actual volumetric flowrate through them. The higher the volumetric flowrate, the higher the efficiency.

 

Note also that there is a difference between polytropic efficiency and adiabatic (isentropic) efficiency for centrifugal compressors. Some sources quote polytropic, others quote adiabatic, and others just quote efficiency without indicating which type is meant.

 

Moreover there are mechanical losses to be taken into account in calculating compressor or turbine powers, so if a compressor vendor quotes 80 % polytropic efficiency then it will actually be several percent points less after taking all other losses into account.

 

To answer your question, I would say that a polytropic efficiency of 75 % is conservative enough for a 20 MW centrifugal compressor,

and a 70 % isentropic efficiency is conservative enough for a 20 MW steam turbine.

 

 

Attached are some centrifugal compressor data from old Elliott brochures.

Note impact of intake volume on polytropic efficiency. Cfm = cubic feet per minute.

 

Attached File  Elliot compressor efficiency.jpg   192.06KB   1 downloads

 



#18 daraj

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Posted 17 December 2021 - 08:03 AM

Thanks Pingpong, this will be useful



#19 latexman

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Posted 19 December 2021 - 07:19 AM

You can't operate a steam turbine with 100 bar steam having only 7 degrees superheat as that would ruin the turbine due to enormous condensation inside the turbine. You will need more than 100 degrees superheat at the inlet, not a miserly 5 - 7 degrees.

 

Excellent point!

 

The basis to the steam turbine efficiency method I gave a link to above is:

  1. 100o F of superheat, no matter the pressure
  2. Pout/Pin = 0.25
  3. 3600 RPM turbine speed

Based on the correction factor curves given for superheat, pressure ratio, and turbine speed.






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