Steam Turbine Problem 2

[edebs71]

Hi

We have a HRSG system that uses the hot exhaust gas of a gas turbine to generate steam. This steam will be used to drive a steam turbine to generate ectectricity.

I know: the turbine inlet mass flow rate, Temperature, and pressure of steam.

How do I know the outlet pressure from the turbine and which turbine to use.

I'm still new in this so any Help will be appreciated.

 

[poetix99]

You would do very well to get some basic background on: Steam Turbines
Combined Cycle systems
Power Plant engineering
Closed cooling water systems

You, as the user, specify the exhaust pressure of the steam turbine. However, this is not arbitrary; it depends upon what heat sink is available for the turbine exhaust.

The vast majority of steam turbines in a "combined cycle system" are condensing turbines. This allows maximum practical power generation, by maximizing the amount of steam energy that is released within the turbine as it expands from relatively high pressure (usually 600 psig (40 barG, or higher), to a rather low pressure (2 psia (0.14 barA) or lower).

Maintaining a pressure that is lower than the surrounding atmospheric pressure requires a SURFACE CONDENSER. Condensers are heat exchangers, and require high flow rates of cooling water to maintain the vacuum at the turbine exhaust. Cooling water is typically a closed-loop with heat rejection to the environment via cooling towers, etc. The exhaust pressure that is possible depends upon the cooling tower capacity and the cooling water temperature.

If you want to size the cooling system to accommodate a certain turbine exhaust pressure, you can easily estimate the turbine exhaust enthalpy for an assumed efficiency (search the web for simple little estimator downloads, or ask a turbine manufacturer for a budgetary quote with some basic thermodynamic info). Then, a condenser manufacturer could size the condenser, and tell you the cooling water flow for your specified water temperature. The cooling water temperature is limited by the temperature of the heat sink to which you are ultimately rejecting heat (atmosphere, river water, sea water, cooling pond, etc)

The whole process to cost effectively choose turbine/condenser/cooling-system can be iterative.

You say that you have a GT and the HRSG already. It is not clear to me how these items could exist (especially the HRSG) without someone having taken some consideration of the cooling system, condenser and steam turbine.

*****

If there is an additional "downstream" use of all of the steam turbine exhaust steam, such as district heating, or in-plant processes, you might want a "back-pressure" steam turbine. This is not typical of combined cycle plants for power generation, but you didn't say whether the GT was in a process plant, or a power plant.

*******

With all due respect; there is nothing wrong with you learning about combined cycle power plants; we all must somehow, sometime, but I hope for your sake that there is someone among you who is competent to assist and supervise you in your learning process. This web-site, as good as it is, must not be your sole source of information.

 

[edebs71]

poetix99,

Thank you for the info.

 

[arto]

Lotsa good basic info here:

http://www.gepower.com/prod_serv/products/tech_docs/en/all_gers.htm

 

[edebs71]

Thank a lot "arto"

Excellent Site.

Cheers

 

[rmw]

Do a search on this site, on all the pertinent buzz words, HRSG, Combined cycle, gas turbine, etc., because there are excellent links for HRSG info in some of the threads.

One that comes to mind is:

http://www.hrsgdesign.com./design0.htm

rmw

 

[toothless]

Using the exhaust steam in a downstream process is the most efficient method and is referred to as "cogeneration". This way the heat value of the steam is used for a useful purpose and not lost in the cooling tower as is with a condensing turbine. Do you have a "customer" for your steam, and will they return hot condensate back to you?

 

[edebs71]

Thanks rmw for this website.

toothless,

The steam generated from the HRSG will be used for process and to run a steam turbine to generate electricity.
The condensing steam from the steam turbine will be condensed and than pumped to the HRSG.

Cheers