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rom the Power plant operations department...

Turbine control

What is the difference between constant pressure mode and sliding pressure mode operation of a steam and can you give theuitability for each one?

There are people here (CSA and others) who will probably give better explanation, but here it goes.Operation mode of steam turbine depends on the control concept that is implemented on steam power plant. Unit's power isgenerally controlled by manipulating of steam flow through turbine. Turbine's steam flow depends on steam pressure and sizeof input area through which steam enters turbine. This area is manipulated (control) by position of turbine's control valves.Therefore, there are two operational modes (in general):

- mode with constant pressure in front of turbine where turbine's pressure is controlled by changing control valves position and- mode with sliding pressure in front of turbine, where control valves are maintained at some constant position (for example 80

or 100% opened). In such case steam pressure is controlled by boiler control loops (by manipulating coal feeders).

Very closely related to this operational modes are so called "control concepts" (maybe this is not 100% correct Englishtechincal term, but I cannot find better right now).Concept of "boiler leading" means that steam pressure in front of turbine is controlled by control valves (constant) whilepower output setpoint is associated with boiler controller that manipulate coal firing.

On the other side, there is "turbine leading" concept in which, power output setpoint is associated by control valves andtherefore every demand in power is immediately followed by changing of control valves position. Then, steam pressure iscontrolled by boiler.Please not that this concept is not fully "sliding mode" because, control valve position is not constant, but this concept isusually called sliding pressure mode, because steam pressure is changing and boiler pressure controller needs to adapt to these

changes.

As I said, I hope CSA will jump in and clearify this a little bit, but I hope you how more complete picture now.

Regards

My recent experience with steam turbine controls is limited to being exposed to them during combined cycle plantconstruction and commissioning, so I'm not able to offer very much in the way of help with this topic. I am hoping jojo or

someone else will jump in here and provide some more information.

It's my understanding that when a steam turbine is operated in sliding pressure control its control valve(s) are moved to the fullopen position and the power output of the turbine-generator is a function of the steam pressure and temperature which isavailable from the boiler. I think this is done for plants which don't run at "base load" (rated output) to increase steam turbineefficiency slightly by eliminating any drop in temperature of the steam which would occur when it passed through a valvewhich is "throttled" (partially open).

I believe constant pressure control for combined cycle plants refers to when the steam turbine control valve(s) is(are) throttledto control the pressure of the steam at the steam turbine inlet. If the firing rate of the boiler (in a combined cycle power plant,this is a function of the gas turbine exhaust temperature and flow) is held constant, the steam pressure can be controlled byopening and closing the steam turbine control valve(s); of course, the power output of the steam turbine varies as the control

valve(s) is(are) varied. Or, if the firing rate of the boilers varies (such as when gas turbine load is varied), the steam turbinecontrol valve(s) can be used to control the pressure at the steam turbine inlet. I think this mode is used when the steam is alsoused in another process and it's pressure needs to be fairly constant, but the gas turbines are run at various loads which meanstheir exhaust temperature and flow is varying. Again, when the steam turbine control valves are throttled, the power output ofthe steam turbine varies.

I'm sure my understanding is very limited, and may not even be entirely correct. I'm also not sure if the terms are used

Posted by prabhakar on 17 August, 2008 - 12:50 pm

Posted by Mikas on 17 August, 2008 - 10:41 pm

Posted by CSA on 19 August, 2008 - 1:14 am

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similarly in conventional steam plants (such as natural gas- and coal-fired plants) and combined cycle plants (where gasturbine exhaust is used to generate steam). Something else I have come to understand about combined cycle plants is that thedesigners of them have all manner of slight "tweaks" (changes; improvements; modifications) to the plant design to try toimprove efficiency and that sometimes these tweaks have names which don't really seem to be descriptive or even entirelycorrect in the strict interpretation of the meaning of the words used to name them. So, it's possible that my understanding maybe slightly off because of my exposure to one of these plants which used a term which was not entirely descriptive of themodified process.

I don't have a copy of the book, but the Babcock & Wilcox "Steam: Its Generation and Use" engineering text/reference manualis one of the best on the subject of steam generation and boiler cycles and steam plant design. It's pretty common in many

libraries (though not always the latest version), and it's available on the 'Net via the following link:

http://www.babcock.com/library/pdf/steambook.pdf

It should be in the "library" of every fossil fuel-fired steam-producing plant in the world.

Google and Dogpile (http://www.dogpile.com) are two good Internet search utilities which might yield some good results ifyou're patient and can try various word groupings (which is what most people don't do when using search engines for technicalmaterial; one needs to think of as many permutations of titles and functions to try, and even search some of the lesser-relevantsites to find even more possible search entries).

There are also many good engineering handbooks which might have more and better descriptions. http://www.alibris.com has

been recommended here on control.com before; one can find new and used copies of books anywhere in the world on this site.

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