I am working with an EPC company who builds power plants in India. We are building a combined cycle power plant.
In this regard, I want to know in detail about the sliding pressure control of the steam turbine or is there any useful web site that can provide me this information?
There is limited information on the net - MHI have a few things and so do fisher control valves (both from dogpile search engine). If you find any other information I wouldn't mind a copy as I'm looking into the feasibility of sliding pressure at a fix pressure station.
If you need information on sliding pressure control, you should contact your Fisher representative directly, they have a book called power primer. This is a red cover book and is very good in explaning some of the controls in Power Plants. There is a whole chapter on sliding pressure control.
Read this book, if you have questions you can call me, I teach the furnace and boiler course for Emerson (Emerson owns Fisher Controls)
Ben Janvier, principal process control consultant
Emerson Process Management
> If you need information on sliding pressure control, you should contact your Fisher representative directly, they have a book called power primer. This is a red cover book and is very good in explaning some of the controls in Power Plants. There is a whole chapter on sliding pressure control. <
I read that book, however there is still a question I don't understand. That is, in term of sliding pressure operation and control, what is the difference between subcritical and supercritical steam generation unit?
I just finished a 3x3x1 sliding pressure STG plant. To summarize the entire philosophy in a nutshell:
The STG inlet control valves simply run wide-open (100%) at all steady-state conditions. This means the inlet steam pressure(s) "float", or end up being whatever they'll be (ie. a function of inlet steam flow/pressure/temp).
In my 3x3x1 plant, that means the HP steam pressure varies from 40 BARG (in 1x1x1 operation) to 120 BARG (in 3x3x1 operation). How do you get the STG main steam valves to stay 100% when they're running in pressure control? You simply provide the STG control system with an external pressure set-point that is always lower than the actual header pressure (say 5%). That's it. (NOTE: Just rate-limit the set-point changes so any sudden changes in steam pressure will be taken care of by the STG.)
Feel free to contact me for any control philosophy/imlementation details at firstname.lastname@example.org
> The STG inlet control valves simply run wide-open (100%) at all steady-state conditions. This means the inlet steam pressure(s) "float", or end up being whatever they'll be (ie. a function of inlet steam flow/pressure/temp).
This is a bit of a generalization. Although the control system and system descriptions refer to sliding pressure operation as Valves Wide Open (VWO) they are not technically pressed against the bump stops. The valves still modulate slightly to maintain turbine speed. If the valves were against the seats, any increase in load would slow down the turbine, and the valves would be incapable of maintaining speed until the steam flow increased.
Second, a sliding pressure steam turbine, in any installation, has a point in its part load operation where it automatically transitions from sliding pressure control into inlet pressure control. A general rule of thumb is that this occurs at approx. 40%-60% of the design throttle pressure. This is mandatory to prevent instabilities that arise at low loads from turbine blade stalls and boiler swell and surge.
Steam turbines designed for sliding pressure operation typically have two main steam stop and control valves. They do not have the typical steam chest for partial arc admission, nor Curtis/Rateau type control stages.
The control of system output is directly related to the amount of firing control provided. In a combined cycle, this control is provided by CTG load and duct burner firing heat input. The result is that output is held fairly constant at the expense of fluctuating pressure and flow. Most plant control systems have high enough dead bands on the indication of pressure and flow, that the fluctuations go unnoticed.