I need all of your help to understand what is Boiler Follow mode, Turbine follow mode and Co-ordinated control used in Thermal Power Plants and what is the difference between these three?
If anyone can explain with some example that will be great.
Which mode normally used and why ?
As far as I know in Boiler Follow Mode, Boiler is the master control and it controls the pressure and load demand is given to the Turbine Governer. In Turbine follow mode Turbine is the Master Control and controls the pressure where as load demand is given to the boiler. I am unable to understand this clearly. So If anyone can explain this a bit with example that will be appreciated.
Boiler Follow Mode: The boiler is divorced from the generation control, which means the steam turbine utilizes stored energy in the boiler to provide immediate load response. The boiler must then change firing rate to bring pressure back to setpoint.
Turbine Follow Mode: Turbine control valves maintain a set pressure while the boiler fires to maintain load. Drawback here is a slower generation response. There are variations with this scheme, in that the turbine control valves can be fully opened at higher loads to minimize the energy penalty associated with the DP loss across them. In that case, it has been called sliding-pressure control, or even cascade control.
Coordinated Control: In general, you provide various logic schemes to move the steam turbine valves for quick load response, as well as fire the boiler for the anticipated energy requirements of the boiler (generally via an energy balance equation).
Names being what they are, there are variations to all types of control, so you primarily need to focus on the type of control in question and go with whatever it is being called.
Could you please explain a bit about the constant/fixed pressure and sliding pressure operation of the boiler?
What I understand about sliding pressure control is in sliding pressure control the pressure inside the turbine is not fixed and it varies as per the process. It means the pressure at the downstream of the turbine control valve is always varies as per the steam pressure in the main steam line. So whatever is pressure of the steam in the main steam line, the same pressure will also be there in the turbine. If that is the case how sliding pressure control can be a part of Turbine Follow mode?
In turbine follow mode the steam turbine control valve controls the pressure. So as per the set point given to the turbine governor they maintain the pressure. In that process they need to open or close to keep the pressure constant. So I am little bit confused how sliding pressure control is a part of Turbine follow mode. In fact I believe Boiler follow mode looks like to be the sliding pressure control.
Please clarify this a bit .
Good explanation sir. but I hope you can give some more clarity on the individual modes by mentioning its use in different cases and also I want the control system block diagram model (including the PID controllers and curve look-up tables, where they are used) for implementing all the three modes.
The Turbine follow mode (Constant Pressure Control Mode):
As name indicates, in turbine follow mode the boiler and its auxiliary maintains its parameter to maintain set point given to turbine, which may be in term of load reference or Turbine inlet pressure reference. Thus the turbine output (MW) or Turbine inlet pressure remains almost constant i.e. the turbine governing valve opens-closes to regulate turbine inlet pressure.
Boiler Follow mode (Sliding Pressure Control Mode):
In boiler follow mode, the boiler parameters are set as reference and output of boiler is used to produce power from turbine. Generally the boiler operates near maximum operating capacity, thus inlet pressure at the turbine varies as at boiler outlet. Thus the turbine output will vary as per the boiler parameters i.e. the turbine inlet pressure will vary(slides) as per boiler outlet pressure and mostly turbine governing valve remain full open.
It is the combination of both of above to achieve maximum output of plant using maximum efficiency of all equipment at all running condition.
Just for my query: I think you are working with Alsthom/Ansaldo Power Plant.
turbine master (turbine follow mode)
boiler master (boiler follow mode)
if you put both turbine and boiler master in auto then it is called Coordinate control.
if you put turbine master only in auto then it will see the main steam pressure and accordingly it will throttle the turbine control valve (it will not bother load variation)and it will try to maintain the main steam pressure.--in this condition let us consider you are not varying coal feeding in boiler.
if you put boiler master only in auto then it will see the main steam pressure and accordingly it will vary the coal feeding and try to maintain the main steam pressure.--in this condition let us consider you are not varying turbine control valve position.
so if you put turbine master only in auto, it will try to maintain MS pressure. if you put boiler master only in auto, it also will try to maintain MS pressure.
Now if you put both turbine and boiler master in auto then it is called coordinate control. Once again boiler master will see MS pressure with respect to operator pressure set point and it will vary the coal feeding accordingly, but turbine master will see load with respect to operator load set point and it will throttle turbine control valve.
so in coordinate mode turbine master will see load and throttle the turbine control valve, but if turbine master only in auto (boiler master in manual) it will see MS pressure and throttle the turbine control valve.
Can you please explain how to get sliding pressure or a constant pressure at boiler outlet??
BFP is the pressure source for boiler, we may ask the supplier to give A pump to give constant pressure output at all loads, and as our flow through boiler varies as per load, so varies our Differential pressure so ultimately we will get Different pressure (not constant pressure) at Boiler outlet.
because of the above am not able to understand how to get sliding pressure or constant pressure.
From your response it is not clear whether you have plant that is already operational and you are modifying controls, or whether you are still at design stage.
In any case, if you are opting for sliding pressure, you have to be careful of steam velocities within the boiler tubes, when your operating pressure is below nominal. At lower pressures steam volumes increase, and so velocities increase, leading to increased erosion within the pipes. If your boiler is already operational and designed for fixed pressure, you need to check with the boiler manufacturer what is the lowest pressure the boiler can be operated at, before erosion due to increased velocities becomes unacceptable.
One other factor to take into consideration is your feedwater valve. You need to check what is the maximum allowable pressure difference across this valve, when you decrease the boiler pressure. The BFP will maintain the same discharge pressure (again unless you are using a mix of feedwater valve and variable speed BFP).
Further to your comments, the simplest form of sliding pressure operation (albeit with limitations) is to open the steam turbine governor valves fully open, and let the boiler/steam turbine pressure settle naturally to a pressure point. Then when you need to change ST output just vary the boiler firing rate.
The following two part article is very well done.
Boiler-Tuning Basics, Part I
Boiler-Tuning Basics, Part II
TURBINE FOLLOW WITH BOILER MASTER ON AUTO:
The BID is derived by comparing the MW Demand and the actual MW and feeding the error signal to a PI controller, where the MWD is also fed as a Feed Forward signal to decrease the controller delay. Similarly the MSPD is derived from the MW demand (through a preset curve) and is sent to EHG, where it is compared with the actual Main Steam pressure to decide the Control Valve position.
BOILER FOLLOW WITH TURBINE ON REMOTE:
The MSPD is derived from the above MW demand signal and is compared with actual boiler outlet pressure. The error signal is fed to A PI controller where the steam flow also acts as a feed forward signal. The output of this controller, which is nothing but the BID, thus increases/decreases as soon as the MW demand is increased/decreased, thus increasing/decreasing the steam flow.
Similar to Boiler follow with Turbine remote, BID is derived from the error signal of MSPD (derived from the MW demand) and the actual main steam pressure. the only difference being the MW Demand acting as a feed forward signal instead of the steam flow. Thus the action taken by this PI controller is instantaneous, without waiting till the variation of steam flow.
Thank you very much for your links.. The explanation is excellent in that article.. Very good concept got trapped in words..
i really wanna understand the PID for boiler master and CMC and its correction factors. that is load correction and pressure correction?
BFM :- boiler follow mode. it means Boiler Follow Turbine. Boiler Control Pressure and Turbine Control Load.Pressure Controlling By either Fuel Master and Air Master.
example:- when u need constant load with Grid then this mode is normally used.
TFM or TFB:- Turbine Follow Mode or Turbine Follow Boiler... In this Mode Turbine Control Pressure and Boiler Control Load. if u need more load then automatically Boiler master increases feeding and acordingly fuel master and air master control...
example:- when u need variable load dispatching with frequency then this mode used ...
But generally BFT mode used
CCS:- Co-ordinates Master control...
in this mode... Boiler master in Auto, Turbine Master in Auto. Both TDBFP CCS in..
hope this will be little bit helpful to you....
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