BASIC PROCESS CONTROL

In a simple control system, a process variable (PV) is measured and compared with a setpoint value
(SP). A manipulated variable (MV, or output) signal is generated by the controller and sent to a final
control element, which then influences the process variable to achieve stable control. The algorithm
by which the controller develops its output signal is typically PID (Proportional-Integral-Derivative),
but other algorithms may be used as well:

This form of simple control may be improved upon and expanded for a greater range of process
applications by interconnecting multiple controllers and/or redirecting measurement and control
signals in more complex arrangements. An exploration of some of the more common control system configurations is the subject of this chapter.



Lessons In Industrial Instrumentation
By Tony R. Kuphaldt
Version 1.13 – Last update January 6, 2011

Coal-Fired Power Plant
Coal-fired units produce electricity by burning coal in a boiler to heat water to produce steam. The steam, at tremendous pressure, flows into a turbine, which spins a generator to produce electricity. The steam is cooled, condensed back into water, and returned to the boiler to start the process over.

Coal Fired Power Plant Concept

Coal Fired Power Plant Animation 

From the concept of Coal Fired Unit, the Boiler is very importance. So, we should be know                the Basic of Boiler.


Basic Boiler 
A boiler is comprised of two basic system. One system is the steam water  also called the waterside of the boiler. In the waterside, water is introduced and heated by transference through the water tube, converted to steam, and leaves the system as steam.


Boiler must maintain a chemical balance. The manner in which this done can interact with the feedwater control system. The amount of blowdown must be considered in the feed water control scheme, especially if the blowdown is continuous. Often, the blowdown flow is divided by the concentration ratio time the feedwater flow. Continuous blowdown is the common method for controlling the chemical concentration . On large boiler this may be done automatically by measuring the boiler conductivity to control the blow down rate. the blowdown rate may also be achieved by combining the conductivity with ratio control of blowdown, ratioing blowdown to feedwater flow. In unity plant, conductivity is usually measured and blowdown is achieved manually. This required on a periodic basis or when the conductivity gets too high.


Conductivity is measured in micro mhos which is equal the reciprocal of 1 mechanical ohm(resistance). The othe boiler system is the fuel air-flue gas system, also referred to as the fireside of the boiler. This system provide the heat that is transferred to the water. The inputs to this system are the fule and air required to burn the fuel. The fule and air chamber is also referred to as the windbox. The outputs are the flue gas and ash.     

Ref: TENNESSEE VALLEY AUTHORITY
       Boiler Control SYStem, g.f.(jerry) gilman