The learning objectives of this section are to be acquainted with the following
boiler control systems combustion control burner management trip system
level control
the main objectives of the combustion control system are to supply fuel oil
according to the boiler load to control the combustion air supply according to
the fuel oil consumption to shut off the burners in case of an emergency a
cascade control system is used to obtain the best possible stability the system
is comprised of several controllers a master controller receives the steam
pressure and a steam flow signal the output signal from the master is the
input signal to the air controller and the fuel oil controller called the slave
controllers the control philosophy is that the slave controller has to take
care of the variations in the master controller output signal before this
effects the master controllers input signal the master input signal or the
process variable is the value that has to be constant in the first place to
maintain the best possible combustion during change of boiler load we need a
high-low selector which also receives a signal from the oxygen controller the
high-low selector makes it possible to increase the air flow before increase in
fuel oil flow when we increase spoil a load as we are reducing boiler load we
want to reduce the oil flow before we reduce the airflow by adding the signal
from the oxygen analyzer we can optimize the a oil ratio further this to
compensate for variations in fewer oil quality
airflow due to soot and reduced air blower capacity tuning of a cascade
control system is usually done the following way
set the slave controller in manual during normal operation tune the control
according to specifications in the operating manual when the master
controller input process variable is correct
adjust the slave controllers setpoint to the slave controller input signal when
this is completed the slave controller can be switched back to auto the master
controller must be in manual during firing of the boiler if it is necessary
to tune the master controller this must be done with the slave controller in
auto or remote setpoint mode carefully follow the instructions in the manual as
each burner has a limited capacity with an optimum range
we usually have several burners on each boiler to achieve the best possible
combustion and a various load conditions the burner management system will fire
and shut off the burners in sequence depending on the boiler load one burner
is selected as the master and the others are slaves a set steam pressure
deviation will activate the system the low and high pressure limits must
reflect the pressure control system settings
to avoid hazards to people and possible damage to the boiler a shutoff or
tripped system is always installed input to the trip system can be the following
signals flame failure combustion air failure air preheater failure high high
water level low low water level high high steam pressure the mergency stop
activated
all the signals are wired in a separate system and are not an integrated part of
the control system the trip system requires separate pressure and level
switches to maintain the correct water level during all normal operating
conditions we need an automatic level control system on boilers with a low
steam production and a high water content the level control system only
measures the water level and controls the feed water flow by a control valve
on boilers with a high esteem production and low water content the level control
system is usually based on signals from the steam consumption and the feed water
supply in addition to the water level a sudden increase in steam consumption
results in a pressure drop in the steam water drum and the water level increases
and vice versa this is why we need to compare steam consumption with the feed
water supply before we introduce major changes the feed water supply