09-11-2012, 02:31 PM
COMPUTER CONTROL OF BOILER OPERATION
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INTRODUCTION
With the cost of energy r1s1ng so rapidly,
more intense scrutiny is given to the purchase of
new boiler control systems, as well as to the replacement
of old and outdated boiler controls. More
advanced boiler controls are capable of maximizing
the efficiency of a boiler steam production process,
and thereby minimize the use of the purchased fuels.
The elaborate systems which were previously difficult
to justify when the cost of fuel was low,
are now the common desire of most utility managers.
Boiler controls have progressed from the
mechanical jack-shaft connections, to the pneumatic
and the electronic analog, and finally to the more
sophisticated and versatile digital systems. However,
the increased capabilities can generally lead
to fuel savings which can recoup the capital investment
in two of three years, and at times even
considerably less. Of course, the most sophisticated
controls cannot be justified for many boiler
controls so some compromises must be made between
capability and cost.
PASADENA BOILER CONTROL SYSTEM
The Diogenes digital control system has been
in operation on the Pasadena 500 psig and 200 psig
boilers for over 1 year. During this time overall
boiler efficiency has increased 11%. See Figure 1
and Table 1. The project also included new combustion
burners, monitoring devices for all key
variables and modifications to the boiler feed
water system. The original system covered the 500
and 200 psig boilers, since the installation of a
new 500 psig boiler has been added as well as the
steam distribution system. Now the entire pressure
let-down system is controlled from one location.
The reliability and response of the steam plant have
been greatly enhanced.
PREVIOUS CONTROL SYSTEM
The previous control system was the basic
analog/mechanical package provided by boiler manufacturers.
Combustion was controlled with a single
master controller. This controller positioned the
air dampers and fuel valves through a mechanical
linkage system. The primary source of set point
was drum pressure with no feedback or feed forward
control.
PRESSURE CONTROL
The pressure controller will control to a
setpoint through its proportional and integral functions,
and through the use of a dynamic steam flow
feedforward signal from the steam flow lead channel.
The relationship between the static and dynamic
portions of the feedforward can be determined by
selection of values in the steam flow lead channel.
When the steam pressure control is not controlling
the fuel flow, its output is forced to track
the actual amount of fuel flow, so that it is prepared
for a bumpless transfer to automatic control.
DRUM LEVEL CONTROL
The drum level control incorporates a three
element control schem~. There is proportional and
integral controller which will control feedwater
flow to the value demanded. The demand for feedwater
flow comes from the drum level controller
which has the lagged value of steam as a feed forward
on its output. This three element control will
achieve a high degree of stability on the drum
level. The setpoint for drum level control is
entered by the operator through the control console.
OPERATOR INTERFACE
Logic capabilities inherent to Diogenes proves
quite valuable in combustion control. At Pasadena,
logic is used primarily to keep the operation safe
or safeguard against human error in judgement. EX:
Control loops cannot be switched from auto digital
to manual or vice versa in the wrong sequence. For
instance, if air flow is switched to manual, gas
flow will automatically be forced to manual. Startup
selection of auto/man is organized in a logical
way. Selection is made sequential from left to
right as located in control console. Any hazardous
condition that could occur is protected by logic
to cause the control system to fire down to minimum
fire and hold till fault is corrected or to
trip the boiler.
The ramp generator employed in the start-up
sequence performs a quite useful function. The
operator has an option to use this when needed or
by-pass the function for faster start-up. For a
cold start-up, after a lengthy shutdown the ramp
generator will assure that refactory temperature
limits are not exceeded on a time basis. This
means that the boiler can be started up safely
without requiring operator intervention and constant
monitoring.