2. Scope of Presentation.
Definition and explanation of Boilers, its common equipment and a control schemes
used in boiler control operations.
Explanation of the principles of the combustion process.
Evaluation of types of control schemes.
Explanation of shrink/well phenomenon and its effects on boiler level control.
Design and evaluation of boiler level control schemes.
3. Boiler.
Boilers are systems used
to heat a fluid (usually
water) in a closed vessel.
It can be boiled, heated,
or vaporized. You can
then use the outcome for
various purposes or
heating applications.
4. Components of a boiler.
Burner- this is where air mixes with the fuel source and combusts.
Combustion Chamber- this is where fuel is burned to heat the water
Heat exchanger- this transfers the heat produced by the burners within the combustion
chamber.
Controls- allows the used to set water temperature, air and fuel supply mixture, internal
pressure and ignition
Supply and return line- supplied line lead from the boiler and deliver the heated water
or steam to distribution points, return lines bring the water back to the boiler
Circulator pump-the component that pushes the hot water out through the supply lines
to the distribution lines.
Exhaust Stack- is designed to safely expel spent fuel away from the building’s exterior.
5. Combustion.
Fire is the visible effect of the process
of combustion – a special type
of chemical reaction. It occurs
between oxygen in the air and some sort
of fuel. The products from the chemical
reaction are completely different from the
starting material.
Combustion is when fuel reacts with
oxygen to release heat energy.
6. Types of combustion schemes.
The three basic types of combustion control schemes are:
Single-point positioning.
Parallel positioning.
Metering.
7. Single- Point positioning.
This is also a single PID loop. The main
jackshaft is modulated based upon the
firing-rate demand. This, in turn, moves
the air and fuel linkage arms, opening and
closing the air damper and fuel control
valve.
8. Parallel Positioning Combustion
Control.
Like single-point
positioning combustion
control, this is also a
single PID loop. Parallel
positioning controls help
a burner to optimize its
fuel-to-air ratio by using
dedicated motorized
actuators for the fuel and
air valves.
9. Metering Combustion Control.
The air and fuel devices
are driven by individual
actuators and now air and
fuel flow meters are
added. Metering
combustion control is
disclosed in which both
the fuel flow rate and the
combustion air flow rate
are metered in a desired
ratio corresponding to a
master firing rate
demand.
10. • Comparison of the three.
A single-point positioning scheme is the least expensive and simplest system available
while a metering scheme is the most costly and complex. It also is the most efficient
option.
A parallel-positioning scheme falls right in the middle of the two regarding cost and
complexity.
A metering scheme is the best system for boiler efficiency.
11. SWELL AND SHRINK.
Steam flow from the boiler is controlled by the downstream process. A sudden increase
or decrease in steam flow changes the pressure in the steam drum and boiler circuit.
The change in pressure will cause a change in both the boiling point and density of the
water and steam. These combined reactions will cause the level in the steam drum to
increase or decrease rapidly. The increase and decrease in the water level caused by the
pressure change are commonly referred to as the swell and shrink reactions.
12. Drum level control systems.
The three basic types of
drum level control systems
are:
Single element.
Two element.
Three element.
13. Single element.
This consists of a proportional signal
coming from the drum level transmitter.
This signal is compared to a setpoint, and
the difference is a deviation value.
14. Two element.
• Consist of two elements.
• Level Element: a proportional signal
coming from the drum level transmitter.
This signal is compared to a setpoint, and
the resultant is a deviation value. This
signal is acted upon by the controller
which generates corrective action in the
form of a proportional value.
• Steam Flow Element: a mass flow rate
signal is used to control the feedwater
flow, giving immediate corrections to
feedwater demand in response to load
changes.
15. Three element.
Consist of three elements
1. Level Element:
corrects for unmeasured
disturbances within the
system.
2. Steam Flow Element:
corrects for unmeasured
disturbances within the
system.
3. Feedwater Flow
Element: responds rapidly
to variations in feedwater
demand.
