Page 409 - Boiler_Operators_Handbook,_Second_Edition

394 Boiler Operator’s Handbook

boiler because the controls were not set up for that 50%. ciple purpose of the system is to make sure it is safe to
The setup of a boiler feedwater control system usu- light a fire and to continue operation of the boiler. The
ally ignores the blowdown (which requires a little more burner management does two things; it “supervises” the
feedwater) so the valve position is set to handle the cor- operation to ensure all operating parameters are within
rect amount of feedwater for the normal steam flow and “limits” and it supervises or performs and supervises
blowdown. The technician setting up the system looks the procedures required to place a boiler and its burner
at the schematic and realizes that the level controller can (or burners) in operation.
add to the steam flow signal to increase flow and raise It can also provide a controlled shutdown of the
the water level but he doesn’t think about what has to be boiler and its burner(s). The bulk of the controls manu-
done to lower the level if it gets too high. factured to serve the purpose of burner management
A 50% bias opposed to the steam signal and bal- are supplied by one of two manufacturers, Fireye (now
anced by the 50% output of the level controller has a net a division of Allen Bradley) and Honeywell. Their de-
zero effect on the steam flow to valve position relation- vices are competitively priced and do an excellent job
ship but allows the level controller to modify the relation- of burner management for small and medium sized
ship up and down by 50%. Without that bias the level single burner boilers. The controller becomes a system
controller output is around zero and it can’t do anything when it is connected to a burner’s controls, level, pres-
to lower the water level if there is a slight upset in opera- sure, and temperature limit switches and a flame scan-
tion (like lower steam pressure that will allow more water ner or flame rod.
to flow into the boiler) that results in a high water level. Accurate detection of a fire is the most important
Three element control accomplishes the same thing function of the burner management system. It has to
as the two element control but it measures the feedwa- ensure there is no fire when it shouldn’t be there, as well
ter flow as a process variable and the feedwater flow as ensure a fire is present when it’s supposed to be, and
controller adjusts the feedwater control valve until the respond accordingly. Detectors are either flame rods,
feedwater flow matches the steam flow plus or minus infra-red sensing, ultra-violet sensing, or more modern
any adjustment from the water level controller. Three multiple frequency sensing units. The detectors, with
element systems are necessary whenever feedwater the exception of flame rods, are all called scanners.
pressures are frequently changed or affected by heavy The basis of flame rod operation is that a fuel and
load swings so the linearity of the control valve can’t be air mixture does not conduct electricity but a flame does.
maintained. The rod has to be positioned where it is in the flame, a
Of course there’s going to be a problem with a two pilot flame on large burners. The flame also has to have a
element system that’s set up right if the steam flow sig- grounding electrode that touches the flame so electricity
nal is lost. The controller won’t be able to open the valve can be conducted from one electrode to the other. The
more than 50%. I don’t consider this a big deal because grounding electrode is normally connected electrically
I don’t think you should be operating a boiler with a to the metal parts of the burner. It can also be another
two element control in single element mode unless it’s flame rod positioned at another point in the flame. The
at very low loads. Modern digital controls sort of solve flame rod itself has to be constructed of a material that
that problem because their instructions include switch- will not melt or oxidize in the fire and the insulation
ing to single element control that can fully stroke the separating it from the metal parts of the burner have to
valve when the steam flow signal is lost or low. Failing be capable of withstanding the high temperatures. Nor-
that and circumstances require you to operate a boiler mally the rod is made of high chrome steel and the in-
without a steam flow signal you can simply find the sulators are ceramic material. The portion of the burner
summer and adjust the bias from a -50% to zero and the management system that identifies the presence of a fire
valve will respond directly to the level controller signal. has to produce a voltage adequate to push a detectable
current through the flame and sense that current and
distinguish between no flame, false signals (like the rod
BURNER MANAGEMENT shorting out on some metal in the burner) and a flame.
The flame scanner is a sensor that detects a fire in
Before I cover the control of fuel and air to pro- the burner by absorbing some of the light energy emit-
duce a flame and add heat to the boiler I’m compelled ted by the fire. Scanners may detect infra-red light or
to cover the burner management system. For years we ultra-violet light, any frequency of light in between, or
called them flame safeguard systems because the prin- a combination. Some are called “self-checking” but that