Page 78 - Boiler_Operators_Handbook,_Second_Edition

Boiler Plant Operations 63

event the minimum fire pressure regulator fails. You’ll operation of an ignitor. Most boilers will be equipped
probably encounter multiple burner boilers without with a gas-electric ignitor. Small boilers frequently use
minimum pressure regulators, and experience the dif- nothing more than an electric spark to light the fire,
ficulties of operating without them. All multiple burner that’s because their burner is the size of an ignitor or
boilers should have them, one for gas, one for oil, for smaller. You can also run into some with oil-electric
more reliable operation. ignitors and a few with high energy electric ignitors
A low fire position is not a certain solution to prob- and other unique methods. The bulk of boilers use a gas
lems when lighting off a boiler. Fixed fire boilers light off electric ignitor and we’ll stick to that mode. Many of
at full fire so there is no switch or adjustment to be made you may choose to call an ignitor a “pilot fire” or “pilot
and they can experience rough light off. A rough lighting light” but I’ll simply talk about ignitors and you’re free
is due to creation of a fuel and air mixture that is outside to use any of those labels.
the flammable range (see the chapter on fuels) which fi- Since ignitors use an electric arc to start the gas or
nally lights when a proper mixture is established. oil fire it’s appropriate to make certain the flame sensor
When firing gas it’s usually because the mixture is doesn’t think the electric arc is a fire. Begin by closing all
fuel rich due to gas leaking past a regulator. A quantity the manual fuel shut-off valves including the ones that
of gas is trapped between the regulator and safety shut- supply fuel to the ignitor. Next, go through several par-
off valves at a higher pressure than normal. When the tial ignition cycles to see if the spark is detected. What’s
shut-off valves open the result is a flow of gas larger an “ignition cycle?” That’s everything you and the
than normal for a few seconds until that buildup of gas Burner Management and combustion controls do from
bleeds off. On oil fired boilers the gun can start empty starting of the fans to igniting the main fuel on the first
with fuel mixing with the air in the gun to produce too burner fired, including a purge.
lean a mixture. On the next try, if the gun isn’t purged, On multiple burner boilers we also have a “burner
the mixture can be fuel rich. A rich or lean condition ignition cycle” which includes waiting then trying to
can be created depending on operation of atomizing light that burner. If the flame scanner “picks up” (the
medium controls. If the boiler doesn’t have smooth igni- system indicates the scanner recognizes a flame which
tion start looking for short term surges or sags in fuel is probably from another burner) your burner supplier
pressure and flow when compared to conditions after a has a problem and you shouldn’t continue to operate the
stable fire is established. boiler until the problem is fixed. Sometimes you can cor-
There’s usually a lot of room for low fire variations rect the difficulty by re-sighting the scanner (adjust it so
because most boilers have fan dampers that simply can’t it points in another direction) but if you do you should
close enough to produce a minimal excess air condition perform this check regularly to ensure the adjustment
at low fire. Those dampers leak so badly that low fire hasn’t failed to prevent sensing a spark as a flame.
is usually established with the dampers in what could We also talk about discrimination and it’s very im-
be considered a closed position and excess air is still portant in multiple burner boilers. The flame scanner for
200 to 300%. A good variable speed drive will provide a burner should not detect the fire of any other burner.
lower excess air at low fire but the flow usually has to be If it does, it can improperly indicate the ignitor, or main
controlled to overcome problems with changes in stack flame, of its burner is on and allow the fuel valves to
draft producing significant changes in the air flow. remain open when, in fact, there’s no fire there. To pre-
Remember, it’s always important that low fire be a vent this, and any false indication of a fire, a Burner
stable condition. With multiple burner boilers where the Management system will normally lock out when a
code limits low fire air flow to 25% of full load air flow fire is detected that shouldn’t be there. That’s anytime
that can be difficult because air fuel ratios can change a flame is detected but the fuel safety shut-off valves
with number of burner registers open. Set procedures aren’t energized. Even with a single burner boiler you
must be established to get the air flowing at the correct should make sure that works. Slip the scanner out of the
amount through the burners to be started; and those burner assembly during the purge period and expose it
procedures must then be followed religiously. to a flame, the burner should lock out.
For almost all ignitors the trial time is ten seconds.
Establish an Ignitor We call it the pilot trial for ignition (PTFI). That means
With low fire (actually light-off) position deter- that ten seconds after the ignitor gas shut-off valves
mined it’s time to actually get a flame going in the open the scanner must detect a flame to permit con-
burner. Except for very small boilers that involves the tinued operation of the boiler. If the ignitor flame isn’t