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For
safe operation of any fuel gas
fired steam boiler
Combustion is monitored and
controlled to avoid creating
an explosion inside the
combustion chamber.
Boiler steam pressure and
water level are monitored and
controlled – high pressure
or low water level could
destroy the boiler.
Boiler feed water is kept
clean and the boiler is
inspected periodically to
avoid internal corrosion or
fouling, which could destroy
the boiler.
Guidelines for safe boiler
design, control, maintenance and
operator training are provided
by NFPA 85 "Boiler and
Combustion Systems Hazards
Code". For a complete
copy of NFPA 85 see www.nfpa.org.
The main items for combustion
safety are these:
|
Item
|
Safety
|
Reason
|
| Air
Purge (startup) |
Before
lighting the boiler
burner, the air blower
is operated long enough
to displace 4 times the
boiler combustion
chamber volume. |
If
natural gas and air are
inside the boiler when
the burner is lit, the
boiler can explode. |
| High
Natural Gas Pressure |
A
pressure switch in the
natural gas line stops
the burner if the gas
pressure gets too high. |
If
the natural gas pressure
control valve fails,
excessive gas can flow
to the burner. If too
much natural gas flows
to a combustor, the
burner could go out or
the flame could be large
enough to cause
overheating. If the
flame goes out and
natural gas continues to
flow, a combustible
mixture could fill the
boiler combustion
chamber (explosion
hazard). |
| Low
Natural Gas Pressure |
A
pressure switch in the
natural gas line stops
the burner if the gas
pressure gets too low. |
If
the natural gas pressure
falls too low, the
burner could go out. If
natural gas is still
flowing, a combustible
mixture could fill the
boiler combustion
chamber (explosion
hazard). |
| Low
Flame Strength |
A
flame detector stops the
burner if the flame
becomes weak or goes
out. |
If
the flame goes out,
natural gas flowing
through the burner could
cause an explosive
mixture to fill the
boiler combustion
chamber (explosion
hazard). |
| Air
Blower Not Running |
A
switch stops the burner
if the air blower is not
running. |
If
the air blower is not
running, the burner
cannot operate. |
| Low
Combustion Air Flow |
A
pressure switch in the
burner air plenum stops
the burner if air
pressure gets to low
(meaning the air flow is
too low). |
If
the air flow becomes
low, the flame may
become weak or go out
even though natural gas
is still flowing
(explosion hazard). |
The main items for boiler
safety are these:
|
Item
|
Safety
|
Reason
|
| High
Steam Pressure |
A
pressure switch in the
steam chamber stops the
burner if the pressure
gets too high. |
If
the burner continues
firing even when steam
flow is low, the boiler
pressure can build and
the vessel can explode. |
| Low
Water Level |
A
level switch stops the
burner if the boiler
water level gets too
low. |
If
the water levels gets
low enough that part of
the boiler runs dry,
that part can overheat,
weakening it and
possibly resulting in an
explosion. |
Special Hardware to Assure
Safety:
|
Item
|
Safety
|
Reason
|
| Natural
Gas Pressure Regulator |
An
automatic pressure
regulating valve steps
natural gas supply
pressure down to the
level needed by the
burner. |
If
the natural gas supply
line pressure rises too
much, the burner may
over fire, putting out
the flame or overheating
the boiler. |
| Natural
Gas Drip Leg |
A
drip leg (low point
drain) located near the
Natural Gas Pressure
Regulator traps debris
in the natural gas
supply. |
If
solid or liquid debris
enters the natural gas
supply valve train,
operation can be
affected. |
| Natural
Gas Block & Vent
Valves |
Two
automatic block valves
with a vent valve
between them are used to
stop natural gas from
reaching the burner when
it is off. The block
valves have position
switches so the control
system knows when they
are close. |
A
single automatic block
valve in good condition
would be enough to stop
natural gas flow, but a
second automatic block
valve provides
insurance. A vent valve
between the block valves
pipes any leakage gas to
atmosphere when the
block valves are closed. |
| Linked
Natural Gas and
Combustion Air Valves |
The
flow control valves for
natural gas and
combustion air are
linked mechanically –
as one is opened or
closed, the other does
the same. |
If
too much or too little
combustion air is
supplied with the
natural gas, the flame
can go out. |
| "Self-Checking"
Flame Detector |
The
flame detector has a
mechanical shutter which
blocks the detector
tubes view of the flame
periodically. If the
detector tube still sees
flame when blocked, the
detector has failed and
the burner shuts off. |
If
a flame detector tube
fails it must be
replaced. Sometimes
these tubes can fail in
the "flame OK"
position – if this
happens the burner could
go out and the detector
would not know. NFPA
guidelines require
self-checking detectors
when the burner operates
more than 24 hours
without shutdown. |
| Boiler
Steam Pressure Emergency
Relief Valves |
Two
special relief valves
vent steam from the
boiler if the pressure
reaches dangerous
levels. |
If
the pressure controller
and the high pressure
switch shutdown circuit
fail, these valves
prevent excessive steam
pressure from destroying
the boiler. |
| Water
Level Gauge Glass |
A
special gauge lets the
operator view the boiler
water level directly. |
If
the level control
instruments malfunction,
the operator can still
monitor water level and
take appropriate action. |
| Low
Fire To Light the Main
Flame |
The
control system drives
the burner main natural
gas control valve to
"low" flow
when the main flame is
to be lighted. |
The
pilot flame is much
smaller than the main
flame and may be unable
to light the main flame
if fuel gas is flowing
at maximum rate. |
| Interrupted
Pilot Burner |
The
small pilot burner turns
off as soon as the main
burner is lit. |
A
single flame detector is
used in this system. It
must be located to see
both the small pilot
flame and the large main
flame. If the pilot
operates all the time,
and a problem with the
main flame caused it to
go out, the system would
not detect the flame
loss. At high burner
firing rates, the pilot
flame may be too small
to reignite the main
flame.
Notes: With a
separate flame detector
for both the main flame
and the pilot flame, the
pilot can remain on all
the time. Also,
with a large pilot flame
(usually at least 10% of
the main flame)
reignition of the main
flame is very likely and
the pilot can remain on
even with a single flame
detector in use.
|
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