Lee Doran
Winter 1994
Category: Operations
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (7 printed pages)
The importance of fuel firing equipment cannot be over emphasized. The majority
of boiler explosions occur in one of two ways: a failure of a boiler pressure
part, or a furnace explosion.
Jurisdictions and insurance companies have long recognized the need for the
proper installation, maintenance, repair, operation and inspection of this
equipment. However, the real world shows us that there are many people involved
with this equipment, including installers, maintainers, operators or inspectors
who might have minimal knowledge of the maintenance and testing requirements
for the different components that make up a fuel train.
There are several organizations such as UL (Underwriters Laboratory), FM
(Factory Mutual), IRI (Industrial Risk Insurers), etc, that publish
requirements for the various components which make up a fuel train for specific
burner output. Also, several codes and standards such as NFPA and ASME publish
requirements for the entire assemblies.
For now, only IRI, ASME CSD-1 (Controls and Safety Devices for Automatically
Fired Boilers 1992) and NFPA 8501 (Single Burner Boiler Operation-1992) shall
be considered.
A typical
fuel train has several
components, each with a specific purpose which is briefly explained below.
-
Manual shut-off valve (MSOV) - the purpose of this valve is to shut off the
fuel supply so that maintenance or replacement of the fuel train may be done.
-
Gas pressure regulator (PRV) - its purpose is to maintain the fuel at a
constant pressure as recommended by the burner manufacturer.
-
Low pressure gas switch - senses a low pressure in the range where the burner
cannot properly operate. This switch must be equipped with safety lock-out
requiring manual reset. This tells the operator to check the gas pressure,
since the burner will not try to restart until the switch is reset.
-
Safety shut-off valve (SSOV) - the first of two valves in series that
automatically opens and shuts off the fuel supply to the burner. On burners
above 2,500,000 BTU/HR, the valve closing time is one (1) second maximum.
-
Vent valve - this is normally an open valve. It energizes to close. Whenever
the SSOV's are closed, the vent valve is open, so if any gas is leaking past
the first SSOV (4), it will vent this gas to the atmosphere, so that even if
the second SSOV (6) may leak, no gas will go past this valve since there is no
pressure differential across the SSOV
-
Safety shut-off valve (SSOV) - the second of two valves in series,
automatically opens and shuts off the fuel supply to the burner. On burners
above 2,500,000 BTU/HR, the valve closing time is one (1) second maximum. This
SSOV in conjunction with the first SSOV (4) and the vent valve (5) comprise the
double block and bleed arrangement and ensure no gas leakage into the
combustion chamber during burner shutdown.
-
High gas pressure switch - the purpose of this safety device is to sense a high
gas pressure and shut down the burner before an unsafe condition can occur. The
high pressure is usually due to failure of the gas regulator. This switch must
be equipped with a safety lock-out requiring manual reset. This tells the
operator that the gas regulator failed since the burner will not restart until
the switch is reset.
-
Manual shut-off valve (MSOV) - the purpose of this valve is to allow testing of
all components of the fuel train under actual operating fuel pressure without
firing the main burner. This testing is done after any maintenance or extended
shut down to ensure all components are working properly prior to actually
firing the main burner.
-
Firing rate valve - valve equipped with a modulating motor controlled by boiler
pressure (steam) or temperature (hot water) and regulates the amount of fuel to
the burner and through linkages, controls the air damper (like the cruise
control on a car), should be located as near the burner as possible.
-
Test valves - the purpose of these valves is to allow testing of the SSOVs for
leakage. This test is required by ASME CSD-l and NFPA 8501 to be done on a
monthly basis.
-
Drip leg - the purpose of this piping arrangement is to trap any debris or
water which may accumulate in the gas piping to prevent fouling of the fuel
train components including burner orifices.
-
Pilot manual shut-off valve (MSOV) - the purpose of this valve is to shut off
the fuel supply so that maintenance or replacement can be done.
-
Gas pressure regulator (PRV) - the purpose is to maintain the pilot fuel at a
constant pressure as recommended by the burner manufacturer.
-
Safety shut-off valve (SSOV) - automatically opens and shuts off fuel supply to
the pilot burner.
Testing of the fuel firing apparatus, including the flame safeguard system, is
relatively easy. Using the fuel train described above as an example, a
qualified technician would:
-
Line up the main burner fuel train with the first MSOV (1) open and the second
MSOV (8) closed and line up the pilot fuel train with the MSOV (A) open. This
allows testing of all components under actual fuel pressure without firing the
main burner.
-
Start up the burner and observe the pre-purge time. The flame safeguard
controls are programmed to provide at least four air changes of the combustion
chamber and gas passes. This also proves air flow (air flow proving switch) and
adequate gas pressure (low pressure gas switch). After pre-purge, the
modulating control will drive the burner air damper towards the closed
position. The flame safeguard controls will then open the pilot SSOV (C) and
ignite the pilot burner. Once the pilot flame is established, the flame scanner
will sense the pilot flame and the flame safeguard will open both main SSOVs
(4) and (6). Observe these valves as they open. At this point, the pilot
ignition system, the flame scanner, the action of SSOV operation, and the high
gas pressure switch have all been proven.
-
Main burner ignition and flame establishment is also timed into the flame
safeguard control depending on burner type and size. Since the MSOV (8) is
closed, no fuel will flow into the main burner. Therefore, at the end of this
trial for ignition period, the flame safeguard control will shut down the pilot
burner. Since the main burner is not firing, the flame scanner will sense a
flame failure, and the flame safeguard control will close the SSOVs (4) and
(6). Observe the action of the SSOVs. This operation proves the flame failure
system and proper SSOV operation.
Points to remember:
-
If the operator/maintainer is not trained to repair or test this equipment, the
services of a competent service company should be obtained.
-
Always refer to the burner manufacturer instructions and safety precautions.
-
Check jurisdictional requirements to ensure proper fuel train arrangement and
installation. Ensure all safety devices required by the adopted codes are
installed.
-
Ensure all safety devices which make up the fuel train are accepted for the
intended service by a nationally recognized testing agency such as, but not
limited to, UL, FM or AGA (American Gas Association). Primary controls (flame
safeguard controls) rebuilt or altered by individuals or companies which do not
have the sanction of a nationally recognized testing agency are not allowed to
be used. (Ref: ASME CSD-l Interpretation 90-2).
Pressure gauges should be installed before and after the gas pressure regulator
and at the burner, in order to monitor pressure to ensure they are within the
manufacturer's recommendations.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.