Fuel Firing Apparatus - Natural Gas
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 of the ASME Boiler and Pressure Vessel Code for current requirements.