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Fuel Firing Apparatus - Natural Gas

Print Date: 12/13/2017 4:27:48 PM

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.

 

 

  1. 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.

     

  2. Gas pressure regulator (PRV) - its purpose is to maintain the fuel at a constant pressure as recommended by the burner manufacturer.

     

  3. 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.

     

  4. 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.

     

  5. 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

     

  6. 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.

     

  7. 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.

     

  8. 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.

     

  9. 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.

     

  10. 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.

     

  11. 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.

 

 

 

  1. 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.

     

  2. Gas pressure regulator (PRV) - the purpose is to maintain the pilot fuel at a constant pressure as recommended by the burner manufacturer.

     

  3. Safety shut-off valve (SSOV) - automatically opens and shuts off fuel supply to the pilot burner.

 

 

 

 

 

Points to remember:

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.