A typical fuel train (such as the one shown in Figure 2 and in the photos) 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.