Fuel Gas Firing – Pilot Burners
Most fireboxes do not need pilots for ignition; the operator lights the burner with a hand-held torch. But for those that do, attention should be paid to the pilot’s heat release, particularly when continuous pilots are used for assuring continuous flames at the burner—firebox safety will depend on their reliability.
Pilots for burners come in three sizes: small, medium, and large. Or, to use the classifications of the National Fire Protection Association (NFPA): Class 3, Class 2, and Class 1, respectively. Burner suppliers tend to provide Class 3 pilots. Be aware of their limitations before approving them.
Class 3 Pilots. The heat release of these very small pilots generally does not exceed 4% of the full load burner heat release. They are designed to light-off gas and oil burners under very restricted conditions.The fuel flow and the combustion air flow must be within narrow limits. Such pilots may not light-off minimum fuel and maximum air flow. Class 3 pilots are suitable only if the fuel and air flow conditions are controlled for light-off. They should be turned off after the main burner is lit-off, and they should not be used to support ignition or to extend the burner control range.
As an exception, if a pilot, however small, can be shown by test to reliably light-off the burner under all possible operating conditions, then it may be treated as a class 1 pilot for that application. The range of operating conditions should be recorded in the safety instruction sheets so that any re-engineering of the furnace has the information available.
Class 2 Pilots. The heat release of these intermediate pilots generally is between 4% and 10% of the full load burner heat release. They should not be used to ignite the main fuel under uncontrolled or abnormal operating conditions. Raising the pilot heat release from 4% toward 10% simply widens operating conditions.
Class 1 Pilots. The heat release of these larger pilots generally is 10% or more of the full load burner heat release. They are designed to ignite any combustible combination of fuel and air. Such pilots can be left burning continuously and can be used both to light-off the main burners and to serve as an alternative to minimum fire devices.