NFPA 86 mandates a timed pre-purge cycle before every burner ignition sequence in Class A and Class B industrial ovens and furnaces. This pre-purge requirement is one of the most critical combustion safety provisions in industrial process heating, preventing ignition of combustible mixtures that may have accumulated during shutdown.
Complete Combustion engineers and documents NFPA 86-compliant pre-purge calculations as part of every TSSA Field Approval package in Ontario and Canada.
During an unplanned shutdown, residual unburned fuel can accumulate inside a combustion chamber or connected ductwork. NFPA 86 requires combustion air be mechanically forced through the entire furnace enclosure before any ignition trial.
The total volume to be purged includes the combustion chamber(s), burner tile zones, exhaust ductwork and stack, and any connected heat recovery equipment.
NFPA 86 requires a minimum of four complete volume displacements of the total purgeable volume.
Minimum purge time (seconds) = Required purge air volume (ACF) ÷ Combustion air delivery rate (ACFM) × 60. This becomes the programmed BMS timer setpoint.
NFPA 86 requires proof that the combustion air damper is in the full-open position for the entire pre-purge duration via a mechanical limit switch or differential pressure proving switch.
Can the purge timer be bypassed for rapid restart?
No. NFPA 86 does not permit the pre-purge cycle to be bypassed or shortened for any reason. Every ignition sequence must begin with a full pre-purge cycle.
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