NFPA 86 (Ovens and Furnaces) and NFPA 87 (Fluid Heaters) recognize that some industrial heating systems are installed where electricity is not available, and heaters must be operated without the benefit of a forced-draft, clean-air purge prior to startup.

Nevertheless, natural draft furnaces can be started up safely by ensuring ventilation doors and exhaust ducts are wide open for a sufficient amount of time prior to ignition. Natural draft ventilation is driven by buoyancy forces, just like the chimney effect that occurs when exhaust from a fire rises up a chimney (i.e., because “hot air rises”). The difference with pre-ignition purge is that the buoyancy forces arise from the difference in gas density of methane and air. One thousand liters of air weighs about 1.2 kilograms, whereas one thousand liters of natural gas weighs less than 700 grams. (By comparison, helium and hydrogen are even less dense, but the density of natural gas is sufficiently low to cause a natural draft purge in a reasonable amount of time.)

The purpose of purging a furnace prior to burner light-off is to remove any combustible gases from the furnace enclosure and thereby prevent accidental ignition of an accumulation of gas from a prior unsuccessful light-off or leaking shutoff valve. When forced ventilation is used, the standards require purging the enclosure with 4 volumes of fresh air prior to light-off. In other words, if a furnace enclosure is 100 cubic meters, and the forced draft fan can be proven to deliver at least 100 cubic meters of fresh air per minute, a purge duration of 4 minutes can be programmed into the startup sequence and the code requirement will be satisfied.

However, when natural draft ventilation is the only available method of purging, determining the length of time for purge is not straightforward. Without a fan, it is more difficult to determine the exhaust gas flow rate, but more importantly, the exhaust flow rate varies with the amount of residual methane still in the furnace. As the furnace becomes more diluted with air (i.e., as the purge process dilutes the initial methane concentration down to lower values) the buoyancy driving force declines, and so does the purge rate. There is no way to ensure a certain number of “fresh air purge volumes” are forced into and out of the enclosure because the volumetric flow rate changes with time.

To overcome this problem, the Section 8.5.1.2 of NFPA 86 requires the purge time to be determined by measurement, at a time when the furnace is at normal ambient temperature. The preferred method of doing so relies on combustible gas analyzers and oxygen analyzers to continuously measure the exhaust flow leaving the furnace until the concentration falls below 25% of the LFL (lower flammability limit) of the fuel gas in air.

This author has modeled the accumulation and dissipation of natural gas in a hypothetical furnace using a large-eddy-simulation software tool called Pyrosim, which is derived from the NIST code FDS (Fire Dynamics Simulator). A video showing the process for a 20 cubic meter furnace is shown here, and a plot of exhaust concentration versus time for the simulation is also shown.

These results are not applicable to any furnace or gas source or combustion system other than the one modeled, and readers SHOULD NOT extrapolate these results to any other furnace or application. The ventilation rate depends strongly on the size of the openings (for exhaust gas outflow and fresh air inflow) and the time required to purge an actual furnace in the field could vary greatly from case to case. Furnace users are urged to consult with a purge specialist to determine the correct purge time for their own applications.

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During a recent explosion investigation, this author discovered a new failure mode that is not sufficiently addressed in NFPA’s trio of industrial heating equipment standards (NFPA 85, NFPA 86, and NFPA 87) that cover Boilers, Ovens, and Fluid Heaters, respectively. The failure mode occurs only in heating systems equipped with natural gas burners and flue gas recirculation (FGR) for control of NOx emissions. The investigation where the failure mode manifested itself happened to be concerned with a boiler explosion, but ovens, furnaces, and fluid heaters are equally capable of experiencing the same problem, if certain factors are in play.
The schematic below identifies the primary equipment that plays a role in the incident scenario. In addition to the boiler, burner, blower and natural gas source, there are two flow valves (FV-001 and FV-002) that control the amount of FGR blended with fresh air that enters the burner. On smaller boilers, FV-001 is set manually during commissioning to approximately 50% open and rarely changed, whereas FV-002 is typically an automatic valve with two discrete positions – closed (no recirculation) and normal (standard recirculation).

NFPA burner safety requirements require a pre-ignition purge at the beginning of each burner startup to help ensure the combustion chamber is free of residual fuel gas or any other combustible vapor. NFPA burner standards have included a purge requirement for at least 50 years and such requirements have reduced the rate of explosions significantly.
The goal of the purge cycle is for approximately 4 volumes of fresh air to be admitted into the combustion chamber to drive out any unwanted combustible gas or vapor. For example, if the combustion chamber has a volume (𝑉 = 100 ft3) and the blower is delivering a flow rate Vdot = 400 acfm the purge time should be 𝑡 =1.0 min. This amount of purge is almost always conservative enough to ensure combustible vapors are diluted to a nonflammable concentration in the firebox. The very first volume of fresh air purge in theory is enough to remove the combustible vapors if a plug flow model is assumed for the air flow inside the chamber. The requirement for 4 purge volumes arises from the fact that the plug flow model isn’t conservative enough if plug flow behavior is not achieved. Hence, a perfectly-stirred vessel model is used instead. The decay of fuel concentration in the firebox is exponential with time, and 4 volumes of purge air will take a 50% fuel concentration down to 1%.
However, if the purge air isn’t comprised of pure air, but rather a mixture of “flue” gas with a high concentration of unburned fuel from the prior unsuccessful burner ignition attempt, the purging process is much slower. The figure below shows the difference in decay rates between the normal case, where the purge air is 100% air, and the compromised case, where the purge air comprises 50% FGR (with residual fuel) and 50% fresh air. When purge is carried out with contaminated air, the number of purge volumes required is 8, not 4.

For the boiler explosion case described above, this author found that FV02 had been unplugged from its power source and the damper was stuck in a partially open condition. After 3 unsuccessful ignition trials in rapid succession, the spark igniter set off an internal deflagration that damaged the vessel walls such that a complete replacement of the boiler was required.