This column extends last month's discussion on the fundamentals of hazardous-classified areas to the exterior areas of high-temperature furnaces.

In last month’s column, some fundamentals of hazardous-classified areas were considered. We showed that a “fuel-inevitable” area (e.g., an open dip tank containing a volatile organic liquid) could benefit from hazardous-area classification, but an “ignition-inevitable” area (e.g., the interior of a furnace) must be designated unclassified. This column extends the discussion to the exterior areas of high-temperature furnaces.

Furnace Exteriors
Unlike the inside of a combustion chamber, the area outside a furnace could be designated hazardous-classified under some circumstances. If there is a non-negligible probability of fuel gas or organic vapor releases into the area, then installation of explosion-proof or purged electrical enclosures might enhance safety for some types of furnaces, as discussed below.

According to NFPA 497, the exterior of most furnaces should not be designated Class I, Division 1 because “normal” conditions do not involve releases of fuel gas or organic vapor. Here, normal includes routine releases of fuel due to either normal maintenance or normal operations.

Conversely, some furnace exteriors might be designated Class I, Division 2 if fuel gas or organic vapor releases are possible under “abnormal” circumstances (e.g., from infrequent leaks or venting). Here, abnormal excludes major catastrophes but could include gasket and seal leakage. When properly designed, installed, inspected and maintained, steel piping with flanged or threaded connections does not pose a high risk of leakage. The National Fuel Gas Code (NFPA 54) and the ASME Code for Pressure Piping (B31) establish the installation and maintenance requirements for such piping components on ovens and furnaces.

Ignition Sources
If the exterior of the combustion system presents any surfaces that are routinely “hot,” the boundaries for any classified areas should exclude those hot surfaces. Hot-surface ignition is a complex phenomenon, and, in some circumstances, surfaces above 1000°F will not effectively ignite a gas-air mixture. In the absence of specific information about particular compounds, surfaces above 350°F may be conservatively deemed hot enough to ignite organic gases and vapors in air.

Similarly, if the combustion system’s enclosure is “not tight” (i.e. flames or hot gases from the interior can escape or outside fuel vapors can be drawn inside), potential sources of fuel gas should be located far enough away from these openings so the hazardous-classified area ends before the opening begins.

Class A, B, C and D Ovens/Furnaces
Many electrically heated and gas-fired ovens and furnaces are constructed with tight enclosures and cool exterior surfaces (e.g., Class-D vacuum furnaces). The areas surrounding such systems may be designated either Class I, Division 2 or Unclassified depending on how frequently the vessel or piping connections are unsealed and resealed. Each time a seal is broken, the release of flammable gas is possible, and electrical components that are not rated for Class I, Division 2 service should be de-energized. The surroundings should be sniffed for hazardous concentrations of organic gases before re-energizing unclassified electrical components.

Conversely, some industrial heating systems are not constructed with tight enclosures (e.g., Class-A ovens or thermal oxidizers with unsealed joints), and other systems present continuous ignition sources (e.g., Class-C atmosphere furnaces with flame curtains or Class-B furnaces with frequently opening doors). Care should be taken to preclude the release of all flammable gases in the vicinity of such “ignition-inevitable” heating systems to minimize the risk of explosion.

Installing Class-I electrical components in ignition-inevitable areas is often pointless, because the areas cannot be designated as hazardous-classified due to the constant presence of ignition sources. In such areas, separation, isolation and ventilation should be employed to prevent fuel gases and vapors from migrating to ignition-inevitable areas.

Readers should be cognizant of exceptions to these examples (e.g., a Class-C furnace that is capable of sequentially releasing flammable gases and exposing a burnout pilot) where Class-I, Division-1 electrical components may be warranted. As with all mechanical and combustion systems, good engineering judgment should be employed to ensure actual risks are mitigated appropriately with reasonable controls.IH