Don't Push the Envelope

May 5, 2011
Take a sensible approach to avoid ignition dangers.

It was a rush job with a lot of political pressure. The refinery's heavy-oil hydrogen compressor was enclosed in a housing surrounded by equipment. The oil company didn't consider an API-500-defined Class I, Division 1 Electrical Area Classification (EAC); by its standards, because the compressor was open on three sides and the bottom was exposed, the area was perfectly safe -- until the roof blew off. Hydrogen had become trapped under the roof. We advised the refinery to install a forced-air roof vent with an interlock and alarm.


EAC, which defines standards for safe limits on electrical ignition sources, is an invigorating challenge technically as well as politically. Although our first EAC survey of this particular refinery was too conservative, we negotiated a reasonable standard we could live with.

Once the hazard has been identified (see "Don't Zone Out On Area Classifications"), you must define the envelopes around the potential leak sources. First, it's important to recognize there are two types of areas: Class I, Division 1, where a risk always exists because the chemical continually is present -- e.g., the interior of a compressor and sometimes the surrounding area if not well ventilated; and Class I, Division 2, where a risk sporadically can occur -- e.g., during maintenance when equipment is removed. A Division 2 envelope always exists around a Division 1 area. (European standards call these areas zones, with Zone 0 equivalent to Division 1.)

An area can be considered unclassified if it satisfies two conditions: the potential for an ignitable NFPA Class I liquid or a flammable gas must be low -- having a flange or pump in an area generally rules that out; and ventilation must be adequate. One exception is the ten-ft region around an open flame such as in a furnace, which is considered unclassified. The exception to the exception is a source like an automatic valve in this zone -- it's out of sight of an operator.

A Division 1 area can be converted to Division 2 with adequate ventilation -- a minimum of 12 air changes per hour -- with air from an unclassified supply. However, a Division 2 area never can be turned into an unclassified one. API 500, states: "Providing ventilation to allow re-classification on an enclosed area from classified to unclassified is not allowed in enclosed areas containing devices handling hydrocarbons." Obviously, good practice extends this rule to non-hydrocarbons such as flammable chlorosilanes.

Buildings can be unclassified even in a Division 2 area -- provided they don't contain sources, are constructed of fire-resistant materials with sealed entrances, and have positive pressure ventilation. Weatherstripping generally suffices. Windows that open are prohibited.

There are two geometric variables in defining a zone around a source: L and R. L is the horizontal distance defined in EAC (plan) drawings usually developed for plant and unit boundaries and equipment. R is the three-dimensional radius around a source. L is important mainly for heavier-than-air (HTA) compounds; R is crucial in defining the expansion volume for lighter-than-air (LTA) compounds.

Another useful classification tool is the Reid vapor pressure. API-500 assigns a greater risk to a highly volatile liquid (HVL) or mixture, i.e., one with a vapor pressure ≥40 psia at 100°F.

Now, let's consider basic API-500 Division 2 definitions: 1) for a HTA liquid vapor, L = 50 ft and R = 25 ft; 2) for an LTA liquid vapor, L = 15 ft and R = 25 ft, above; 3) for an LTA gas, L = 15 ft and R = 25 ft, above; 4) a 10-ft Division 2 envelope surrounds the contours of a source vessel; and 5) a 15-ft zone exists around vents. If the space above a source is blocked by more than 15%, it's a Division 1 area; this rule applies to walls -- a Division 2 area can have a maximum of two walls.

Note that EAC surveys are based on Division 2 envelopes that encompass the Division 1 zone.

Several factors -- volatility, pressure and flow -- require additions to L. For a HVL compound, add 50 ft to L. For a source with high pressure (>500 psig) or high flow (>500 gal/min), add 50 ft. The maximum L can extend as far as 150 ft.

To simplify our survey, we chose to set limits on the 50-ft adder for pressure and flow: if the line size for a liquid exceeded three in. or a gas velocity surpassed 2,200 ft/min and pressure and flow constraints were achieved, then we included the 50-ft adder.

Other factors such as heat are important in classification. If an NFPA Class II or Class III liquid were heated, even below boiling, then you should consider it a potential ignitable vapor.

Dirk Willard is a Chemical Processing Contributing Editor. You can e-mail him at [email protected].

About the Author

Dirk Willard | Contributing Editor

DIRK WILLARD is a Chemical Processing Contributing Editor.