I have written about the confusion within many industries about their "electrical classification" and what the Recognized and Generally Accepted Good Engineering Practices (RAGAGEPs) allow and do NOT allow.  In 2012 we even came across facilities that were using codes from an entirely different industry being applied to their facility, which utilized an entirely different class of flammable material.  In this article I will use the two most popular codes, API 500 and NFPA 497, and touch on the use of ASHRAE 15.  But let me state this up front, NEITHER API or NFPA allow an area to be "unclassified" merely by installing ventilation; ASHRAE 15 DOES permit exclusion of an electrically classified machinery room, BUT it is not a BLANKET EXCLUSION as many have come to believe.

Lets review NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (FREE on-line review) for what it states regarding using ventilation when establishing HAZLOCs.  NFPA defines "Adequate Ventilation" as...

A ventilation rate that affords six air changes per hour, 1 cfm per square foot of floor area (0.3 m3/min/m2), or other similar criterion that prevents the accumulation of significant quantities of vapor-air concentrations from exceeding 25 percent of the lower flammable limit (LFL).

NFPA 497, 4.1.3.2 states:

A Class I, Division 2 location is a location

1. In which volatile flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems or in case of abnormal operation of equipment, or
2. In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are normally prevented by positive mechanical ventilation and which might become hazardous through failure or abnormal operation of the ventilating equipment, or
3. That is adjacent to a Class I, Division 1 location, and to which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors above their flash points might occasionally be communicated unless such is prevented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided.

Pay particular attention to the fact that NFPA 497 does NOT permit excluding, per the definition, the area from being a HAZLOC by the mere presence of a ventilation system.  However, it DOES permit us to limit the buffer zone outside a Class I Division 1 area when there is adequate positive-pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided.

To proove this point further, we can review the many helpful diagrams provided in NFPA 497 that re-enforce this position.

FIGURE 5.9.1(c) - Leakage Located Indoors, at Floor Level. Adequate ventilation is provided. The material being handled is a flammable liquid.

API Recommended Practice 500 Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division I and Division 2 follows pretty much the same path.  In fact, the definition for a Class I Division 2 area is almost identical to the one in NFPA 497, with ONE (1) major difference:  API removed the reference to "mechanical" ventilation in #2 of the definition and allows for adequate "natural ventilation".  I have shown this below in API 500 Section 6.  

Class I, Division 2 Locations—Locations (1) in which volatile flammable liquids or flammable gases are handled, processed or used, but in which the liquids, vapors or gases normally will be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment; (2) in which ignitable concentrations of gases or vapors normally are prevented by positive [mechanical] ventilation, and that might become ignitable through failure or abnormal operation of the ventilating equipment; or (3) that are adjacent to a Class I, Division 1 location and to which ignitable concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by either adequate positive-pressure ventilation from a source of clean air (and effective safeguards against ventilation failure are provided) or separation by a vapor tight barrier.

NOTE In item (2) above, the word “mechanical” has been removed (between “positive” and “ventilation”) from the NEC definition to allow both natural and mechanical means to provide for adequate ventilation.

The code actually defines a Class I Division 2 area in section 3.2.5.3:

Class I, Division 2 location A location in which flammable gases or vapors may be present, but normally are confined within closed systems; are prevented from accumulating by adequate ventilation; or the location is adjacent to a Division 1 location from which ignitable concentrations might occasionally be communicated. Reference 6.2.1.1.b and NEC Article 500.5(B)(2) for a more complete definition.

API 500 Section 3.2.27 defines "adequate ventilation" as:

(natural or artificial) that is sufficient to prevent the accumulation of vapor-air or gas-air mixtures in concentrations above 25 % of their lower flammable (explosive) limit, LFL (LEL). See 6.3.2.

We can also turn to API 500 Diagrams to see that ventilation can "lessen" the classification, but it will not exempt an area solely due to "adequate ventilation".  But that is NOT really needed when using API 500 as your RAGAGEP, as section 6.3.1.2 states the following:

Providing ventilation to allow the reclassification of an enclosed area from classified to unclassified is not allowed in enclosed areas containing devices handling hydrocarbons. Equipment as described by 6.2.4.1 and hydrocarbon-fueled prime movers with fuel gas pressure at 125 psig or less as provided for in 8.2.5 are excluded.

Here is an example of API 500 Diagram for "adequately ventilated" areas...

Lastly, the only RAGAGEP I know of that permits a general exclusion from an area being a HAZLOC resides in ASHRAE 15 (which is referenced in IIAR Bulletins as well).  ASHRAE 15-2010 states the following:

8.12 Machinery Room, Special Requirements

g. When refrigerants of Groups A2, A3, B2, and B3 are used, the machinery room shall conform to Class 1, Division 2, of the National Electrical Code.⁵ When refrigerant Groups A1 and B1 are used, the machinery room is not required to meet Class 1, Division 2, of the National Electrical Code. Exception: When ammonia is used, the requirements of Class 1, Division 2, of the National Electrical Code shall not apply providing the requirements of Section 8.12(h) are met.

h. When ammonia (R-717) is used, the machinery room is not required to meet Class 1, Division 2, of the National Electrical Code,⁵ providing (1) the mechanical ventilation system in the machinery room is run continuously and failure of the mechanical ventilation system actuates an alarm or (2) the machinery room is equipped with a detector, conforming to Section 8.11.2.1, except the detector shall alarm at 1000 ppm.

So I hope this helps those confused about using "adequate ventilation" to exempt an area from being a HAZLOC.  I certainly hope it does not cause more confusion, as I have tried to quote from the codes to show that adequate ventilation can lower the severity of the classification, but it can not exempt the area; unless of course your area is an ammonia machinery room and your RAGAGEP is ASHRAE 15.

This article is my professional opinion.  It is the methods at which I have used for 20+ years classifying HAZLOCs.  I am sure there are many that will disagree with my position and I wish them all the best in their process safey efforts.