November 2017

Special Focus: Plant Safety and Environment

Demystify hazardous area classification and compliance assessments

As safety becomes increasingly paramount at hydrocarbon processing facilities, so too does the importance of addressing and clarifying the confusion associated with hazardous area classification (HAC) assessments and compliance auditing in Class 1 locations.

Johnston, J. E., Eccleston, C. A., Bath Process Safety Management

As safety becomes increasingly paramount at hydrocarbon processing facilities, so too does the importance of addressing and clarifying the confusion associated with hazardous area classification (HAC) assessments and compliance auditing in Class 1 locations. This work will attempt to “demystify” some of the uncertainties surrounding HAC and compliance assessments.

Regulatory background for HAC

In February 1992, Occupational Safety and Health Administration (OSHA) regulation 29 CFR 1910.119: Process Safety Management of Highly Hazardous Chemicals, became law in the US, prompting a great deal of confusion. Over the next several years, industry professionals began the process of sorting through the responsibilities, regulations and impending deadlines that had been imposed upon them. Acronyms such as PSM (process safety management), PSI (process safety information), RAGAGEP (recognized and generally accepted good engineering practices), MOC (management of change) and PHA (process hazards analysis) became commonplace in the vocabulary of process safety professionals nationwide.

Operators were now required to meet stringent deadlines regarding the creation and management of a plethora of process information associated with equipment in manufacturing processes covered by this law—i.e., any covered process. This information would be used to support the exhaustive efforts needed to create a baseline PHA for each covered process by May 1997. Additionally, OSHA required that all PHAs be placed on a 5-yr lifecycle requiring their review and revalidation. Along with the revalidation of PHAs came the task of revalidating all process safety information on the same 5-yr cycle.

As a result of these requirements, every willing, competent employee was tasked with supporting, participating in, facilitating or conducting PHAs. The required information used to support PHAs included materials of construction, piping and instrumentation diagrams (P&IDs), relief system designs and design bases, ventilation system designs, material and energy balances, safety shutdown systems and electrical classifications. Yes, electrical classification was, and still is, legally considered an integral part of required process safety information!

The following sections discuss three major myths surrounding electrical/HAC:

  • Myth 1: Only electrical engineers (EEs) assign area classification to a location
  • Myth 2: Only EEs understand classification information
  • Myth 3: Keeping classification documentation “evergreen” is difficult.

Myth 1: Only EEs assign area classification

The myth is that this assignment is the sole responsibility of the EE. Prior to debunking this myth, it is imperative that one understand the definition and scope of electrical/HAC.

Electrical classification—better stated as “HAC”—is defined as: “…a probability analysis and risk assessment evaluation of a manufacturing or process area possessing a potentially abnormal flammable atmosphere that focuses exclusively on the minimization or elimination of electrical energy as a potential or probable source of ignition.”

A review of this definition allows for clarification of what HAC is not. HAC is not a secondary line of defense against the following:

  • Poor process design
  • Poor facility and equipment maintenance
  • Faulty equipment operation
  • Catastrophic vapor releases.

Often, area classification assessment studies default to the responsibility of EEs, without taking into account the fact that the assessment process itself has nothing to do with the discipline of electrical engineering. Assessing a process facility for HAC is completely dependent on the actual process conditions. Process pressures, temperatures, flowrates and stream compositions are essential elements used to define a hazardous classified location.

Both the National Fire Protection Association (NFPA 497) and the American Petroleum Institute (API RP-500) provide various figures to assist the engineer in the overall assessment process. These figures are predicated on different pieces of actual process equipment, such as hydrocarbon pressure vessels, pumps, compressors, sumps and pressure relief devices, to name a few. The real challenge is the method chosen to determine the extent of the classified area, or “hazard radius.” The hazard radius is the distance a vapor or gas can travel from its source of release to the point where it no longer remains flammable (i.e., its concentration falls below the lower explosive limit, or LEL).

This methodology becomes even more complex, given that most process streams found in a typical process facility are a complex mixture of various hydrocarbons. The assessor is now faced with two choices for determining the hazard radius:

  1. The use of API RP-500 Annex D, which requires first determining the volatility category of the process material.
  2. The use of commercially available gas vapor dispersion modeling software.

Both of these techniques require extensive knowledge of the chemical process. The basic purpose of HAC is to eliminate or minimize the risk of ignition from electrical energy sources, as shown in the HAC assessment process.

The process of HAC assessment

After an assessment methodology is developed, the actual process of classifying the area is ready to begin. A typical assessment study includes a basic six-step process:

  1. Obtain the required documentation as determined from the assessment methodology. PFDs provide information about the process streams. Plot plans will serve as the backgrounds for area classification plan drawings. Material safety data sheets (MSDSs) are the source of process information about each component in the process stream. P&IDs provide a lower-level view of the process for equipment identification and process arrangements.
  2. Field survey the area in question to determine the accuracy of the plot plans and verify the locations of
    all point sources of emissions. Area classification background drawings should show all equipment that might impact the dispersion of the process material. These drawings should also indicate the prevailing wind direction.
  3. Determine the extent of the classified area that surrounds each point source of emissions. The extent of classification diagrams should come from NFPA 497 for petrochemical applications and from API RP500 for petroleum refinery applications and/or gas dispersion modeling software tools.
  4. Develop a composite area classification plan drawing that shows the contribution of all point sources.
  5. Develop elevation drawings to provide clarity where emissions sources are located in multilevel process structures.
  6. Create a detailed assessment report that documents the following information:
    • The rationale used to classify the areas
    • The critical process material information usually obtained from MSDSs
    • A detailed listing of all point sources of emissions that appear on the drawings
    • Special exceptions made when classifying a particular location.

All area classification documents should be placed under the control of a facility MOC process control. As modifications are made to the facility, these documents should be reviewed to verify the impact of these changes on the HAC assessment.

The basic purpose of HAC is to eliminate or minimize risk of ignition from electrical energy sources. The following conditions are necessary to support ignition:

  • A flammable material (gas, liquid-produced vapor or liquid) must be present and in quantities sufficient to produce an explosive or ignitable mixture
  • A source of ignition must be present.

Four deliverables are essential for an HAC assessment to meet this purpose:

  1. The classification: Class 1 for gases, liquids and vapors. For the purpose of this discussion, Class 2 combustible dusts and Class 3 easily ignitable fibers and flying atmospheres are excluded.
  2. The division: In Division 1 locations, ignitable concentrations of flammable gases, vapors or liquids exist under normal operating conditions; in Division 2 locations, flammable gases, vapors or liquids may be present in ignitable concentrations under abnormal operating conditions.
  3. The gas vapor grouping: Group A for the atmospheric presence of acetylene; Group B for the atmospheric presence of hydrogen greater than 30% by volume; Group C for the atmospheric presence of ethylene; and Group D for the atmospheric presence of butane, hexane, methane, propane and natural gas. The most common grouping is both C and D. OSHA requires that all electrical devices carry a label from a nationally recognized testing laboratory, such as Underwriters Laboratory.
  1. The T Code (temperature classification): A T Code on a label affixed to an electrical device simply means that the surface temperature of a device will not exceed the stated temperature under normal or abnormal operation. T Codes are an important part of the overall electrical device classification, and are dependent on the lowest auto-ignition temperature of a pure component of a given gas-vapor mixture. If an electrical device has a T Code that is higher than the lowest auto-ignition temperature, then ignition can and will occur without the presence of an electrical arc. National Electrical Code Table 500.8(C) provides cross-reference guidance on
    T Codes and maximum surface temperatures in °C.

A proper and complete HAC assessment will include all of the addressed deliverables.

Myth 2: Classification information is understood by only EEs

HAC information should address and support the following functions:

  1. The HAC compliance assessment process
  2. The design of electrical system equipment and installation practices
  3. The PHA process.

The HAC compliance assessment is required by OSHA and is performed by a team of professionals. The team utilizes the deliverables obtained from the HAC assessment process using plan drawings, process structure elevations, materials data and details as the basis for the compliance assessment. An HAC compliance assessment is performed to verify that the existing electrical equipment and electrical system installations are in compliance with codes, standards and recommended practices, such as:

  • National electric code (NEC) NPFA 70
  • API RP500 (Recommended practice for classification of locations for electrical installations at petroleum facilities classified as Class 1 Division 1 and Division 2)
  • 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]
  • NFPA 496 (Standard for purged and pressurized enclosures for electrical equipment)
  • API RP540 (Electrical installations in petroleum processing plants).

The compliance assessment study will typically address various issues that relate to installed electrical equipment, as well as to installed electrical wiring and raceway systems.

Common compliance assessment issues

As a result of the confusions that exist, issues arise requiring specific remediation. Common issues found in a typical HAC compliance assessment site walkdown include:

  • HAC documentation does not provide the T Code for heat-producing electrical equipment. Typically, this involves transformers, motors and light fixtures.
  • Installed electrical equipment does not provide the required label information to determine the T Code of the electrical device, or T Code information is missing from the affixed label.
  • HAC documentation does not take into account the proper gas/vapor grouping. This is typically found in instances where impacts from Group B sources are seen.
  • Improper application of enclosure purge and pressurization systems.
  • Lack of understanding of the impact that a protected fired vessel plays in an HAC.
  • Lack of understanding of the impact that a fired vessel with air-aspirated or open-flame burners plays in a classified area.
  • Improper design of building ventilation systems, especially those buildings with a piped-in source of flammable material. Analyzer buildings have the most problematic issues.
  • Improper application of conduit sealing practices that result in a failure to mitigate issues associated with the propagation of a flame front through the conduit system.

PHA process and HAC. One purpose of a PHA study is to evaluate or design safety systems to prevent or minimize the consequences of catastrophic releases of toxic, reactive, flammable or explosive chemicals. These releases may result in toxic, fire or explosion hazards. The PHA process map in Fig. 1 indicates how HAC data is used in the PHA development process.

Many disciplines—including chemical, mechanical, safety and electrical engineering—should use the deliverables from an HAC study.

FIG. 1. PHA process map.
FIG. 1. PHA process map.

Myth 3: Keeping documents “evergreen” is time-consuming and expensive

An illustrative description is provided in Fig. 2 as to how information should flow in a process that is designed to maintain process electrical safety information in a present or “evergreen” state.

FIG. 2. Information flow for keeping HAC documents “evergreen.”
FIG. 2. Information flow for keeping HAC documents “evergreen.”

HAC provides both present and new information that is fed into MOC and PHA processes. This HAC information is also used to determine specifications for process electrical equipment and installations. This information is then used to develop design specifications for engineering and installation specifications for construction.

Myth 3 is a widely held belief, but it is not true if the HAC documentation is included in the MOC process. Many changes are made in the manufacturing process that have no impact on the HAC, and so no formal changes to the record documents are needed.


A review of important points conveyed in this work includes:

  1. Myths associated with HAC can be debunked once the topic is understood from a practical perspective.
  2. The concept of “electrical” HAC assessments is not related to the term “electrical.” The methodology of HAC assessments is based on chemical processes. The ability to perform a proper HAC assessment is predicated on assigning someone who is knowledgeable in understanding the chemistry of the process.
  3. OSHA has established that electrical/HAC assessments are an integral part of required process safety information, and that the proper execution of these assessments is law in the US.
  4. The four deliverables from an HAC assessment are key in the design and selection of electrical equipment and the elimination or minimization of electrical energy as a source of ignition.
  5. The ability to properly perform an HAC compliance assessment requires a highly trained, experienced and knowledgeable professional who is well versed in NEC article 500, NFPA 497 and API-RP500. HP

The Authors

Related Articles

From the Archive



{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}