Hazardous areas exist in every power plant.Following proper design rules and anticipating how the plant will be operated can avoid creating mixtures of gases that could explode or catch fire. Quantifying electrical safety risks and reducing them to an acceptable level is a team effort. The designer must follow applicable codes and standards when engineering the plant, and operators must run and maintain it within those guidelines. Does your staff know where the hazardous areas are in your plant? Are hazardous area classifications made years ago still appropriate under current standards? Cataloging risks Hazardous area classification is a rigorous method of determining where an explosive environment may be present. The codes and standards used in this process provide guidance for selecting, building, and installing electrical equipment in that area. In general, hazardous areas (HAs) are defined by: • The type of hazard or hazardous material. • The likelihood of its presence in flammable concentration. • The auto-ignition temperature of the hazardous material. An explosive atmosphere can be created by a gas or vapor, dust, or even fibers. Methane-based hazards dominate the gas-fired generation sector, whereas spontaneous combustion of coal dust plagues plants fired by that fuel. The different zones defined by codes for classifying particular hazardous areas are listed and discussed in the online supplement to this article. The probability that any hazard exists in combustible concentration is determined by the specifics of the plant or system under consideration. For example, a natural gas vent line is much more likely to contain such a hazard than a lube oil line—unless, of course, the oil line’s flanged joint is leaky. Finally, plant design also must protect against auto-ignition of combustible substances. A good example of this type of hazard is a flammable gas coming into contact with a hot surface. Codes define various temperature classes to guide designers as they specify equipment. In the field By design, natural gas-fired power plants are fueled by an extremely combustible substance. But fuel lines and the turbine deck are hardly the only HAs in the plant. The prerequisites for developing an area classification for a gas-fired plant are understanding the characteristics of all of hazardous materials present in it and defining where those materials exist (or may spread due to a leak). This allows mapping out the plant’s HAs. defines hazardous (classified) areas as those where fire or explosion hazards may exist due to the presence of flammable gases. The above conditions constitute the well-known “fire triangle” (Figure 1). and classified areas—are all synonymous with EAC. The approach described in this article focuses on a combustion turbine-generator (CTG) plant. vapors. • The release of energy is sufficient to ignite the mixture.com. Burns & Roe is not responsible for classifications prepared by others using these guidelines. He can be reached at 201-986-4339 or rsaini@roe. although the same methods are applicable to any type of power plant and its systems. electrical area classifications explain how to select and install electrical equipment and wiring—right down to the wiring method—to minimize the likelihood of ignition of a flammable or explosive mixture. NFPA 70. the term EAC is used to avoid confusion with other nonelectrical classifications. For example. The National Electrical Code (NEC). is principal engineer and can be reached at 856-638-6920 or cemma@roe. combustible dust. the source of energy is understood to be the facility’s electrical system. Disclaimer: The guidelines mentioned in this article describe approaches used by Burns and Roe Enterprises Inc. PE. In the context of electrical equipment. is senior principal engineer for Burns and Roe Enterprises Inc. or ignitable fibers. The information is provided solely to educate power plant designers and staff. PE. In the context of EAC. or liquids.com. Charles Emma. hazardous locations. all three of the following statements must be true: • Flammable or combustible material is present. —Ram K. The result of the process is usually called the EAC of the facility. • The material is mixed with air in the proportions required to produce an ignitable mixture. hazardous (classified) locations. .Each HA should be classified according to applicable industry codes and standards. Taxonomy and terminology For a fire or explosion to occur. In the following guidelines. Electrical area classification (EAC) is the process of determining the existence and extent of hazardous locations in a facility containing any of those substances. the following terms—area classification. Saini. 1. ethyl ether Group D: Acetone.S. The designations facilitate the approval and listing of equipment suitable for use in hazardous locations. or liquid is present. butane. the NEC has permitted hazardous locations to be categorized using either of two “concepts” or methods: Class I. The information presented in this article is based on the “Class. heat. and III. Division” concept.) Group (material properties) Hazardous chemicals and flammable materials are given group designations based on their propensity to explode or catch fire. and 2 (NEC Article 505).Air. NEC Article 500 segregates chemicals and materials as follows: • • • • Group A: Acetylene Group B: Hydrogen Group C: Cyclopropane. propane. The second method complies with International Electrotechnical Commission (IEC) standards and has been used in Europe. and fuel must be present to start a fire. II. vapor. Zone 0. Hazardous locations are categorized by class. Areas are classified in accordance with the type and properties of the combustible material and the likelihood that an ignitable mixture will exists. Class II: Denotes areas where combustible dust is present. fuel oil . (Class III areas rarely exist in CTG facilities. Since 2002. hexane. Class III: Denotes areas where ignitable fibers are present. group. natural gas. National Fire Protection Association (NFPA) Recommended Practices 497 and 499 and other industry sources also group chemicals in this way. 1. The first method has traditionally been used in the U. Division 1 and 2 (NEC Article 500). and division as follows. Class (material type) Class I: Denotes areas where flammable gas. and Class I. • Group E: Combustible metals Group F: Carbonaceous materials. Division 2. Accordingly. and cost overruns. A typical area classification for propane gas would be: Class I. including coal dust • Group G: Flour. HVAC engineering. building arrangements. special-purpose mechanical equipment with long lead times (motors and instrumentation and control systems and components. 450C AIT. pressures. Auto-ignition temperature The auto-ignition temperature (AIT) of the hazardous material in a facility must be known to complete its EAC. Nonclassified or unclassified: Denotes areas where the presence of hazardous material is so infrequent that Electrical Area Classification is deemed unnecessary. process operations and conditions. Here: • Class I indicates the presence of vapor. for example) must be specified and ordered early. NFPA 497 provides AIT values for various combustible substances. EAC dos and don’ts An EAC is best performed by a project team whose members are knowledgeable about classification. confusion. fire detection and protection systems. They include the properties and behavior of materials. • Group D indicates that propane is a member of this group. The requirements are intended to prevent electrical equipment from being the ignition source for a flammable mixture. and . a facility’s EAC must be known before any electrical equipment can be specified. or installed. Many variables must be included in area classification equations. Division 2: Denotes areas where flammable or combustible concentrations exist under abnormal operating conditions or are not likely to exist under other conditions. temperatures. maintenance procedures. chemicals used. delays. volumes. and plant safety. adequacy of mechanical ventilation and gas detection systems. Once an area has been classified. • 450C is the auto-ignition temperature of propane. Division 2 indicates that the vapor is present only under abnormal conditions. • Obviously. the installation itself must be explosion-proof. plastic Division (the likelihood of hazardous atmosphere) • Division 1: Denotes areas where flammable or combustible concentrations exist under normal operating conditions or are highly likely to exist under other conditions. electrical equipment and wiring. rework. applicable codes and standards. designed. the NEC provides very specific and stringent requirements about the electrical equipment and associated wiring that can be installed within that area. piping and equipment construction. flows. Failure to determine the EAC for the facility and such equipment in a timely fashion can result in unsafe installations. weather. starch. On many CTG power projects. Group D. and NFPA 499 covers Class II areas containing dusts. metering points. should produce a consistent and safe classification. These practices assume the ability to identify and locate sources of hazardous material. and liquids. Eschewing this approach in favor of classifying an entire facility using a broad-brush approach (a process referred to as “blanket classification”) almost always leads to a more dangerous and ultimately more costly facility. Careful evaluation of all data in accordance with established techniques. testing. areas with large quantities of process equipment and piping that handle flammable/combustible materials can. along with the application of sound engineering judgment. 3. nine-step outline of NFPA practices. support. section. as an area. Locate material release sources. be considered a release source. The NEC calls for each room. and maintenance areas. sampling points. such as open process points. and certain process operations. piping and instrumentation diagrams. Although many other companies and industry organizations have written specific procedures for Area Classification. The NFPA way What follows is a brief. ventilation. This is especially true for a power plant. The sources can be necessarily open process points or unintentional leaks in the process. Once sources are located. 2. vapors. Confirm the need for classification by assessing the likelihood of the presence of flammable and combustible materials. including: codes. They also provide diagrams for typical situations that give recommended distances from the source for the extent of a hazardous location. flow rate. and reference. building construction. and vents. Assemble pertinent information. As a practical matter. See NFPA 497 and 499 for additional guidance in identifying sources. practices. equipment arrangement drawings and plot plans. process flow diagrams and material and heat balance chart. the practices provide guidance in determining the degree of the hazard (Division 1 or Division 2) and its extent. might result in a few Division 1 areas and several much larger Division 2 and nonclassified areas. control valves. . List all flammable and combustible materials and their pertinent properties. Although the diagrams take into account pressure.operator interfaces. and commissioning. NFPA 497 covers Class I areas containing gases. it must be remembered that they are only typical and require sound engineering for application to specific situations. and safety procedures. the NFPA Recommended Practices for Classification are most applicable to power plants. To determine an electrical area classification: 1. A properly conducted EAC. The actual practices and associated references should be consulted when performing any EAC. or area of a facility to be considered individually in determining its classification. operating. in a well-designed and constructed facility. 4. such as ignition temperatures and flash points. volume. drains. standards. process and operating descriptions. pump seals. which normally contains several different process. 1-1999: Safety Requirements for the Storage and Handling of Anhydrous Ammonia ANSI C2-2002: National Electrical Safety Code – Section 127 ANSI/ASHRAE 15: Safety Code for Mechanical Refrigeration (for ammonia) Factory Mutual (FM) Underwriters Laboratory (UL) standards Occupational Safety and Health Administration (OSHA) standards . or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas American National Standards Institute (ANSI) ANSI K61.S. Determine the extent (or distance from the source) of hazardous areas by reviewing the plant’s equipment layout drawings. a. Consider using recognizable boundaries (walls. Prepare the recommended documentation for the EAC project team to review and approve. 8. Gases. Storage and Handling of Liquefied Natural Gas NFPA 70-2005: National Electrical Code (Chapter 5) NFPA 497-1997: Classification of Flammable Liquids. floors. This approach will greatly simplify both design and installation. 6. Determine the degree of hazard (Division 1 or Division 2) by assessing the probability of presence. U. ceilings. and column lines) to delineate Classified Areas. Determine an area’s Class and Group from the chemicals present. standards National Fire Protection Association NFPA 30-2003: Flammable and Combustible Liquids Code NFPA 50A-1999: Standard for Gaseous Hydrogen Systems at Consumer Sites NFPA 59-2001: Standard for the Production. 7.5. 9. Applicable industry codes and standards The following codes and standards are particularly applicable to Hazardous Classified Areas in CTG power plants. Ignition gas systems. gas relief points. gas distribution piping. the fuel oil booster pump and leakage tank. Fuel oil systems.b. At-risk systems and equipment The main components and systems in a CTG power plant that are vulnerable to fire or explosion follow. d. the seal oil unit. Other Numerous other materials are available for reference through industry sources. piping to ignition burner and combustor. Plant battery systems (including battery rooms). the filter and scrubber station. Ammonia systems. and turbine auxiliaries equipment/compartment. Fuel oil becomes flammable when heated above its flash point. These include propane gas cylinders. gas analyzer system. and gas relief points. main shut-off valve. gas flow metering. gas compressor station. These include fuel oil tanks. European standards International Electrotechnical Commission (IEC) standards IEC 79-10 (Part 10): Classification of Hazardous Areas IEC 79-14 (Part 14): Electrical Installations in Explosive Gas Atmospheres IEC 79-15 (Part 15): Electrical Apparatus with Type of Protection “n” c. . These include the hydrogen gas cylinder station. the gas control valve block. the fuel oil treatment system. pumps. CTG vendor documentation Major CTG vendors often supply Area Classification documents pertaining to their systems and equipment. fuel oil metering. the gas safety relief valve. These include the gas-receiving station. the gas unit. the fuel oil unloading and forwarding pump station. Fuel gas systems. and piping. Hydrogen gas for generator cooling systems. and diked areas. the fuel oil control valve block. and fuel oil piping to burners. These include storage tanks. the gas preheater. the control cubicle. Propane gas (ignition) system • The ignition gas cabinet. indicators. Fuel gas system • Flange gasket connection points on local piping. hydrogen gas. Fuel oil system • Fuel oil transfer pipe joints and coupling in the fuel oil�unloading area and forwarding pump stations. alarm switches. and ammonia gas systems. vaporizer. and sumps. • Pressure relief devices. Anhydrous ammonia system • Storage tanks. tubing. filling and suction valve blocks. • Fuel oil tank piping. Bounding hazardous classified locations Using the references listed above. • Gas relief points above the turbine building roof. determine the extent of the Classified Area for each type of leak source shown in the table. Some are identified by the CTG vendor. oil piping (both outdoor and at the combustor). • Open process points. with screwed couplings and glands. Hydrogen gas system • H2 gas storage cylinders. pump blocks. sample points. vents. • Battery rooms (hydrogen gas is released from lead acid�flooded cell batteries as they charge). or at equipment. • Seals (unless seal-less) at pumps and control valves. . propane gas (ignition). burners. in the turbine auxiliary equipment area. and heaters. and diked areas. • The H2-cooled generator.Potential leak sources Following are some of the most common potential leak sources in fuel gas. fuel oil. • Gas relief points located in the open air above the turbine building roof for equipment located within the turbine area. • The H2 cooling unit and separator. • Flange gaskets or screwed connections in main headers. Source: Burns and Roe Enterprises Inc.Hazardous materials and where they typically exist in a power plant. Note 1: Adequately ventilated per NFPA 497 means a ventilation rate that affords either six air . and the plant’s operations staff to ensure their understanding of the affected areas. they should be discharged outdoors at a minimum distance of 10 feet (for natural gas) and 15 feet (for hydrogen) from any electrical equipment. The documentation also provides the necessary guidance to engineers working on facility design. and 3 are the plan and elevation drawings used for classifying the hazardous areas around a combustion turbine-generator. not mentioned in the table. These guidelines should be used as a minimum requirement for Classification of Electrical Areas where such materials are located and processes are performed. EAC must be carefully documented because it serves as the basis for electrical equipment requirements in classified areas. To avoid undue expense. A deluge water spray system should be installed for automatic/manual actuation. the construction contractor. as well as to insurance and inspection personnel. Equipment manufacturers’ recommendations for the Area Classification of specific equipment should be followed. respectively. the potential leakage sources. especially if they are more stringent than the guidelines provided in the table. The design engineer should prepare a hazardous location design basis document defining the plant hazards. Support drawings and documents Like any engineering study or analysis. or extending classified areas to recognizable boundaries. When there is more than one leakage source in an area (for example. take precautions to verify that boxing in overall areas. Figures 4 and 5 are similar drawings for the roof area of a hydrogen-cooled generator and an ammonia vaporizer skid. The protective measures ensure the safety of the O&M personnel working in the facility. Sound engineering judgment should be applied to determine if additional areas. Figures 1. along with properly implemented protective measures. its alarm should be sent to the fire alarm panel in the plant control room. Note 4: Ammonia detectors should be installed around the diked area and connected to the plant’s distributed control system. etc.changes per hour. a manifold serving several instruments or valves). does not include electrical equipment that would otherwise not be included in the hazardous area. Note 2: If vent pipes are being discharged in an otherwise nonhazardous area. and the extent of the classified areas. 2. Documentation should include: . Such documentation. Note 3: A continuously operated exhaust fan in the battery room is required to remove any accumulation of hydrogen gas discharged from the batteries. or if there are several pieces of equipment with potential leak sources. reduces the possibility of an explosion or fire. light fixture. should be identified. The design basis document and the associated plant drawings that show the extent of the horizontal and vertical boundaries of each classified area should be discussed with the owner. Hazardous area classification drawings should be prepared for each plant and should include similar details for the use of the plant construction contractor and the operations staff. 1 cfm per square foot of floor area. Loss of electrical power to the fan should be alarmed in the plant control room. the area should be boxed out as an overall three-dimensional shape covering the limits of the extreme leakage points. or another similar criterion that prevents accumulated vapor-air concentrations from exceeding 25% of the lower flammable limit. and scrubber. along with the rationale for making the determination. practices.) and how and where the materials are handled. etc. as well as their pertinent properties (flash point. normal and leakage sources. . and boundaries. references. A discussion of all the assumptions made during the analysis.• • • • • • A listing of all the flammable and combustible materials used in the facility.The EAC for the area around a heat-recovery steam generator. Brief descriptions of the process and its O&M and cleaning details. 2. A listing of each room or area and its determined area classification. A complete set of EAC drawings indicating all of classified area’s process equipment. standards. A list (including dates and/or editions) of all the codes. and other data used to prepare the classification. Courtesy: Burns & Roe Enterprises Inc. density. fuel gas heat exchanger. ignition temperature. Hazardous area classification for the area around a combustion turbine-generator. including the hydrogen manifold and connections. Courtesy: Burns & Roe Enterprises Inc.3. . Hazardous area classification for the roof above a hydrogen-cooled generator. Courtesy: Burns & Roe Enterprises Inc.4. . Instrumentation enclosures may be the general-purpose type if they are part of intrinsically safe systems and if they are installed per NEC Article 504. Cable trays and cables. Rigid metal conduit is the allowable wiring method in Division 1 areas. provided that specific rules in NFPA 496 are followed. . Purging and pressurization of enclosures are permitted to prevent the entrance of flammable and combustible material. Courtesy: Burns & Roe Enterprises Inc. Rather than providing specific design guidelines. the following brief list of NEC rules is intended only to convey an awareness of the complexity of electrical design in hazardous areas. • • • • • • • • • Explosion-proof and dust-ignition proof equipment is required in Division 1 areas and for certain types of equipment in Division 2 areas. the NEC provides stringent requirements for the design and installation of electrical equipment within it. These rules should be followed very carefully. making exceptions only if absolutely necessary. General-purpose equipment is permitted for certain applications in Division 2 areas. once the EAC for an area has been determined. Electrical equipment must have temperature ratings or operating surface temperatures below the AIT of the hazardous substance present. Designing for hazardous areas As mentioned earlier. Experienced and knowledgeable engineers are needed to apply the NEC rules to specific situations. are permitted in Division 2 areas.5. under certain conditions. Equipment approved by Underwriters Laboratories or by another appropriate agency is preferred in hazardous locations. It is preferable to locate electrical equipment outside the hazardous area.Hazardous area classification for the area around the ammonia vaporizer skids and fuel gas valves and devices at a combined-cycle power plant. Burns and Roe is not responsible for classifications prepared by others using these guidelines. . The information is provided solely to educate power plant designers and staff.Disclaimer: The guidelines discussed in this article describe approaches used by Burns and Roe Enterprises Inc.