Purged and Pressurized Systems

This is a continuation of my last column, which was about the different types of purged and pressurized systems used in modern control rooms. Electrical control components and instrumentation devices and other equipment located in control rooms at refineries, chemical plants and similar facilities also are suitable for use in hazardous or classified locations. If purged and pressurized systems are installed in these rooms, the National Electrical Code (NEC) permits general-purpose equipment intended for use in unclassified locations.

The NEC recognizes equipment in National Fire Protection (NFPA) 496 that can be used to reduce, limit or even eliminate hazards by providing adequate positive-pressure ventilation from a source of clean air. Such ventilation must be coupled with effective safeguards against ventilation failure. Requirements included in Chapters 4, 5 and 7 of NFPA 496-2003, Purged and Pressurized Enclosures for Electrical Equipment, provide the necessary guidelines for preventing the entry of flammable vapors or gases into the control room. Naturally, the enclosures in the control room must be capable of housing the electrical components.

A control room typically contains data processes, processing control instruments, communications, HVAC equipment and power for lighting, power panels and more. However, many factors must be considered when laying out a control room.

In order to calculate the necessary volume of air in a control room, including access requirements, the number of people and type of equipment to be housed must be known, as well as the location of the room, which is relative to the direction of the prevailing wind. It also is necessary that the location of the processing units (relief values, vent stacks, emergency relief systems, etc.) be identified and logged.

Next, the control room must have a positive pressure of at least 25 Pa (one-tenth inch of water) and must be maintained with all openings closed. However, the Code permits this minimum air pressure to drop to a lower level when doors and other apertures remain open, providing a minimum air velocity of 60 ft. per min. (0.3 m per sec.) through the openings.

Determining the location

The location of the control room as well as the relation to the source of gases or vapors must be evaluated with special attention paid to the direction of the prevailing wind. For example, one side of the room may face a location generally free from trace amounts of flammable vapors or gases, or the height of the fan intake may be used to provide a clean source of air. If ducting is needed to reach an uncontaminated source, it must be noncombustible material, free of leaks, and protected against damage or corrosion. Sensitive pressure switches and other devices must be installed to appropriately measure these low values.

Type X equipment

If a control room in a Class 1, Division 1 location contains equipment that functions safely only in an unclassified location, then Type X equipment must be installed. Type X purging reduces the inside area to an unclassified location while the outside area is classified as a Class 1, Division 1 location. Power must be disconnected immediately when the positive pressure fails. A sensing device (cutoff switch) must be used to detect failure of the system at the discharge end of the fan. This type of device energizes an audible or visual alarm located in a constantly attended and monitored position (see the Ex. to Sec. 4.10.1 of NFPA 496, which permits an alternative method).

The electrical power circuit feeding the positive-pressure air system equipment must be tapped ahead of the main service-disconnecting means supplying the control room. The air-flow-monitoring switch, disconnecting means and motor for the air system fan must be suitable for the unclassified location. Provisions for repressurization are permitted if the air system should fail. Properly installed detectors ensure the atmosphere surrounding the electrical equipment and accessories is nonflammable. It also is possible to use a purge timer to prevent reapplying the power too soon after the pressurizing air system is restarted. The time period should be set to allow the room to be swept with at least four changes of air.

Types Y and Z equipment

Type Y purging reduces the classification within a control room from Class 1, Division 1 to Division 2. This purging system is normally used for equipment that operates after the positive air pressure has failed. Type Z purging reduces Class 1, Division 2 to unclassified. If the location of the control room and/or equipment is suitable for this type of purging, it is not necessary to disconnect the power immediately upon failure of the positive-pressure air system. Although, for safety reasons, equipment should be disconnected as fast as possible.

Be familiar with these basic rules, and for those seeking more detailed information, see NFPA 496.  EC

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.




About the Author

James G. Stallcup

Code Contributor
James G. Stallcup is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.

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