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Purging and Pressurizing Methods
National Fire Protection Association (NFPA) and the Instrument Society of America (ISA) define several techniques for protecting equipment. Most equipment requires only basic pressurization in Class II areas or purging in Class I areas. Ventilation and dilution are advanced protection methods for heat producing or flammable gas analyzing equipment.
Purging common equipment in Class I Areas As strictly defined by NFPA 496, purging is a start-up process of Class I area pressurizing which removes flammable vapors from a protected enclosure. This is accomplished by exchanging a known volume of protective gas while maintaining a minimum positive enclosure pressure of 0.10 inches (2.5 mm) of water. The 2003 edition of NFPA 496 recommends 4 volume exchanges for all enclosures and 10 volume exchanges for all motors.
Pressurization of common equipment in Class I & II Areas Pressurization prevents the entrance of flammable gas or combustible dust into protected enclosures. In Class II areas, this is accomplished by manually removing any dust and then applying a protective gas supply to maintain a positive enclosure pressure of 0.50 inches (12.7 mm) of water. In Class I areas, this is accomplished by purging, and by then maintaining a minimum positive enclosure pressure of 0.10 inches (2.5 mm) of water. Power can then be applied to the protected equipment under conditions established by the Division rating.
Ventilation - Hot equipment in Class I & II Areas Ventilation provides protection and also removes or dissipates heat from electrical devices within a protected enclosure. Ventilation is commonly used to cool equipment or reduce enclosure surface temperatures. Ventilation requires high airflow and is commonly performed with blowers for high voltage switchgear devices.
Dilution - Analytical equipment in Class I Areas Dilution provides protection and also continuously removes or dissipates flammable gases within a protected enclosure. Dilution may require the use of nitrogen to blanket the enclosure. Otherwise, a higher flow of instrument air will likely be required.
Pressurization Standards NFPA 496 is the American standard for design, marking and performance of enclosures and pressurization systems. ISA design standards address construction, installation and testing of protected enclosures and pressurization systems.
History of NFPA and ISA design Standards.
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