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Pressurization System Designs- Choosing a System
There are four primary factors that determine which purge system is appropriate for your application:
Area Classification
Area classification determines the type of purge system needed. For Division 1 areas, the equipment inside the enclosure determines whether a Type X system (equipment rated for general-purpose) or a Type Y system (equipment rated for Division 2) may be used.
Equipment Ratings
Equipment ratings inside the enclosure are important in evaluating which purge system to use in a Division 1 area. If the Division 1 area contains at least one general-purpose component, a Type X system is required. If all devices in the enclosure are rated for Division 2, then a Type Y system can be used. Special conditions exist for enclosures such as gas analyzers and chromatographs that contain a flammable gas. Refer to
NFPA 496 for more information.
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| Example of a 3003 top-mounted panel |
Enclosure Size
The size of the enclosure determines the size of the purge system. How the system is mounted depends on the position of doors, windows and cable entrances.
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Power Requirement
For Type X systems, the control unit operates the power disconnect to the enclosure. If the power requirement for the enclosure exceeds the contact ratings on the control unit, a control relay must be added. If the control relay is located in the hazardous area, it must be rated for that hazardous location. As power increases inside the enclosure, high temperatures become a problem. Refer to
NFPA 496 for more information.
Requirements for Alarms
For Type Y and Type Z purge systems, audible alarms or visual indicators must be used to alert operators that pressure inside the enclosure is below the NFPA minimum.
Alarms are connected directly to the enclosure and monitor the differential air pressure between the enclosure and the environment outside. These alarms are activated by a reduction in flow or pressure within the protective enclosure and have a direct connection to the enclosure, eliminating the need for an alarm on the protective gas supply.
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- The alarm must be located where the operator can see it easily
- The alarm must take its measurement from the enclosure only
- Alarms located in the hazardous area must be rated for the area
- Valves cannot be connected between the alarm and the enclosure
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