Purging Control Panels to Reduce Classification
By Tom O’Marra
Hazardous Locations, such as Class I, Division 2 or Class II, Division 2, are common areas to find equipment in the industrial control industry. These are locations, as defined by The National Electrical Code (NEC), where flammable gases or liquids are utilized (C1D2) or in the case of Class II, Division 2, combustible dust.
Under normal circumstances, these combustibles are contained, but should an equipment malfunction occur, they have the potential to become a source of ignition.
Any equipment utilized in these hazardous areas needs to be specified appropriately to ensure they have the proper approval ratings for the given environment. This is easier to achieve with control valves or pressure switches but can be difficult with control panels as it would not be practical or economic to source every component contained within to have a hazardous area classification rating.
One of the best options, in this case, is to install a purging system on the control panel. The purge system utilizes instrument air or other inert gases to create a constant positive pressure inside of the panel to keep out any combustible gases or dust. An explosion proof exhaust vent gets installed on the panel and allows the ambient air in the panel before the purge process begins to be vented to atmosphere during the purge process.
A pressure switch is utilized to engage once the control panel is up to the internal pressure as set by the user. Total time to get up to pressure depends on the size of the control panel. Should the pressure deviate to low above the setpoint, the pressure switch will open and indicate to the user that they are losing pressure withing the panel. Many times, the pressure switch will be tied to an alarm light or a shutdown circuit to stop operation while the cause of the pressure loss is remedied.
Utilizing these purging systems allows the control panel’s internal area classification to be reduced, either to down one Division or Zone or to Non-Hazardous. There are different types of purging definitions dependent on the hazardous area classification.
Type X: Type X purging reduces the zone classification with a control panel from Division 1 to Non- Hazardous.
Type Y: Type Z purging reduces the zone classification with a control panel from Division 1 to Division 2.
Type Z: Type Z purging reduces the zone classification with a control panel from Division 2 to Non- Hazardous.
Control panel purging systems can be analog control based, with a pressure gauge, or be fully digital with color touchscreen. The purge units themselves come in various electrical and ambient ratings to best suit the overall application. Units can be flush mounted in a panel door or mounted externally.
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