Everyone once in a while we come across purged and pressurized enclosures during our PSM/RMP audits involving processes that contain/process flammable liquids or gases. These purged and pressurized enclosures are used in lieu of using enclosures that are either intrinsically safe or explosion proof. But this method of protecting the process from ignition sources is NOT as simple as some would hope; in fact the method comes with its very own Recognized and Generally Accepted Good Engineering Practice (RAGAGEP), NFPA 496: Standard for Purged and Pressurized Enclosures for Electrical Equipment. Here are some requirements that may be a surprise to some:
#1 Not all enclosures are permitted to be used in this manner
Suitability of identified equipment shall be determined by one of the following:
- Equipment listing or labeling
- Evidence of equipment evaluation from a qualified testing laboratory or inspection agency concerned with product evaluation
- Evidence acceptable to the authority having jurisdiction such as a manufacturer’s self-evaluation or an owner’s engineering judgment.
#2 Enclosures MUST be constructed of material that is not likely to be damaged under the conditions to which it may be subjected.
#3 Enclosures MUST be protected from excessive pressure of the protective gas supply.
#4 Enclosure MUST be constantly maintained at a positive pressure of at least 25 Pa (0.1 in. of water) above the surrounding atmosphere during operation of the protected equipment.
#5 Where the protective gas supply is used to supply other than Type X pressurized equipment, an alarm MUST be provided to indicate failure of the protective gas supply to maintain the required pressure.
#6 ALL pressurizing system components energized in the absence of the protective gas MUST be identified for the classification that would exist in the absence of the pressurizing system.
#7 Piping for the protective gas shall be protected against mechanical damage.
#8 If compressed air is used, the compressor intake MUST be located in an unclassified location.
#9 The electrical power for the protective gas supply (blower, compressor, etc.) shall be supplied either from a separate power source or from the protected enclosure power supply before any service disconnects to the protected enclosure.
#10 A permanent marking MUST be on the protected enclosure in a prominent location so that it is visible BEFORE the protected enclosure can be opened.
#11 Protective Gas Supply Valve MUST have the following in a permanent marking: WARNING: PROTECTIVE GAS SUPPLY VALVE, This valve must be kept open unless the area atmosphere is known to be below the ignitible concentration of combustible materials or unless all equipment within the protected enclosure is de-energized.
#12 Where the protective gas is other than air, the following WARNING shall be posted: WARNING: Protective Gas Release Poses Potential for Asphyxiation
Specific Location Requirements
Type Z Pressurizing
This method reduces the classification within an enclosure from Division 2 to unclassified. With Type Z pressurizing, a hazard is created only if the pressurizing system fails simultaneously with the area outside of the enclosure becoming ignitible due to the release of normally contained flammable liquids or gasses, or combustible dust. For this reason, it is not considered essential to remove power from the equipment upon failure of the pressurizing system.
#1 Detection MUST be provided to indicate a failure to maintain a positive pressure within a protected enclosure.
#2 Failure to maintain the positive pressure within a protected enclosure MUST be communicated by an alarm or an indicator.
#3 Any protected enclosure that can be isolated from the protective gas supply shall be equipped with an alarm.
Exception: The protected enclosure shall be permitted to be equipped with an indicator where the isolation is done with a valve(s) that complies with the following:
- The valve is IMMEDIATELY ADJACENT to the protected enclosure.
- The valve(s) is intended for use ONLY during servicing of the protected enclosure.
- The valve(s) is marked as stated in #11 above.
#4 Where an alarm is used, the following shall apply:
- The alarm MUST be located at a CONSTANTLY ATTENDED location.
- The alarm actuator MUST take its signal from the protected enclosure and can NOT be installed between the enclosure and the protective gas supply.
- The alarm actuator MUST be mechanical, pneumatic, or electrical.
- Electrical alarm actuators MUST be identified for a Division 2 or Zone 2 location as applicable.
- NO valves are permitted between the alarm actuator and the enclosure.
#5 Where an indicator is used, the following shall apply:
- The indicator MUST be located for convenient viewing.
- The indicator can NOT be installed between the enclosure and the protective gas supply.
- The indicator MUST indicate either pressure or flow.
- NO valves are permitted between the indicator and the enclosure.
- The protective gas supply MUST have an alarm that is located at a CONSTANTLY ATTENDED location.
Type Y Pressurizing
This method reduces the classification within an enclosure from Division 1 to Division 2. Equipment and devices within the enclosure must be suitable for Division 2. This requires that the enclosure does not contain an ignition source. Thus, a hazard is created within the enclosure only upon simultaneous failure of the pressurizing system and of the equipment within the enclosure. For this reason, it is not considered essential to remove power from the equipment upon the failure of the pressurizing system.
#1 All of the requirements for Type Z Pressurizing apply
#2 Equipment within a protected enclosure MUST be approved for Division 2 or Zone 2 locations.
#3 Electrical alarm actuators MUST be identified for a Division 1 or Zone 1 location as applicable.
#4 Ventilated equipment that would develop temperatures higher than the marked temperature class (T Code) upon failure of the ventilation MUST be AUTOMATICALLY de-energized when the flow of protective gas STOPS.
Type X Pressurizing
This method reduces the classification within an enclosure from Division 1 to unclassified. Because the probability of a hazardous atmosphere external to the enclosure is high and the enclosure normally contains a source of ignition, it is essential that any interruption of the pressurizing results in de-energizing of the equipment. Also, it is essential that the enclosure is tight enough to prevent an escape of molten metal particles or sparks.
#1 A device MUST be provided to AUTOMATICALLY de-energize ALL circuits and equipment within the protected enclosure that are NOT identified for Division 1 or Zone 1 upon failure of the protective gas supply to maintain positive pressure.
Exception: Power shall be permitted to be continued for a short period IF immediate loss of power would result in a more hazardous condition, and IF BOTH audible and visual alarms are provided at a constantly attended location.
#2 The device provided to de-energize power upon failure of the protective gas supply to maintain positive pressure MUST be actuated by flow or pressure sensors.
#3 The flow or pressure sensor MUST be identified for the location in which it is installed.
#4 NO valves are permitted between the flow or pressure sensor and the protected enclosure.
#5 The sensor shall take its signal from the protected enclosure and MUST NOT be installed between the enclosure and the protective gas supply.
PLEASE note this is just scratching the surface of NFPA 496. The Code also provides a design for pressuring control rooms and analyzer rooms. But as you read this posting I hope it became clear that using purged and pressurized enclosures are NOT a means to fully eliminate the need to design an area as a Hazardous Location!