Purged and Pressurized Enclosure Protection Techniques in HAZLOCs

With OSHA’s Process Safety Management standard coming up on its 27th birthday, we are seeing companies still grappling with managing their Hazardous Locations (HAZLOC). When the process involves a flammable liquid or gas, it is still fairly common to find a HAZLOC containing permanently installed equipment that is not rated for the HAZLOC. And by equipment, I am not talking about mobile equipment or small components… I mean we still find electrical panels the size of a big screen TV inside the HAZLOCs. And as usual, the facility scrambles to find a solution to abate the recognized hazard; however, often times the abatement falls short of meeting the most recognized RAGAGEP – NFPA 70 (e.g. the National Electric Code). Specifically, I want to discuss the use of purged and pressurized enclosures as a “Protection Technique” as this seems to be the root of evil for many of these facilities who rely on contractors “expert advice” which leads them astray.

NOTE: I am NOT an electrical expert and do not attempt to even install ceiling fans at my home. I hire certified electricians for my electrical work; however, as a safety engineer, I have over 25 years working with NFPA codes related to flammable hazards, specifically HAZLOCs.

So let’s start with what NFPA 70 says about the “Purged and Pressurized Enclosure Protection Technique”…

500.7(D) Purged and Pressurized. This protection technique shall be permitted for equipment in any hazardous (classified) location for which it is identified.

Using this protection technique is VERY COMMON when an electrical cabinet/enclosure not rated for a Class I location is found within a Class I HAZLOC. Companies want to keep the cabinet there and to do so they need to assess their options. Many come to the conclusion that merely putting some plant air into the cabinet will abate the hazard of this improper cabinet being located in a HAZLOC. In some situations, they may be correct; however, in most situations, they will be wrong.

We turn to another NFPA standard for some specific guidance for this protection technique… NFPA 496: Standard for Purged and Pressurized Enclosures for Electrical Equipment

And in this standard, we begin to see the rationale for the different purging and enclosure needs based on the type of HAZLOC the cabinet is located within. We have three (3) types of arrangements we can use:

1. Type X Pressurizing
2. Type Y Pressurizing
3. Type Z Pressurizing

Each of the types of pressuring techniques has its proper application and LIMITATIONS.

Type X Pressurizing enables the use of equipment suitable for unclassified locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 1 or Zone 1 locations.

Type Y Pressurizing enables the use of equipment suitable for Division 2 or Zone 2 locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 1 or Zone 1 locations.

Type Z Pressurizing enables the use of equipment suitable for unclassified locations within the protected enclosure where the equipment would otherwise be required to be suitable for Division 2 or Zone 2 locations.

 

Let’s start with Type Z Technique as it is the lowest risk protection technique, as it enables the use of equipment suitable for UNCLASSIFIED LOCATIONS within the protected enclosure where the equipment would otherwise be required to be suitable for DIVISION 2 HAZLOCs.   Type Z Pressurizing requires some type of FAILURE INDICATOR when the positive pressure within a protected enclosure is lost. This failure indicator is usually an audible and/or visual alarm; however, a visual gauge can also be viewed as an acceptable indicator, as in Type Z, the indicator is NOT required to de-energize the protected equipment upon detection of the failure to maintain a positive pressure within the protected enclosure.  When using the Type Z technique, any protected enclosure that CAN BE ISOLATED FROM THE PROTECTIVE GAS SUPPLY MUST be equipped with an ALARM. However, there is an exception where the isolation is done with a valve(s) that complies with the following:

1. The valve is immediately adjacent to the protected enclosure.
2. The valve(s) is intended for use only during servicing of the protected enclosure.
3. The valve(s) is marked with the following text:

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.

 

Where an ALARM is used in Type Z, the following shall apply:

1. The alarm shall be located at a CONSTANTLY ATTENDED LOCATION.
2. The alarm actuator shall take its signal from the protected enclosure and shall NOT be installed between the enclosure and the protective gas supply.
3. The alarm actuator shall be mechanical, pneumatic, or electrical.
4. Electrical alarm actuators shall be identified for a Division 2 or Zone 2 location as applicable.
5. NO valves shall be permitted between the alarm actuator and the enclosure.

Where an INDICATOR is used in Type Z, the following shall apply:

1. The indicator shall be located for CONVENIENT VIEWING.
2. The indicator shall NOT be installed between the enclosure and the protective gas supply.
3. The indicator shall indicate either PRESSURE OR FLOW.
4. NO valves shall be permitted between the indicator and the enclosure.

 

Type Y Technique would be for those occasions where a cabinet/enclosure that is suitable for a Class I, Div 2 HAZLOC but is being used inside a Class I, Div 1 HAZLOC.

All of the requirements for Type Z are required, as well as the following:

1. Electrical alarm actuators shall be rated/approved for a Division 1 or Zone 1 location
2. Ventilated equipment that would develop temperatures higher than the marked temperature class (T Code) upon failure of the ventilation shall be automatically de-energized when the flow of protective gas stops.

 

The Type X technique carries the highest risk of the three (3) techniques as it enables the use of equipment suitable forUNCLASSIFIED LOCATIONS within the protected enclosure where the equipment would otherwise be required to be suitable for a Class I, Division 1 HAZLOC. Thus Type X Pressurizing technique requires:

1. A device shall be provided to AUTOMATICALLY DE-ENERGIZE ALL CIRCUITS AND EQUIPMENT WITHIN THE PROTECTED ENCLOSURE that are NOT rated for Division 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 the 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 shall be actuated by FLOW OR PRESSURE sensors.  There can be NO VALVES between the flow or pressure sensor and the protected enclosure. The sensor shall take its signal from the protected enclosure and shall NOT be installed between the enclosure and the protective gas supply.
3. Equipment, such as motors or transformers, that can be overloaded shall be provided with devices to detect any increase in temperature of the equipment beyond its design limits and shall de-energize the equipment automatically.  Exception: Power to the circuits shall be permitted to be continued for a short period if the 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.
4. For ventilated equipment, the flow of protective gas shall provide SUFFICIENT COOLING EVEN DURING OVERLOAD CONDITIONS, or the equipment subject to overloading shall be provided with devices to detect any increase in temperature beyond its design limits and to de-energize that equipment automatically.  Exception: Power to the circuits shall be permitted to be continued for a short period if the 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.

5. A means shall be used to prevent energizing of electrical equipment within the protected enclosure until at least four enclosure volumes of the protective gas (ten volumes for motors, generators, and other rotating electric machinery) have passed through the enclosure while maintaining an internal pressure of at least 25 Pa (0.1 in. of water).

6. If the enclosure can be readily opened without the use of a key or tools, an interlock shall be provided to immediately de-energize all circuits within the enclosure that are not approved for the location when the enclosure is opened. 

7. The interlock, even though located within the enclosure, shall be approved for external area classification.

8. Protected enclosures that contain hot parts requiring a cool-down period shall be designed to require the use of a key or tool for opening.

    

 

When using purged and pressurized protection techniques, the gas being used to pressure the cabinet/enclosure MUST meet some quality and flow requirements, such as:

1. be free of contaminants and shall contain no more than trace amounts of flammable vapor or gas.  Air of normal instrument quality, nitrogen, or other nonflammable gas shall be permitted as a protective gas.
2. protective gas supplies shall be designed to minimize chances for contamination.
3. piping for the protective gas shall be PROTECTED AGAINST MECHANICAL DAMAGE.
4. Where compressed air is used, the COMPRESSOR INTAKE SHALL BE LOCATED IN AN UNCLASSIFIED LOCATION.
5. Where the compressor intake line passes through a classified location, it shall be constructed of noncombustible material, designed to prevent leakage of flammable gases, vapors, or dusts into the protective gas, and protected against mechanical damage and corrosion.
6. 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.
7. Where “double pressurization” is used (e.g., a Division 1 enclosed area pressurized to a Division 2 classification that contains ignition-capable equipment also protected by pressurization), the protective gas supplies shall be independent.
8. The protected enclosure shall 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.  Where the protective gas supply is used to supply other than Type X pressurized equipment, AN ALARM SHALL BE PROVIDED TO INDICATE FAILURE OF THE PROTECTIVE GAS SUPPLY TO MAINTAIN THE REQUIRED PRESSURE.
9. All pressurizing system components energized in the absence of the protective gas shall be identified for the classification that would exist in the absence of the pressurizing system.

 

When using purged and pressurized protection techniques, the cabinet/enclosure must have certain markings/labels:

1. A PERMANENT MARKING shall be on the protected enclosure in a prominent location so that it is visible BEFORE the protected enclosure can be opened.
2. The marking shall include the following information:
   (1) WARNING — PRESSURIZED ENCLOSURE — This enclosure must not be opened unless the area atmosphere is known to be below the ignitible concentration of combustible materials or unless all devices within have been de-energized.
   (2) The external area classification for the protected enclosure
   (3) The pressurization type (e.g., Type X, Type Y, or Type Z)
   (4) The temperature class (T Code) or the operating temperature in degrees Celsius as determined in Section 4.6

WARNING:  Power must not be restored after enclosure has been opened until enclosure has been purged for ____ minutes at a flow rate of ____________.  

The minimum pressure shall be permitted to be used in place of the flow rate where the pressure is a positive indication of the correct flow.