Enclosure purging and pressurization
By Chris Romano
Purging and pressurizing systems is one of the most versatile explosion protection methods. These systems are based on the principle that in Zone 1 or 2, Division 1 or 2, the gas mixture in the ambient atmosphere, which may ignite under certain circumstances, is removed from the housing by an initial purge process. After the purge phase, sufficient compressed air or inert gas is supplied to compensate for leaks in the enclosure. This permanent overpressure, achieved using compressed air or inert gas, prevents any potentially explosive atmosphere in the ambient air from entering the enclosure.
During the rapid exchange purge phase, an internal pressure of approximately 3.5 - 12 mbar (1.3” to 5.0” Water Column) is achieved. During operation, this internal pressure reaches 0.6 - 3 mbar (0.3 to 1.2” Water Column).
For applications in Zone 22/Class II (dust), the purge process is omitted because purging would raise explosive dust. Instead of pre-purging, the interior of the housing is inspected for dust and cleaned manually if dust is present.
The pressurizing system is particularly suitable for installed equipment that is not approved for use in hazardous areas. Once installed in a purge enclosure, it can then be used directly in the hazardous area.
Purge/pressurization products are designed to supply one or more protected enclosures with clean instrument air or inert gas. This process removes flammable gases or prevents the accumulation of ignitable dusts within the protected enclosure(s). This method of protection is not limited by the quantity, configuration, power requirements, or location of the protected equipment. These systems are commonly used for all applications involving basic electronics, electrical equipment, motors and switchgear.
Purge/pressurization vs. explosion-proof
Explosion-proof enclosures are well known for their size, weight, and price as a means of protection. Another disadvantage is they allow for the explosion to happen but contain it within itself, provided the bolts are ALL torque down properly.
Purge/pressurization can take standard enclosures and make them safe for installation in hazardous areas as a means of protection, and, unlike explosion-proof, it does not allow for an explosion to occur.
The disadvantage of purge/pressurization enclosures is their operation requires a constant air or other inert source. Also, for very small enclosures such as instrument housings, there is a cost advantage in using explosion-proof, but in any enclosures over 1 cubic foot, purge/pressurization will have the advantage.
There are four primary factors that determine which purge system is appropriate for a particular application:
- Classification of the area
- Ratings of the equipment inside the enclosure. Is there a containment system within the enclosure that houses hazardous gas or liquid (gas analyzers)?
- Type of enclosure, enclosure size, position of doors, windows, and any accessories
- Power requirement to the equipment inside the enclosure
The area classification determines the type of purge system needed. For Zone 1/Division 1 areas, the equipment inside the enclosure determines whether an Ex px/Type X system (equipment rated for general-purpose) or an Ex py/Type Y system (equipment rated for Division 2/Zone 2) can be used.
The rating on the equipment inside the enclosure becomes important in evaluating which purge system to use in a Zone 1/Division 1 area. If the Zone 1/Division 1 area contains at least one general-purpose component, an Ex px/Type X system is required. If all devices in the enclosure are rated for Zone 2/Division 2, then an Ex py/Type Y system can be used. Special conditions exist for enclosures such as gas analyzers and chromatographs that contain a flammable gas.
Another consideration is for analyzers that are taking in a hazardous gas or liquid, examining it, and then putting it back into the process. Because there is a potential for leakages of this hazardous material inside the enclosure, dilution or the use of inert gas may be required. Some conditions may require the Zone 2/Division 2 area to use an Ex px/Type X system because of the type of leakage.
Refer to EN60079-2/EN61241-4/NFPA 496 2008 for more information.
Explosion-proof enclosures are designed to contain an explosion if an electrical device ignites flammable substances within the enclosure, thus preventing ignition of the surrounding atmosphere. These enclosures are commonly used for circuit breakers, mechanical switchgears, and high-powered equipment. The failure to properly tighten all bolts and screw covers on these enclosures is the greatest problem facing end users.
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.
The size of the enclosure is determined by the free volume within the enclosure. Normally, the equipment mounted inside the enclosure cannot be subtracted to the free volume. The volume of the enclosure is required in determining the purge time, which is based on 4 volume changes for N.A. standards and 5 volume changes for IEC and EN standards and the flow rate of protective gas through the enclosure. If a motor is being purged, then the requirement for N.A., IEC, and EN, standards is 10 volume changes. However, the stator of the motor can be subtracted to the free volume of the enclosure.
For Ex px/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. If after purging pressure to the enclosure drops below the minimum required value, then the enclosure power must be disconnected and cannot be engaged until after a successful purge and enclosure is pressurized.
For Ex pz,py/Type Z,Y systems, power to the enclosure can remain on if an alarm is activated indicating loss of pressure. If an alarm is not used, then power to the enclosure must be disengaged.
Many applications are existing enclosures designed for safe areas that need to be placed in hazardous areas. They may already be installed in the plant or are getting ready to ship to a customer, only to find out that the area is hazardous. One method is explosion-proof. This means taking everything out of the general-purpose enclosure, sizing up an explosion-proof enclosure, waiting until it comes in with the proper conduit, feeding through explosion-proof controls on it, and then checking to see if it will fit in the desired/original position.
In this application, a stainless steel enclosure features an access door for control adjustments and maintenance, along with a very unique audible and visual alarm system.
The other method is to install the proper purge system to the existing enclosure. Purge systems can make it easy to take a general-purpose enclosure and make it safe for hazardous area mounting. Wiring to and from the pressurized enclosure still has to be protected from the area, but the enclosure is easier to retrofit.
New applications are easier because they can be planned and worked into the schedule. When selecting an enclosure or retrofitting an existing enclosure, some design criteria should be considered:
- IP65/Type 4 protection should be considered for larger enclosures because they can hold pressure better than less environmentally protected enclosures.
- If using a plastic enclosure, EN standards will look at the dielectric of the plastic for its electrostatic discharge. Not all plastic enclosures can be used in hazardous areas.
- Any conduit, cable gland, fittings, windows, tray doors, or any access points from the enclosure will be a leak point of pressure. The better these are sealed, the less purge gas is required. If using inert gas or bottled gas, this could be very important.
- Users will need a purge gas supply, which is not always available.
- Whatever source of purge gas is being used, make sure the capacity of the source is big enough to handle the purging of the enclosure for the duration required.
- Make sure the purge gas supply lines running to the enclosure are big enough to handle the flow required for purging. Most manufacturers will have suggested tubing/piping size in their manuals.
- Make sure all the equipment within the enclosure is vented so these individual devices can be purged.
In this application, a dual pressurization system is mounted above two identical devices that are separately protected to allow independent access. Both devices feature TYPE 4 cases, which makes them suitable for purging as is.
ABOUT THE AUTHOR
Chris Romano is the Product Portfolio Manager for Purge & Pressurization Systems at Pepperl+Fuchs. He is an expert on hazardous locations and purge/pressurization systems. He has presented training seminars and technical papers to organizations like ISA and NACE.