NFPA 496 is the standard for Class/Division classifications, and IEC 60079-2 is the standard for Zones. But these are just the purging and pressurization standards, which are only a piece of the puzzle. The wiring, temperature, and material of the enclosure and equipment within also need to be considered. For example, the wiring to and from the enclosure is not part of the purge system and will have to be protected. The area classification determines what can be used and which standards have to be followed.
Purging is required to eliminate all flammable gas inside an enclosure. To achieve safety, a certain volume of protective gas per free enclosure volume must be achieved. NFPA 496 requires four volume changes, and IEC60079-2 requires five volume changes. The flow rate through the enclosure is measured or certified to make sure the enclosure is fully purged.
After an enclosure has been purged, pressure must be maintained within the protective enclosure. To pressurize an enclosure, a low flow rate of protective gas is introduced to keep the enclosure pressure above the minimum safe pressure. Its value depends on NFPA 496 or IEC60079-2 standards. A constant flow of protective gas for pressurization is required for most enclosures because of leakages from doors, windows, conduit/cable glands, etc.
For Hazardous Dust Environments:
The hazardous dust must first be physically cleaned out of the enclosure. Once cleaned, the enclosure is sealed and pressurized. Pressurization is usually a low flow rate of protective gas, which keeps the enclosure pressure above the minimum safe pressure, depending on NFPA 496 or IEC60079-2 standards. A constant flow of protective gas is required for most enclosures because of leakages from doors, windows conduit/cable glands etc. Purging is not done because pressure relief vents usually require a spark arrestor that cannot exhaust dust particles.
For Hazardous Dust and Gas:
Enclosures must first be cleared of all combustible dust and then purged of all flammable gas. The pressurization component remains the same, but the minimum safe pressure may vary. Usually, hazardous dust requires a higher safe pressure in the enclosure, depending on the standard used for pressurization.
After successful purging and/or cleaning out and pressurization of the enclosure, equipment can be operated safely. If the pressure drops to a level below the safe minimum, action is required. Depending on the standard being used and the area classification, action can range from de-energizing the protective enclosure immediately or after a pre-determined time (for shutdown procedures) to sounding a visible or audible alarm for an operator to take action but leave the protective enclosure energized.
For all systems, a pressure relief vent is required to exhaust excess pressure within the enclosure. Pressure relief devices are also required for dust-only environments, because a regulator could fail and excess flow could enter the enclosure.
Protective gas is used to purge and pressurize enclosures. In most plant environments, this is compressed air. Some applications use bottled air, and some use an inert gas like nitrogen. When considering a protective gas supply, several parameters must be considered:
During startup, it is assumed that the inside of the enclosure has flammable gas in sufficient levels to create a fire risk when the equipment is operating.
Purging is the method of flushing all flammable gas out the enclosure so that the levels are below the lower explosive limit (LEL) of hazardous gas. Several variables influence the time needed to make an enclosure safe, including flow rate in/out of the enclosure, size of the enclosure, number of volume exchanges required, the architecture of the inside of the enclosure, and the enclosure strength. If the protective enclosure has several compartments, the protective gas line going into the enclosure may have to be placed in strategic locations to make sure all dead spots are flushed out. The length of purge time is determined by the standards used, but it is typically:
Purging usually requires a controlled flow of protective gas into the enclosure and a suitably sized pressure relief vent for the exhaust. It is good practice to put the vent at the bottom of the enclosure and the protective gas supply at the top, if the hazardous gas is heavier than air. However, it is more important to make sure the entire volume of the enclosure is flushed out. When certifying enclosures, the certification body may require a test to make sure the amount of time indicated really is enough time. Be prepared to ask the certification agency if this test is required.
Large enclosures can be problematic for purging, because the opening pressure of the enclosure protection vent will be elevated. Pressure is based on surface area, so a larger surface means more force:
Using 30 scfm for purging flow, the enclosure pressure will be around 4 in wc or 0.1445 psi.
Using 12 scfm for purging flow, the enclosure pressure will be around 2 in wc or 0.0725 psi.
Sometimes larger enclosures can be reinforced for the higher pressure.
Enclosures are pressurized after being purged or cleaned of dust. Pressurization keeps the hazardous atmosphere from getting back into the enclosure. A constant flow of protective gas into the enclosure is required to prevent leakages from the door, windows, conduit seals, cable glands, etc. The flow rate for pressurization is usually lower than the purge flow rate and the opening pressure of the relief vent, but there may be instances where it will need to be higher than the opening pressure of the vent so that it will cool the enclosure.
When setting the pressure for an enclosure, make sure the pressure is higher than the exact minimum safe pressure of the system but below the breaking pressure of the pressure relief vent. For example, if the minimum safe pressure is set to 0.25 in wc, and the breaking pressure of the vent is 1.0 in wc, set the pressurization pressure in the enclosure to 0.5 in wc. With this higher pressure, there is a better chance that drops in supply pressure will not give an alarm or shut off the system.
Most electronic equipment can be protected, but slight modifications may be necessary before use. Equipment inside an enclosure that is not suited for the hazardous atmosphere will be part of the purging and pressurization system. If the piece of equipment is completely sealed, then it must be opened up so that the protective gas can flow through the equipment to purge it. Consult the required standards for minimum openings. Electronic components like microchips, transistors, caps, etc., do not always need to be ventilated. If a piece of equipment needs to be modified to meet this requirements for purge and pressurization, always check with the manufacturer to make sure the product warranty and safety will not be compromised.