In my columns on hazardous locations, I didn’t get around to equipment. Hazardous location classification provides opportunities to practice ingenuity in design. First, some background information from NFPA’s National Electrical Code (NEC) handbook.
For many years, Class I and Division 1 classification meant the design was going to call for explosion-proof equipment, steel conduit and strategically placed conduit seals. Article 100 defines explosion-proof equipment as “equipment enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.”
Equipment that can cause arcs or sparks that can ignite the atmosphere should generally be kept out of a hazardous location. Sometimes, it can’t be avoided, so the equipment must be identified for the appropriate hazardous location. Today, there are more options for avoiding ignition-capable arcs or sparks.
Explosion-proof equipment has been available longer than most of the other protection methods. Explosion-proof equipment usually consists of conduit entries and flanged joints. It is practically impossible to make threaded joints gastight. The conduit system and apparatus enclosure “breathe” due to temperature changes, and any flammable gases or vapors in the room can enter the conduit or enclosure over time, resulting in an explosive mixture. An arc could produce an explosion.
If an explosion occurs within an enclosure or conduit system, the burning mixture or hot gases must be sufficiently confined to prevent the ignition of any explosive mixture that could exist outside of the equipment. To prevent a rupture, which could release burning or hot gases, the enclosure must have sufficient strength to withstand the pressure generated by an internal explosion. During an explosion, gases escape through any paths or openings that exist in the enclosure.
Explosion-proof enclosures are designed so that escaping gases will be sufficiently cooled as they exit through openings that are long in proportion to their width. Two examples of this are the screw-on type junction box covers (Figure 1), and the tight tolerance, wide-machined flange between the body of the enclosure and its cover (Figure 2). The function of the joint is the same whether it is flanged, threaded, rabbeted or any other type designed for this purpose. The purpose is to cool the escaping gas.
The clearance between flat surfaces can increase under explosion conditions because the internal pressures created by the explosion tend to force the surfaces apart (Figure 3). The amount of increase in the joint clearance depends on the stiffness of the enclosure parts, size, strength, spacing of the bolts and the explosion pressure. When there are no internal pressures, measuring the joint width and clearance does not indicate the actual clearances under the dynamic conditions of an explosion.
Explosion tests are usually needed to demonstrate the acceptability of the enclosure design. Figure 3 illustrates the need to properly install all provided bolts, screws, fittings and covers. It is very common to find that bolts are missing or mismatched. If bolts are missing, it is essential that the manufacturer’s specified bolts are used for replacement.
Early in my career, I reviewed an assembly that was going into a gasoline distribution facility. The process equipment consisted of several explosion-proof enclosures. The area was classified as a Class I, Division 2 location. The authority having jurisdiction would not accept the installation.
When I met the customer, he handed me several equipment nameplates that had the manufacturer’s name and a UL listing log and indicated he wanted me to place the nameplates on all of the enclosures. I told him I couldn’t do that and he needed to contact UL. What I could give them was a field evaluation of the installation.
After I cleared that up, I examined the equipment. Right away, I noticed all the equipment flanges were damaged. Many had scratches or scuff marks on them, which could compromise the flanges’ ability to cool escaping gases. Flanges of explosion-proof equipment need to be protected from damage. I also noted that pipe dope had been applied to conduit threads. Pipe dope could limit the ability of burning gas to escape, which could cause the enclosure to rupture. Clearly, this installation was not going to pass inspection. Discarding the equipment would be costly and delay the project.
We brainstormed a solution. I suggested that most of the existing equipment could be used if a purged and pressurized systems were designed. Purging is defined as “the process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level.” The standard for purging is NFPA 496, Purged and Pressurized Enclosures for Electrical Equipment.
I learned the facility had a source of clean air from a safe location, so we had a potential solution. A purging system would use all of the existing enclosures, which were no longer explosion-proof.
Each enclosure is usually maintained under pressure. Purging is used to remove flammable concentrations of gas or vapor. Once that happens, the enclosure or enclosures are maintained at a positive pressure as pressurized enclosures.
Pressurization is defined as the process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust or an ignitable fiber.
Pressurization can be used in both Class I and II locations. It can also be used in Division 1 and 2 locations for both classes and in Zone 1 and 2 locations. However, it is not permitted in Zone 0 locations.
NFPA 496 provides requirements for three types of pressurizing.
- 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 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.
Since Type X pressurization reduces the classification within the enclosure from Division 1 (or Zone 1) to nonhazardous, this would permit ignition capable equipment to be installed in the enclosure. Failure of the protective gas supply requires an equipment power supply that is not suitable for the location to be automatically de-energized. However, power to equipment can remain on for a short period if immediate loss of power could create a more hazardous condition.
Since Type Y pressurizing reduces the classification in an enclosure from Division 1 (or Zone 1) to Division 2, all of the wiring methods and equipment permitted for Division 2 locations are permitted in the enclosure. This allows a lot of equipment that is normally nonarcing and non-sparking. The equipment must also maintain the temperature limits for the hazardous environment.
Type Z pressurization reduces the classification in the enclosure from Division 2 to nonhazardous.
Since this was a Class I, Division 2 location, at least Type Z pressurizing was required.
When considering using pressurization, it is important to consider the effects. In other words, don’t replace one problem with another. An enclosure with an internal pressure can be a hazard if it is over pressurized. Precautions must be taken to protect the enclosure from overpressurization by the protective gas supply. The precautions may include excess pressure relieving devices. Pressurized enclosures must be maintained at a positive pressure of at least 0.1 inches of water above ambient pressure to keep the hazardous environment out.
Air, nitrogen and other nonflammable gases are permitted to be used. However, air is the most likely gas to be used as it is easier to obtain and use than more exotic inert gases. For this installation, compressed air was used. Where compressed air is used, the compressor intake must be located in a nonhazardous location. NFPA 496 refers to “air of normal instrument quality.” The goal is air that is free of contaminants, such as oil. The gas supply lines must be protected from mechanical damage.
Pressurization can supply a number of individual enclosures. Where any of the enclosures can be isolated, it necessary to have an alarm that will warn that an enclosure has been isolated. Pressurization is also used to protect control rooms. In some cases, individual enclosures might be pressurized in addition to the control room. This may occur where the classification of a room is reduced, but the equipment may require further hazard reduction where such double pressurization is necessary, and the two systems must be independent of each other.
Purged and pressurized enclosures require the protective gas system, protection of the piping and a monitoring system to ensure that the system is properly functioning. However, it can be a practical solution if properly rated equipment is not available, or if the flanges for explosion-proof equipment are damaged or likely to be damaged. There are a number of protection techniques recognized in the NEC that can simplify the design of an installation and reduce the cost.