With lives and property at stake in a hazardous location, it's important you use only the highest quality sealing fittings from reputable manufacturers. A classified area is not the place to cut corners.
Sealoffs prevent explosions from spreading through conduit systems and igniting outside atmospheres. When properly installed and filled with a UL-listed sealing compound, they create a physical barrier that minimizes the passage of gases from traveling freely through the conduit. They stop or localize explosive pressures from traveling through conduits. Moreover, these barriers limit the passage of vapors from classified to nonclassified locations.
Why the need for sealoffs? A conduit system isn't vapor-tight, so the gases inside and outside a conduit can be of the same concentration. When activated, an arc- and spark-producing device; such as a switch or breaker; can ignite these gases in the enclosure and conduit system. For this reason, unless the device is factory-sealed, you must install sealoffs within 18 in. of any enclosure containing arcing or sparking devices. Use a drain seal where moisture may accumulate in the fitting to provide an explosion-proof path for water to exit while still sealing the conduit to UL standards.
How do you install them? Installation of a sealing fitting is straightforward. First, run the conductors through the fitting. Then, pack the fiber filler into it. (The fiber filler makes a dam that prevents the cement from leaking while in its fluid state.) In a horizontal run, you place the fiber filling at both ends of the fitting; in a vertical run, you fill only the bottom end. Finally, pour the sealing compound into the fitting to a depth equal to the trade size of the conduit, having a thickness of at least 5/8 in.
After the sealing compound hardens, you screw in a removable close-up plug into the pouring opening. One note of caution: The NEC states you can't install any splices or taps within a sealing fitting.
What choices do you have? Although their purpose is the same, sealing fittings come in a wide variety of designs to meet the exact requirements of specific applications. Besides being available in designs for vertical and horizontal conduit runs, sealing fittings come in different materials. Grayloy(tm) iron sealing fittings; also known as "breakout" seals; are favored in applications where you may want to fracture and remove a sealing fitting when retrofitting a conduit system. Malleable iron sealing fittings are ideal for the most demanding hazardous location. Extremely strong, malleable iron has a high tensile strength that enhances performance because it will not fracture under excessive internal or external pressures. The higher yield strength of malleable iron also permits the product to have a thinner cross-section, which results in a closer turning radius for easier installation.
Aluminum sealing fittings are popular in applications where corrosion is possible or weight is a consideration, such as on offshore drilling rigs and marine locations. You should use aluminum conduit to reduce corrosion producing conditions. Because conduit runs may be close to one another or closely positioned to a wall, you might appreciate a fitting with a close turning radius. Or, you might want a wide pouring mouth for the sealing cement. Fittings are available that have separate pouring mouths, which can be rotated 90 Degr and placed into one of four directions. You can use them in vertical and horizontal conduit runs.
Sidebar: Sealoffs and the NEC
Sealing fittings or "sealoffs" are a requirement in areas classified as hazardous by the National Electrical Code (NEC). Art. 500 .defines a hazardous location as any area where gases, dusts, or fibers have the potential to reach ignition temperature or to mix in the right proportions to cause an explosion. Examples of these types of applications would include petrochemical and chemical plants, waste treatment centers, pharmaceutical plants, textile plants, grain mills, paint factories and offshore oil rigs.
Other regulations concerning sealoff use are in the NEC. You should read Sec. 501-5 (Sealing and Drainage), subsections (a) through (f) for detailed requirements. You can also refer to manufacturer-provided Code reviews for details. The EC&M book Understanding NE Code Rules on Hazardous Locations is also a good source.
What happens with an explosion? As an explosion travels through a conduit run, the gas in front of it creates an effect known as "pressure piling," which can produce tremendous mechanical and explosive pressure. The sealoff provides a barrier so that these explosions and vapors don't continue to travel throughout the conduit system. By localizing the gases in the enclosure, vapors and flames can cool below outside ignition levels as they dissipate through the explosion-proof construction of an enclosure.