Explosion-proof drains

Figure 17-28. Standard explosion-proof drains and breathers. [Courtesy of Crouse-Hinds Elecfrica Construction Materials, a division of Cooper Industries, inc.)...

To prevent the accumulation of moisture in conduits and enclosures, drains should be installed at all low points. In classified areas, breathers and drains must be explosion-proof. Figure 17-28 shows typical explosion-proof breathers and drains. 

Specifically note that Type 7 (explosion-proof) enclosures and their associated conduit systems are neither gas or liquid tight. Consequently, corrosive gases such as hydrogen sulfide and water from rain or internal condensation can accumulate with the enclosure. Premature failure of electrical devices and interconnections often results when preventive measures such as drains, air purges, and dual rated enclosures are not used to remove or exclude these corrosive elements. Type 7 enclosures are intended for indoor use. 

FIG. H-20 Shop photograph of CSA-approved electric hot pipe tank heaters showing terminal boxes of explosion-proof Ex(d) design. Serrated fin tube outer pipes. The Internal heating eiements are removabie without draining tank. (Source Armstrong Engineering Associates.)... 

Small towers sometimes are reinforced PVC, and there is no need for a lining. A potential problem with PVC or other nonconductive material is the accumulation of a static charge. The explosion of a set of three PVC dryers in a mercury-cell plant was attributed to static discharge in the presence of an explosive gas [27]. The root cause of the incident was the failure of the power supply to the mercury pumps, which allowed mercury to drain from the cells and uncover the steel bottoms. This led to the formation of large quantities of hydrogen. The investigators concluded that towers should be constructed of acid-proof conductive material that can be held at ground potential. 

---