Systems integration company Wood Group Mustang uses all types of hazardous area protection methods depending on the installation environment and other factors (Figure 1). David Dalke, systems and networks expert at Wood Group Mustang, explains some of their considerations:
Explosion-proof housings can be a maintenance problem. "Maintenance is an issue as the housing needs to remain in good working order to function properly," he notes. "The actual integrity of protection of explosion-proof housings is a real soft spot, as human factors increase the risk of bolts being lost or not properly re-installed following maintenance."
Maintenance is also needed for purging. "A key strength of purged systems is improved accessibility for periodic maintenance, but principal problems of purged enclosures are the cleanliness and moisture content of the purge gas source—and monitoring, maintenance and testing of the cabinet supervisory alarms," he says.
"Filter systems for the purge gas supplies require routine inspection and maintenance. In some facilities where inert purge gases other than air are used (typically nitrogen), additional safety considerations, labeling and potentially monitoring may be required depending on the size of the cabinet, its location and the capacity of the purge supply, especially if the cabinets are located inside buildings or shelters."
"Virtually every one of our clients has had some sort of purging, and almost all our North American projects use explosion-proof housings on transmitter installations," he says. "Most of the time, Wood Group Mustang's Automation Group installs purge boxes in classified areas or all the components are rated to be within the classified area."
"While intrinsically safe (IS) systems don't require as much maintenance, they are difficult to design. Added space for the barriers and separation is required, and engineering is needed to perform IS calculations to confirm they meet intrinsically safe installation requirements," Dalke explains. "Passive barriers can cause a significant voltage drop if not designed properly and require a low-impedance ground system. Life-cycle inspection and verification are needed to assure the integrity of the protection system."
At a chemical plant on the U.S. Gulf Coast, the facility standard was to use non-incendive protection in Class I, Div. 2 areas and explosion-proof solutions in Class I, Div. 1. But for a truck and rail loading overflow protection system, requirements for a lightweight, easy-to-manipulate overfill protection system led the plant to install lower-cost, intrinsically safe level sensing systems. Use of intrinsically safe design also allowed for improved integrity of protection in the Div. 1 area, where the sensor cable was subject to mechanical abuse with frequent connecting and disconnecting of the loading arm and rail/tank car connections.