Automation and instrumentation components and systems must often be installed in process plant areas classified as hazardous according to IEC or NEC. These projects present challenges in terms of design, installation and maintenance.
For all such installations, there are four main options as shown in the table: explosion-proof, intrinsically safe, non-incendive or purged. Each approach has its advantages and drawbacks, and many installations will be best served by a combination of two or more methods.
Perhaps the newest development in hazardous-area installation is more widespread use of the non-incendive or non-sparking approach, which requires AEx nA-rated components. "We are seeing numerous installations using AEx nA (non-sparking equipment) located in Zone 2 areas in relation to remote I/O and remote PLC installations," says David Dalke, discipline technical authority for systems and networks, Wood Group Mustang. "The biggest issue with this type of installation is making sure all internal components are certified for this protection within the enclosure." Wood Group Mustang uses various compliance approaches. (See "Maintenance and Engineering Considerations in Hazardous Areas," for more details.)
Explosion-Proof Relies on Brute Strength
This is the oldest, most familiar and simplest method of protection. "An explosion-proof housing is designed to prevent combustible gases or dust from coming in contact with internally mounted components that produce energy," explains Len Laskowski, principal technical SIS consultant, Emerson Process Management. "If enough energy is present and an explosion occurs, the housing is designed to contain it to prevent further explosions in the area."
Because it's been around for a long time, there are many components available with explosion-proof housings and ratings, from instruments to valves to motors. But housings can get very large and expensive, extreme care must be taken during installation, and ongoing maintenance is required.
Derek Sackett, marketing specialist, Phoenix Contact, points out that the main advantages of explosion-proof or explosion-containment technology are the ability to use higher powered devices within enclosures and higher powered end devices. "Also, at least in the U.S., end users seem to be more familiar and comfortable with the installation requirements," he notes.
One of the disadvantages, he says, is maintenance difficulty. "Opening an EX enclosure or device is not permitted without shutting down power to contained circuits—complicating troubleshooting, maintenance and inspection. Because EX proof/containment technology is based on the mechanical integrity of the enclosures, periodic inspections are needed. Maintenance personnel and technicians also must be properly trained on requirements."
Under ATEX and IECEx regulations, EX Proof/containment technology is not viewed as a safe enough technology for Zone 0 areas.
Henry Menke, marketing manager, Balluff, says Balluf's explosion-proof components are used with hydraulic actuators on large refinery control valves. "Advantages of this approach are that a higher level of electrical energy can be present in the hazardous location, and the explosion-proof housing is by necessity tough and robust, a welcome benefit in rough industrial environments."
Neal Cammy, engineering manager at BLAC, a manufacturer of severe-service, high-reliability electro-hydraulic linear valve actuators, agrees. "BLAC's electro-hydraulic actuators are mainly used in oil refinery applications, such as controlling reactor product temperatures. India requires the equivalent of ATEX Zone 2, IIb certification for hazardous area operation, so at an FCC unit in India, we use Balluff explosion-proof linear position sensors (Figure 1)."