Many industrial machines, skid-mounted process assemblies, and robots must be rated for use in hazardous areas. In some cases the hazardous material is inherent to the equipment process, and in others at least part of the equipment must be placed in a hazardous area of a customer plant.
In either case, machine designers have three basic choices to make a system safe for hazardous areas. The first is a traditional solution: explosion-proof enclosures, conduit, and wiring methods. This solution contains explosions that may occur,caused by live circuits in a flammable atmosphere,inside heavy-duty enclosures, sealed conduit, and prescribed wiring installation procedures.
|Intrinsic safety technology is widely seen as safer than explosion-proof enclosures or purge systems because the solution is prevention rather than containment.|
A second choice is to purge enclosures with inert gas, a solution also accompanied by special conduit and installation procedures. Purging allows OEMs to house electrical equipment in lesser-rated enclosures. The enclosure itself is less expensive with this method, but the purge system itself adds cost.
A third choice for coping with hazardous areas is the use of intrinsically safe (IS) components and IS design methods.
Devices that are IS certified are designed to be unable to release enough energy,either thermal or electrical,to ignite flammable material (gas, dust/particulates). Intrinsically safe standards apply to all equipment that can create one or more of a range of defined potential explosion sources.
Intrinsically safe technology is widely seen as safer than explosion-proof enclosures or purge systems because the solution is prevention rather than containment, and because continued safe operation does not rely on maintenance of seals and/or purge systems.
The drawbacks of IS are the added expense of components, complexity of design and unfamiliarity. Many North American system designers are not familiar with IS solutions and are more comfortable employing the brute force of explosion-proof enclosures. When this unfamiliarity is coupled with the inherent complexity of IS system design, the combination can result in low acceptance rate.
The expense is becoming less important--IS component costs are declining relative to explosion-proof enclosures and purged systems.
Complexity of design remains an issue, but the nature of OEM design and deployment reduces concerns. Unlike end-user designed systems, OEM products are typically designed once and replicated many times. Therefore, design complexity and costs can be spread out over many units. This can make IS an excellent choice for applications where IS components cost less than explosion-proof or purged enclosures.
Even if costs are equivalent, IS solutions often can be justified because of benefits to an industrial OEM's customers in life cycle costs. "IS equipment weighs less, is easier to repair and calibrate, and is simple to replace or upgrade, as compared to explosion-proof and purged systems," says David Hohenstein, department manager for hardware and marketing, Pepperl+Fuchs (www.us.pepperl-fuchs.com).
Rockwell Automation says its cost-of-ownership studies show IS to be less costly than traditional explosion-proof methods, arguing that explosion-proof enclosures have dozens of fasteners that must be inspected frequently and that must be opened to gain access. "Processes need to shutdown before opening to prevent exposing live circuits to the hazardous atmosphere," states Patricia Moyer, product marketing manager for I/O at Rockwell Automation (www.rockwellautomatiom.com). "These lock-out and tag-out procedures are costly and time-consuming. IS systems can allow maintenance and replacement without shutdown."
Beyond capital costs, maintenance expenses and performance, there's the issue of safety and compliance. Prevention of an explosion is safer than a containment solution, and IS systems are recognized by most end users and regulating bodies as the safest alternative for installations in hazardous areas. More safety- conscious regions such as Europe long have favored IS.
According to Hohenstein, no explosion anywhere in the world has ever been attributed to an IS system.
OEMs wishing to harmonize equipment design for a worldwide market can employ IS solutions with confidence. Intrinsicaly safe technology is recognized across the globe as an accepted and preferred methodology for coping with hazardous areas.
More-recent advances include IS remote I/O systems and IS fieldbus connectivity solutions. These products allow data to be sent to and from hazardous areas on a single cable instead of dozens of home-run wires. These I/O and fieldbus connectivity products can be used in hazardous areas without the use of purged or explosion-proof enclosures or standalone barriers.
Industry trends indicate IS solutions will grow in popularity. "IS has been accelerating in North America due to the lower cost of components and increased safety as compared to traditionally used explosion-proof and purged systems," agrees Davis Mathews, product manager of instrumentation at Phoenix Contact (www.phoenixcon.com).