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)."
Intrinsic Safety Considered Safest
This protection method is generally considered the safest, as it's the only method accepted for use in IEC Zone 0, and is, of course, accepted in all other classified areas. "Intrinsic safety is based on the principle of limiting energy to a level below that required to ignite hazardous gas or dust," says Robert Schosker, product manager and team lead for process automation, Pepperl+Fuchs. "The energy limitation also pertains to thermal energy. Therefore, in normal operation or in the event of a fault, no sparks or thermal effects may occur that could lead to the ignition of a potentially explosive atmosphere."
Figure 2 shows how barriers are used to limit the amount of energy delivered to components in intrinsically safe areas.
Sackett points out that one of the big advantages of using intrinsically safe technology is the option to use safe area wiring practices. "Besides cabling, that includes less expensive, off-the-shelf, safe area devices that fall under the ‘simple device' category," he says. "So if you plan on using some type of safe area signal conditioner, isolator or converter, you might as well use an intrinsically safe version."
Charlie Norz, product manager, Wago, agrees. "Intrinsically safe I/O modules provide users with all the benefits of a modern fieldbus technology," he says. "The modules combine digital and analog I/O with intrinsically safe barriers, making it easy to engineer, and it reduces wiring costs."
One of its customers in the brewing industry used a number of Wago standard and intrinsically safe (IS) I/O modules to optimize operation of the fermentation stage of its brewing process (Figure 3). "The application used a wall to separate the hazardous and non-hazardous locations. The customer chose to mount our intrinsically safe digital input modules in a non-hazardous location, and intrinsically safe sensors and actuators located in the hazardous area were wired directly to these I/O modules."
The main drawback of the IS approach is the limited availability of components, as those requiring power above 300 mA at 24 Vdc for operation can't be made intrinsically safe. Another drawback is the amount of time required to design an intrinsically safe system, as relatively complex entity parameters and other calculations are required.
Kelli Malloy, leader, U.S. process automation, Turck, says intrinsic safety is gaining popularity in the United States and is widely accept worldwide, in part because it is considered the safest protective mechanism for classified areas. "The installation cost is considerably less than explosion-proof enclosures, and the ability to install it in an instrument tray, even in Class I, Div. 1 areas, is a huge cost savings," Malloy says.
Mike Bond, senior program manager at Optimation, agrees: "I think the U.S. uses IS just as much as Europe does," he says. "My observation is that there are more cord connections to IS devices in Europe, whereas the U.S. still prefers conduit systems." For more on how Optimation uses IS and other protection methods, see the sidebar, "Optimation Opts for Dual Protection."
Non-Incendive Allows Higher Power
Non-incendive has similarities to intrinsic safety, as both methods limit the amount of energy released to a level insufficient to cause ignition of the surrounding atmosphere. But a much wider range of components is available with non-incendive ratings as compared to intrinsically safe certification, including variable-frequency drives (VFDs) rated up 100 hp. On the other hand, this method isn't suitable for Zone 0, as intrinsic safety is.
"Non-incendive installations have gained popularity, especially with digital bus technologies, because they are relatively inexpensive to install," says Malloy. "Ease of installation using quick disconnects and junctioning, expandability and low equipment cost are a huge benefit to users. Bus technologies and non-incendive installations are often done in instrument trays rather than conduit, another cost savings.
"There are two drawbacks to this technology. First, interpreting the electrical code and FM requirements can be confusing because of ambiguity in the language and potential for misinterpretation. Second, non-incendive installations do not account for abnormal operating conditions, and this can be a source of consternation for some users if plant standards require multiple layers of protection. However, the methodology is safe and has been used widely in Class I, Div. 2 installations."
Vacon provides non-incendive VFDs that have UL approvals for installation in Class I, II and III, Div. 2 hazardous locations. Its VACON X5 HazLo drive was used in a Class I, Div. 2 location (Figure 4) instead of an explosion-proof enclosure, providing a 66% weight reduction and a 35% cost reduction.
When in Doubt, Purge
Purged enclosures are continuously supplied with clean instrument air or an inert gas, either of which won't allow ignition to occur when combined with the energy emitted by the components within the enclosure.
"Purged enclosures allow for use of higher powered devices and also devices that don't have or couldn't get proper approval for use within a hazardous area. But purging does add cost to the enclosure system, a source of clean instrument air or an inert gas is required, and hot permits are required to open the enclosure," explains Sackett.
"Using devices with no extra approvals for hazardous areas can keep costs down, helping to offset the added cost of the purged enclosure," continues Sackett. "Also, filling the internal area of the enclosure with either clean instrument air or a clean inert gas has the two-fold effect of increasing the lifetime of electrical components, and preventing an explosion by isolating the electrical components from the corrosive hazardous material."
Laskowski offers an application example. "One of Emerson's customers is a North American refinery using a DeltaV SIS distributed control system with purged enclosures for the logic solvers in a classified area in the field," he explains. "In addition to hazardous conditions, the area is exposed to weather extremes, so the purged enclosures are heated and cooled. The system has been in use for years with no operational problems."
Purged enclosures are often used to house local HMI displays. Louis F. Szabo, business development manager at Pepperl+Fuchs, notes that one of its customers had older display systems in Class I Div 1 areas that were beginning to fail, and the original supplier had long since gone out of business. "P+F installed its Div 2-certified KVM display to replace the old HMI. This certification is important, as non-certified equipment will not get past the safety inspectors, risk assessment personnel and insurance underwriters," says Szabo.
Wireless Avoids Issues
Ian McDonald, wireless solution architect at Emerson Process Management, points out that most hazardous area solutions are complex and expensive. "When instruments must be installed in hazardous areas, all of the wiring must be installed and maintained to the relevant standards to prevent ignition of explosive environments," he notes. "The costs of protecting wired 4-20 mA and fieldbus instruments in hazardous areas can be significant."
However, wireless instruments avoid most of these problems. "WirelessHART devices certified to the relevant protection level for the hazardous area can reduce the amount of equipment installed in the hazardous area, such as the wiring and supporting infrastructure needed with a traditional wired installation," he explains. "WirelessHART process transmitters are battery-powered, which eliminates the need for power wiring, and battery power is safe for use in most hazardous areas."
In the end, all four solutions have their advantages and disadvantages, and opinions are often based on familiarity. "I see the U.S. market continuing to rely on proven and accepted technologies such as explosion-proof," says Matt Behrns, marketing manager, Hoffman Enclosures. "IS solutions require up-front engineering design, and purge requires compressed air sources. Overall, the oil and gas market, which is the primary user of hazardous location solutions, is very conservative and relies on tried and true solutions."
Others say IS is coming on strong and becoming as popular in the U.S. as traditional technologies. Because of its low cost and simplicity, the non-incendive approach is growing in popularity for Class II locations, and purged enclosures provide a means to use most any component, even in Class I locations.
And most process plants will end up using a variety of protection methods, driven by such factors as the specific area classification, the availability of acceptable components and the engineering expertise of their personnel.