Machine builders designing systems for installation in hazardous areas must carefully consider the safety of the machine's operator interface as well as hardware components that will be exposed to the extreme environment. Components to consider might include operator interfaces such as PC-based HMIs, LCDs, HMI panels, panel PCs and thin clients, along with hardware such as controllers and distributed I/O and auxiliary devices like warning lights.
"Most of the controllers and I/O we deal with, including those from Allen-Bradley, are rated for Class I, Div. 2, hazardous areas and can be used right out of the box in those types of classified areas,” says Jeff White, director of technical services at Interstates, a system integrator that provides turnkey electrical engineering, construction, instrumentation, and control systems solutions, headquartered in Sioux Center, Iowa. “In this sense, they do not differ from traditional devices."
That said, even though the hardware can be installed out of the box, you must be aware of what you need to do to make the whole installation safe, adds Jamie Schmidt, lead project engineer at Interstates. "Although the I/O components might be rated for a specific area such as Class I, Div. 1, or Class I, Div. 2, you must follow a certain set of rules to ensure safety in putting the components out there,” she warns. “For example, one of the systems we use is Allen-Bradley's Flex Ex I/O. Rules mandate that you must use specific power supplies to supply the intrinsic power to the I/O. There are also rules for the communications going through the modules to provide an intrinsically safe solution, for putting the components together for the infrastructure and for connecting your field devices to the components. Basically, the field devices have an intrinsically safe rating that needs to be matched to the I/O point to maintain the intrinsic rating."
Additionally, intrinsically safe components may or may not be required, depending on the area's class rating. "For hazardous area installations, you must also consider whether it is a Class I or a Class II area," continues White. "Class I areas involve a hazardous or explosive gas, and Class II involves hazardous dust. For Div. 1 installations for gas areas, the common options are explosion-proof installations, purge and pressurizing and intrinsically safe (Figure 1). When you talk about a Class II, Div. 1, instead of explosion-proof, you're looking for dust-ignition-proof installations."
Interstates designs systems for installations in plants that are based on many factors, says Schmidt. "For example, in deciding whether to use an explosion-proof approach as opposed to an intrinsically safe one, we base our decision on the customer's application and try to follow that approach for the whole plant," she says. "It's also important to account for a company's maintenance practices. For example, after we install equipment in an explosion-proof box, maintenance personnel can't just open up the box if hazardous gases are present. Personnel need to use devices that detect the presence of any hazardous gases. When a hazardous gas is present, operators must shut down the equipment before opening the box. In contrast, an intrinsically safe approach typically involves barriers installed in a nonhazardous-rated area that limit energy to the field device. In this case, personnel can work on the devices live because there will not be enough energy to cause an explosion. The intrinsically safe approach thereby allows more flexibility in some maintenance practices."
Additionally, warns Schmidt, you must ensure you use tools approved for a hazardous area when working on a live system. "It is still a hazardous area, so meters and testing equipment must be approved for that area," she says. "Therefore, maintenance must be aware of the safe working practices—it's not just a free-for-all, because there are still considerations to keep the system safe."
When it comes to components available for Class I, Div. 2 environments, there are more options. "Here, you can consider non-incendive or hermetically sealed equipment,” says Schmidt. “Some of these components can be installed without needing to use intrinsically safe installation practices."
Trends that industry is seeing include an increased use of thin client installations. "Also, the use of mobile HMIs will continue to increase for Div. 2 areas, although they are not yet being rated for use in Div. 1 areas," says White. "Additionally, HMI installations are increasingly using touchscreens, instead of mechanical pointing devices and keyboards."
No More Gaps
When it comes to operator interfaces for hazardous areas, Clark Kromenaker, product marketing manager at Omron, says factors that affect how well HMI panels and panel PCs will survive in the environment are the overall device design, how the faceplate is made and materials. "The whole unit is designed industrially, but the faceplate and touchscreen are designed specifically in terms of how that unit is mounted into the enclosure and what environmental compounds it needs to resist," he says. "Gaps between the screen and the bezel in an operator interface intended for an office environment would not be a big deal. Industrial environments obviously require a precise design with no gaps."
Also distinguishing the faceplate from a traditional design is the material used. "Material that can withstand corrosive-type environments might refer to a NEMA 4X-type of applications," says Kromenaker. "The material could be stainless steel or a certain type of plastic that resists corrosion. There are even more material choices when it comes to NEMA 4- type designs, which must just be water-resistant, not corrosion-resistant. There are a lot of plastics, for example, that you could use for a NEMA 4-type design with some gasketing and sealing because it doesn't have to be corrosion-resistant."
Additionally, when a touchscreen HMI is used, Omron typically recommends 1 in of water pressure for a purging system. "Too much pressure might balloon the touchscreen,” continues Kromenaker. "The purging system basically provides a positive pressure in the enclosure, keeping any flammable gas from getting inside where it could potentially be exposed to a spark and cause a fire. Many suppliers including Omron provide HMI products with a Class I, Div. 2 or a Class II, Div. 2 rating. A big trend is the design of face plates for HMIs that can resist a higher purge pressure.”
Overall, an important trend is to move as much equipment out of that hazardous area as possible. "Perhaps you can get away with just a display and a touchscreen and have the controls remotely located," continues Kromenaker. "In these applications, fiberoptics or wireless comes in handy because the technologies don't use electricity to connect to the HMI, possibly generating a spark. As HMI technology advances into the future, very low-power touchscreens along with very low-power displays should increasingly come into play to make for intrinsically safe designs."
Device Ratings Are Critical
When specifying interface devices for installation in hazardous areas, two of the most important considerations are the device’s rating and the agency that applied it, explains Mike Baucom, CEO, Bebco Industries. "Although it sounds obvious, this information is often difficult to determine when browsing a website or reading product bulletins" he says. "It might be necessary to contact a manufacturer directly and cut past sales jargon such as 'suitable for' or 'rated for.' Quite often, you’ll find there’s no basis in fact for a statement. Getting a complete disclosure is often tedious, but critical. It’s even more critical if you’re designing a product. What you’ll discover is certain manufacturers may not authorize their third-party agency to share their certification file information with other third-party agencies, such as the one you chose to certify your product, thus hampering your ability to attain a certification if the product contains various devices with certificates from different agencies.”
See Also: Operator Interface in Hazardous Areas
Additionally, continues Baucom, many operator-interface devices such as keyboards, PC-based HMI and LCDs, are now available in ruggedized versions. These devices are robust, are resistant to most impacts and can then connect to a PLC's distributed I/O controllers and other devices located in a protected enclosure (Figure 2). "However, even if a device has a hazardous-area rating, it may not be rated for impacts,” says Baucom. “Impact ratings are required by the IEC and will soon follow here in the United States, as we harmonize with these international standards. Engineers and designers should therefore consider secondary forms of protection such as a purgeable access door for interface devices that are not specifically rated for impacts.”
Benefits of Purging
The options for installing devices which are not suitable for the prevailing hazardous substances or impacts are primarily limited to either a purged enclosure or an explosion-proof housing, says Baucom. However, there are sharp contrasts in the level of protection afforded by these technologies. "Explosion-proof housings are rugged and self-contained but won't protect devices from condensation and heat buildup that are common in extreme environments," he says. "In contrast, purged enclosures offer a safe, cool and pressurized environment that can be cooled by supplemental devices to render an office-like environment in which the devices can work for years without incident. However, they will require a source of clean instrument air, which must be considered before finalizing a design.”
Lastly, Baucom says that technology is now addressing most Div. 2 area needs. “As a complement to Div. 2- rated interface devices, manufacturers now produce a large range of power control devices, such as VFDs, relays and PLCs that are also rated for Div. 2, and the list is growing every day,” he says. “These off-the-shelf components can satisfy Div. 2 area process control applications, without the need for purging or explosion-proof enclosures. Regarding Class I, Div. 1 areas, most sophisticated operator-interface devices are not suitable for these locations where the threat of the hazard is considered as continuously present. The market is extremely limited for Div. 1 applications, so there is really no incentive for manufacturers to aggressively pursue it."
Light the Way
It might also be wise to consider the design of auxiliary components such as LED lights and signaling lights for hazardous locations because the devices are likely tied to the same circuitry as the HMI, says Paul Mizuki, marketing manager, Patlite. "If, for instance, the operator is not standing in front of the HMI when an error condition occurs in the hazardous environment, the signal device makes an audible or visual indication that something might be wrong," he says. "The light can be remote or installed on top of the HMI control panel."
The physical design of the company's visual and audible signaling devices are different to suit intrinsic or explosion-proof applications, says Mizuki. "For example, lights for explosive Zone 1 or Zone 2 environments are enclosed in a robust, flame-proof, enclosure,” he explains. “They are made of a thick, borosilicate glass and an aluminum alloy frame, so they will contain any fire or explosion that may occur inside the enclosure without allowing any of it to escape outside the enclosure. The alarms and the lights are considered off-the-shelf components, meaning users can just order the part and need not do anything extra to it."
A recent trend has both incandescent bulbs and xenon tubes moving to LED technology, says Mizuki. Lights are also increasingly multicolor, so, instead of having to purchase three different units and wire them separately, users can just buy one unit and then just wire the one unit. "Also, the future will see more and more devices that use messages such as 'Machine Number 1 is having Issue A,' which tells maintenance exactly what is wrong with the equipment or process. Along these lines, we currently provide products that play MP3 messages and are programmable by the end user.”