Eye on Hazardous Area Sensors

Sensors pervade our lives. From the appliances in our homes to the automobiles that move us from place to place to the automation and control systems that operate and monitor our industrial processes, sensors are everywhere. Like most components, however, they are part of the larger whole—elements in systems growing ever more complex.

05/01/2007


Sidebars:
Sensing safely in a hazardous area
Making the difference

Sensors pervade our lives. From the appliances in our homes to the automobiles that move us from place to place to the automation and control systems that operate and monitor our industrial processes, sensors are everywhere. Like most components, however, they are part of the larger whole—elements in systems growing ever more complex. When used in hazardous areas, sensors demand a thorough understanding of the application and rigorously careful selection.

There are so many variables to consider with hazardous areas, observes Karmjit Sidhu, vice president of business development at American Sensor Technologies, that applying sensors safely under these circumstances becomes a formidable undertaking. Adds Art Pietrzyk, a TÜV-certified functional safety expert, Rockwell Automation, “We’re mitigating the hazards—but a hazard is still a hazard. What’s changed is the approach we’re taking. Sophisticated equipment and more consistent standards are leading the way to improved safety.”

Hazardous area sensors embrace a multitude of devices used in a variety of ways in any number of applications. However, several common denominators apply to all, most notably the protection schemes under which they must be selected and installed, and the standards that help ensure their safe use.

Classic approaches

Most devices operate safely if they are properly selected and applied. The key is to know and understand the application. A device that is Class I, Div. 1 approved for a hazardous location may not be suitable for an area that involves, for example, a toxic medium. “It is misapplication and misunderstanding,” cautions Sidhu, “that leads to trouble. The end user needs to know what he is selecting for use in what. Know what’s in that tank! Don’t rely on what it says on the outside! Your application might not require an approved device, but it might be prudent to use one.”

To perform safely, sensors used in hazardous areas must be designed specifically for the environments in which they operate. Typically, this is achieved in one of three primary ways: by selecting products that are intrinsically safe, using explosion proof devices, or applying a purge-and-pressure system. (As applications differ, those applying sensors in hazardous areas should consider this information only one source among many and supplement it with research relevant to their specific situations.)

Sensors designated intrinsically safe (IS) have insufficient energy to cause ignition of the rated hazard. Compliance testing ensures that an IS-rated device has been tested and a determination made of how much energy would cause a reaction. IS devices normally use an “external apparatus” called a safety barrier designed to limit the amount of current, and hence energy, that can pass in the circuit under any fault condition.

A passive barrier establishes protection mechanisms that prevent overvoltage and limit current. Should a short circuit occur on a 4-20 mA line, the barrier prevents ignition. Every intrinsically safe device must be powered off a barrier, says Ed Herceg, chief applications engineer at Macro Sensors, but “all barriers are not the same. There are many types. You have to match the barrier to the entity parameters of your device to make sure you’re using the right kind of barrier for the device. Otherwise you may not be protected. However, I have heard and believe to be true, that to date, there has never been an explosion or fire in a hazardous location traceable to a failed intrinsically safe device.”

In Class I, Div. 2 hazardous locations, non-incendive devices, equipment, and field wiring practices are sometimes used instead of intrinsically safe devices. A sensor can be contained in an explosion-proof housing , so that if an incident occurs, a containment vessel made to withstand the force of the expected reaction will not allow the force or any fire to propagate. Explosion-proof technology is well established. When properly installed, such devices provide a viable alternative in cases where a suitable sensor for the application is not offered in an intrinsically safe version.

Purge-and-pressurize systems may also be used to prevent sensors and other devices from reacting in a hazardous area. There are several variations, but essentially these systems introduce non-flammable (inert) gas such as nitrogen or carbon dioxide throughout the conduit, components, and equipment to eliminate the possibility for flammable material to enter. An adaptation would be a sealed system that allows no room for flammable materials.

Essentially, “a sensor placed in a hazardous area either must have its electronic components contained in an explosion-proof housing or be equipped with intrinsically safe electronics,” says Les Schaevitz, president, Everight Precision. “Often, it is less expensive to put an intrinsically safe component into an atmosphere than to install an explosion-proof housing, although both options are fairly costly. Beyond expense, an intrinsically safe sensor [with barrier] is often preferred because it eliminates the possibility of explosion. Many people don’t want a potential problem, even if it is contained in an explosion-proof housing.”

Alphabet soup of standards

Users seeking sensors for hazardous areas must look for devices that bear the stamp of approval from the conformance testing organizations that dominate in the region in which they operate. In North America, sensors intended for use in hazardous areas are tested and rated by such independent testing laboratories as UL (Underwriters Laboratories), FM (Factory Mutual), and CSA (Canadian Standards Association) for compliance with such standards as the National Fire Protection Association’s National Electrical Code (NFPA 70). The NEC designations familiar to most U.S. operations essentially divide hazards into three types: vapors and gases (Class I); dusts, such as coal and flour (Class II); and fibers (Class III). Divisions describe the level of exposure: present under normal conditions (Div. 1) or not normally present (Div. 2). Further breakdowns (Groups) are made according to specific substance (hydrogen, acetylene, etc.)

In Europe, CENELEC (European Committee for Electrotechnical Standardization) used to dominate the standards scene, but more recently, it has been overtaken by ATEX directives (Equipment Intended for Use in Potentially Explosive Atmospheres). Japan has its own standardization organization, and a number of regions in Asia have their own standards. The IEC (International Electrotechnical Commission) comes as close as any to promulgating international standards. It is dedicated, it states, to “international standards and conformity assessment for government, business, and society for all electrical, electronic, and related technologies.”

The upshot is that in a shrinking global economy, an abundance of standards adds confusion to sensor selection, application, design, and marketing. “Part of the problem,” explains Macro Sensors’ Herceg, “is that you can have products approved for sale in the U.S. and Canada but they cannot be marketed in Europe or Brazil. That’s creating a lot of impetus for change. People who have an approved product today may find it is not marketable in many regions tomorrow because of changing standards.”

“IEC has come the closest to developing a global standard,” says Leslie Neill, product manager for safety switches and explosion proof hazardous location switches, Honeywell Sensing and Control. “Although Europe ratified the ATEX directives that have been becoming a dominant force over the last few years, IEC more recently has offered the IECEx standard in an attempt to harmonize approvals around the world. A number of 'umbrellas’ cover a number of areas of standardization, but they all overlap. IECEx appears to be an attempt to put an umbrella over all the umbrellas, if you will. However, only one country thus far, I believe, has adopted it as the only national standard in effect, and that’s Australia.”

Harmonization

The regulatory environment may be well established with regional certification systems around the world, adds Everight’s Schaevitz, “but it has to move in the direction of harmonization because the industries covered are increasingly global in nature. We believe the ATEX model is going to lead the way with those seeking harmonization because it was promulgated as an 'international' community standard. Also, the EU member countries that created ATEX have long standing economic and cultural ties with nations around the world looking to more fully integrate into the global economy...and of course, nobody does bureaucracy like the Europeans.'

Presently, the European Union (EU) mandates ATEX certification on products and will not accept other approvals—although UL, FM, and CSA certifications are considered by most to be as or more stringent than ATEX. Similar to the Class/Division designations common in the U.S., ATEX Zone classifications include Zone 0 for areas where ignitable concentrations of flammable gases or vapors are present continuously or for long periods of time under normal operating conditions; Zone 1 if hazards are intermittently present, and Zone 2 for situations in which hazards are present only occasionally or only under abnormal conditions.”

“In the last decade or two, there’s been a slow adoption of the Zone system of classification,” notes Tim Adam, technical team manager, hazardous locations for FM Approvals, member of the FM Global group. “There are three zones as opposed to two divisions. At this point, FM Approvals certifies according to zone requirements as well as the North American class/division designations.”

Adam finds recent customer feedback reflects a need for global standardization as manufacturers move to sell in an international market. “We’ve expanded to take that into account,” he says. “We approve for the USA and also for Canada. And we’ve opened an office in the UK which certifies under the ATEX directives so we can approve equipment for sale into Europe. We recognize that time-to-market is very important to our customers, and we focus on making sure that what we do is done quickly.”

Most agree standards have become variations on the same theme and support the need for the globalization of standards overall. “Some products can be sold only in Europe, while UL/CSA would never approve them for use in North America, and vice versa,” offers Bob Nickels, director of strategic marketing, Honeywell Sensing and Control. “We have an ATEX version and a UL/CSA version of the same switch. This can become an inventory nightmare, and there’s significant cost involved in obtaining all these approvals. If we want to introduce a new product, we must decide where it is to be sold,” he goes on. “Then design engineers must design to all the requirements. After the product is developed, it must go through the approvals process for each place it will be marketed.”

Companies favor harmonization of standards primarily for this reason, insists Nickels. “The physics of a device stay the same. The changes are in the ways of describing a product and of doing the testing. If we can agree on those things, the need for multiple, parallel, redundant testing goes away. Once the fundamental requirements as dictated by the physics are met, vendors should be free to sell that product anywhere. Going forward, I believe the industry will look at all standards in a global way,” he says.

Changes ahead

Both globalization and harmonization are influenced by technological advances. Changes in sensors have taken place in the control circuitry rather than the devices themselves, points out FM Approvals’ Adam. “What the equipment can do is much greater. In response, we must consider more. We need to be sure these enhancements don’t lead to additional risks.”

Sensor technology is a mature technology, admits Honeywell’s Nickels. “The principles of operation remain the same, but the surrounding and support components have advanced. Thirty or forty years ago we didn’t have solid state devices. Every sensing function was done with an electromechanical contact of some kind. Now we have communications networks and systems expressly designed for hazardous areas. They have reduced system costs and allowed more sensors to be used in plant or in a system.”

Camilo Aladro, product marketing manager, Rockwell Automation, echoes Nickels. “Twenty-five years ago, switching functions were done primarily with electromechanical devices. New standards like IEC 61508 [Functional safety of electrical/electronic/programmable electronic safety-related systems] were developed for newer microprocessor-based smart devices. Because microprocessors work on micro voltages, they are almost inherently intrinsically safe, at least in the computing part. And voltages in microprocessors have dropped; they used to be at 5 V, now they are 3.3 or 1.8 V.”

Hazards are never going to go away, Aladro continues. “A hazardous gas will always require careful handling. We must address the way we approach hazardous locations, and mitigate the hazards that are there. There are smart devices available today to do that. Sensors are being designed and re-designed to incorporate intelligence to do things like check calibration. The result is intelligent instrumentation that tests itself to ensure it is working properly.”

Hazardous area sensors are a critical part of the SIS, concludes Rockwell’s Pietrzyk. “Today, we are adopting process standards that are performance based, not prescriptive based. Performance-based standards start with a risk analysis/assessment, and determining the risk level just makes good sense. Along with protecting people, safety is also good business because if your equipment doesn’t blow up, you can remain productive.”


For more standardization, visit these Websites:

www.ansi.org (American National Standards Institute)

www.cenelec.org (European Committee for Electrotechnical Standardization)

www.csa.ca (Canadian Standards Association)

ec.europa.eu/enterprise/atex/guide (ATEX)

www.fmapprovals.com (Factory Mutual)

www.iec.ch (International Electrotechnical Commission)

www.isa.org (Instrumentation, Systems, and Automation Society)

www.nfpa.org (National Fire Protection Association/National Electrical Code)

www.ul.com (Underwriters Laboratories)


Author Information

Jeanine Katzel is a senior editor at Control Engineering. Reach her at jkatzel@reedbusiness.com .


Sensing safely in a hazardous area

This instrumentation cabinet and associated sensors control a very large gas compressor in a Class I, Div 2 hazardous area, says Doug Rutz, general manager at QComp Technologies, Greenville, WI. The OEM equipment is designed to compress several combustible gases for a variety of applications. “The 316 stainless steel panel is exposed to the elements with only a simple snow roof for protection,” he explains. “It was actually built for an installation in Montana. The inside of the cabinet is heated, but the outside can see temperatures to -30 °F.”

The cabinet panel must meet IP65 (NEMA 4X) standards because the instruments are exposed to wind-driven snow, rain, and dust. Although the meters, from Precision Digital, are non-incendive rated for Class I, Div. 2 applications, the HMI, which requires the use of a custom made 316 stainless steel cover, is not. To ensure safety, the 75-cu-ft area behind the panel is purged with compressed air.

“We like the meters,” says Rutz, “because one model can be configured for all our metering needs. The Loop Leader indicator can be rescaled without being removed from the panel. If a customer wants to change the range of the meter, it is a relatively simple thing to do from the front panel. We use the same meter for indicating pressure, suction, temperature, and motor current.”

Source: Precision Digital and QComp Technologies. For more from Precision Digital, visit

Making the difference

Sensors for hazardous areas marketed globally must be approved, rated, or certified for use in the region of the world in which they are sold. Illustration shows the visual differences between a UL-CSA approved product (in this case a

The devices are essentially the same switch, but differences include a metal clamp on the left of the BX device, required to necessitate the deliberate action of opening the body of the switch with a special tool, and a grounding screw required at the top of the switch for visual identification of grounding. Unlike U.S. sites, European sites are not automatically grounded, as they are not necessarily attached to fixed, steel conduit. The BX and LSX lines of explosion proof limit switches are intended for valve actuation applications in petrochemical and off-shore drilling facilities in North America and Europe. (Source: Honeywell)



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