Intrinsic Safety: Thinking Outside the Explosion-Proof Box

Unlike explosion-proof schemes, which aim to contain explosions inside enclosures, intrinsic safety keeps them from ever happening at all. Consider whether it makes sense for your production environment.

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As you develop your safety plan for a hazardous production environment, you might want to move beyond the explosion-proof methods that have been popular for so many years in the U.S., and start thinking outside the box. Particularly for process industries, it could be time to take a closer look at intrinsic safety (IS).

Process automation is the sweet spot for intrinsic safety. “That’s where intrinsic safety shines,” says James Wilkinson, senior applications specialist and technical support lead for MTL Instruments, describing analog signals from level measurement on a tank, for example, going back to a control panel. “Process automation is intrinsic safety’s world.”

That’s because intrinsic safety makes the most sense at low energy levels, where voltages are 24 V or less, and currents are 300 mA or less. That makes it a good fit for field instruments such as thermocouples, RTDs, pushbuttons, simple transmitters, low-power solenoids. For variable-frequency drives, switch gears, Coriolis meters—anything high-voltage or high-current—explosion-proof makes more sense.

“If you’re running a plant where 90 percent of it is low-voltage, low-power instrumentation, that’s when you need to sit back and take a look at intrinsic safety,” says Robert Schosker, product manager—team lead for intrinsic safety, HART, power supplies, signal conditioners and surge protection for Pepperl+Fuchs.

Intrinsic safety is suited to the most hazardous locations—Class I, Div. 1 for North American NEC standards; and Zones 1 and 0 for Europe/IEC. It is used in a variety of hazardous industries—in oil refineries, upstream oil and gas, petrochemicals, pharmaceuticals, even food and beverage.

Intrinsic safety is the No. 1 safety movement in Europe, Wilkinson points out, but has been slower to catch on in North America. “There is a movement to intrinsic safety, albeit a slow movement,” he says. “People here in North America do not understand it. Older engineers don’t understand what it is and how it works; the cost savings and its overall ease of use.”

How intrinsic safety differs

When it comes to protecting hazardous environments, there are three basic methods used: explosion containment, which lets the explosion occur but confines it to a given area, preventing it from reaching the surrounding area; segregation, which physically isolates the electrical parts from the explosive danger; and prevention, which limits the electrical and thermal energy to safe levels.

Intrinsic safety falls within the third category. But there is a general lack of knowledge in the field about what intrinsic safety is and how it needs to be set up, Wilkinson says.

In short, an intrinsically safe system is one whose energy levels are so low that they cannot generate an arc or spark and therefore cannot cause an explosion. This differs from an explosion-proof system in which the explosion is simply contained within an enclosure so that it doesn’t reach the hazardous materials that might create a combustible mixture in the atmosphere.

Such hazardous compounds as hydrogen, ethylene, propane or methane will ignite differently, but they still all follow a basic ignition triangle, which requires fuel, oxygen and an ignition source, notes Randy Durick, director of the network and interface division for Turck. All protection methods eliminate one or more of the triangle components. Intrinsic safety works by eliminating the ignition source. “Being limited by an intrinsically safe barrier, that spark doesn’t carry enough power to ignite,” he adds.

Engineers who’ve been around the business for a while often feel more comfortable with explosion-proofing, says Dani Alkalay, director of marketing for MTL. “I have this enclosure so if an explosion occurs inside, I can contain it. Intrinsic safety is more electrical. You design the circuit so even if there is gas in the air, even if there is a fault condition, it won’t have enough energy to ignite. Engineers need to be more electrically oriented.”

But explosion-proof systems have their own hazards, particularly since they don’t actually avoid creating an explosion. “The housing is designed to contain the explosion. In a fault condition, it may create an explosion, but the container will contain the explosion,” Durick notes. But with corrosion, nicks or cuts to the container, or if screws are not screwed in all the way, a much larger explosion can occur outside the container, he adds.

IS barriers

Even though some engineers find it difficult to trust that a little zener diode barrier will stop an explosion, it will do just that. For intrinsic safety, the zener barrier provides a simple method for keeping energy levels in check. A key drawback, though, is that the barrier must be connected to a special IS ground.

“They require a very high-integrity earth ground, and are more error-prone,” Durick says, explaining that they can start building up impedance by becoming corrosive, for example. “That can create potential for energy to not get shunted to ground appropriately.”

A newer alternative is an isolated barrier, which creates an optical isolation area between the safe area (the control room, for example) and the active area, Durick explains. It provides galvanic isolation and does not require dedicated grounding. “They’re a bit more expensive, but are much easier to install and maintain,” he says.

They do, however, typically require a separate power supply. Galvanic isolators are also more application-specific because they must be configured for either digital or analog use; not both, as is possible with zener barriers.

Another benefit of an isolated barrier, however, is density packing. “You can put two instruments per isolator, sometimes four,” Wilkinson notes. “The zener barrier is one device per barrier. So instead of 100 zener barriers, you can buy 50 isolators, and save money and space.”

These, along with other system requirements, are aspects that scare some folks away, concerned that the system is too complicated or expensive. A mistake some people will make is to take an explosion-proof instrument, put a barrier in front of it, and call it an IS circuit. “It doesn’t work that way,” Schosker says. “You have to do some entity calculation, checking voltage, current, capacitance, inductance, power… People find that a little daunting.”

However, an IS system is not that complicated and there is plenty of help available in the industry for setup. “We can quickly and easily do that entity matching for you,” Durick says, explaining that FM Global, which provides commercial and industrial property insurance, and underwriting and risk management solutions, helps U.S. organizations identify parameters for setting up IS circuits. “They tell us what entity parameters our devices are approved for. Customers can do entity matching themselves or can get our help.”

There are essentially three devices that plants need to be concerned with for intrinsic safety: a power supply, IS barrier and measurement device. “You just need to ensure that as you pick your devices, you pick those that are compatible with each other based on entity parameters,” says Ted Dimm, director of global marketing for Honeywell Process Solutions. “I don’t want to oversimplify, but there’s not really more to it than that.”

Cost savings

Intrinsic safety is worth any additional expense for certified barriers, particularly in upstream oil and gas applications, because it can improve safety over explosion-proof setups, says Roberto Zucchi, global product manager, pressure measurement products, for ABB. But while the need for additional hardware such as a safety power supply source leads to perceptions that intrinsic safety is more expensive than explosion-proof schemes, that’s actually not the case.

The conduit for explosion-proof schemes is a considerable cost. “Depending on the length of the conduit, this probably would be a decision point,” Zucchi says.

In fact, you don’t have to use any special field wiring, Wilkinson says. “The biggest thing is the cost savings and ease of maintenance because you don’t have special wire or conduits, so it’s just an overall easier system to work with.”

“With intrinsic safety, since the energy is limited and not stored, those wires do not have to be protected in the same way,” Dimm says. “With IS, the wires, even if you cut them, couldn’t cause a problem.”

Although more and more people are realizing the installation costs they can save from intrinsic safety, a lot of people still are not reaping the benefits of those savings, according to Schosker, who says they’re still using conduits instead of wire trays, and are continuing to use shielded wires unnecessarily. “They don’t take full advantage of all the benefits of intrinsic safety, and its general-purpose wiring methods,” he says. “If they did, they would really see the true benefits of it.”

Having worked in grain refineries, where he used explosion-proof instrumentation, bending conduit, pouring seals, and everything else that was necessary, intrinsic safety “definitely makes a whole lot of sense,” Schosker says. “The other end of it is downtime, trying to eliminate downtime. With intrinsic safety, I don’t have to shut down my whole process. I can pull out the electronics, slap in the new electronics, and my instrument’s up and running again.”

Choosing a device

Although in the early years, there might have been some variance in the quality of IS field instruments, these days specifications are pretty much the same from vendor to vendor, Schosker says.

The major suppliers of field instruments are designing their products to meet both explosion-proof and IS requirements. While, in theory, this can make it easier for customers to stock their shelves with instruments and decide later in the field which type of device they need, this is really for the benefit of the instrument supplier.

“Most manufacturers today will manufacture their devices so that they meet both the criteria for explosion-proof as well as intrinsic safety,” Dimm reiterates. “And the enclosure itself is designed so that if there is a problem, the explosion would be contained. It’s relatively easy to develop a product that meets both criteria.”

These days, it’s not that difficult to pick out IS devices because most follow standards and generally the same specifications. There are different but similar IS standards around the world, so you should be aware of what you’re looking for.

Although you might check that your instruments have the proper ratings for your location—whether that’s UL, CSA, etc.—there’s a bit more to look for. “They need to actually get in and look and verify that it has the ratings for the area that they’re going into, like Class I, Div. 1, or a zone environment,” Schosker explains.

In North America, the categorization of hazardous areas is done in accordance with NEC article 500. “Class tells you if it’s gas or dust; division tells you the probability of the hazard being present; and group tells you the type of hazard,” Durick says.

Class I, Div. 1 areas contain dangerous concentrations of flammable gases, vapors or mist continuously or occasionally under normal operating conditions. “If I want to run pressure measurement with an analog device at 4-20 mA, I could make it explosion-proof,” Durick says. “But it opens up the opportunity for intrinsic safety instead.”

For Class I, Div. 2, which are areas probably not containing dangerous concentrations of flammable gases, vapors or mist under normal operating conditions, intrinsic safety still opens up other possibilities, Durick adds.

“Division 1 is really what we’re looking at for intrinsic safety,” Dimm says.

Europe operates more according to a three-zone model. Zones 0 and 1 line up most directly with Div. 1, but not exactly. Zone 0 is the most dangerous, and any instrument used there must be incapable of having enough energy to ignite a fuel mixture.

“We try to design devices to comply with all the IS standards; all the global IS requirements,” Zucchi says.

Again, it makes it easier for the supplier to create devices that can fit everywhere, but this might not be important to the user. “If he knows where the project is going to be located, he doesn’t care if it has all the international standards,” Zucchi points out. “If he’s in Canada, he makes sure it complies with what’s in Canada.”

Setting up the circuit

Having the properly rated IS devices is important, but it’s just as important to set up your IS circuit properly. “We see people around the world buying an IS device, but then locally doing something that doesn’t fit,” Zucchi warns. “They’re connecting wiring, but not to intrinsic safety specifications. Or they’re installing a device that’s not IS-certified. Or they’re buying the IS device, but not using the barrier.”

The point is that users need to be sure they’re using the devices according to specifications. Even painting a device to company colors can change its properties. “A device could be painted with a specific oxy paint with a specific thickness in order to not have electrostatic charges on top of it,” Zucchi says, noting that he has seen customers repaint to their own colors multiple times before. “It could lead to a generation of spark, and it could void the protection.”

Whether or not an IS system makes sense also depends on legacy systems. “If they have an existing explosion-proof system, the cost and downtime to pull all that stuff out and put intrinsic safety in will be prohibitive,” Wilkinson explains. “Usually you’ll find intrinsic safety more often than not will be in a new application or new installation. Sometimes people will pull out explosion-proof if it’s come to the end of its lifecycle, but it’s kind of rare.”

Wilkinson says the key to getting intrinsic safety more embedded in North America is making sure the younger engineers understand its benefits. “When you look at hazardous location areas, intrinsic safety is the one methodology that can go in the worst of the worst, and globally,” he says.

Sidebar: Choosing Your IS Device

 

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