Designing Sensors for Hazardous Environments

Some of the most common applications for hazardous duty sensors are in the oil and gas industry. Rotary encoders are frequently used to provide crucial feedback about moving machinery on the rig. Flammable agents are common in applications near the wellbore, so most electrical components on the rig require special hazardous area certification. Hazardous duty sensors are also used in applications such as paint spray booths because of the volatiles coming off the paint; grain silos because the dust environment is very explosive; chemical factories; explosives factories; and believe it or not, for cosmetics because the very fine powders that are used are highly explosive.

Classifications

Designing for hazardous areas has been well codified over the years. For example, you have to know whether or not the hazardous material is a gas, or a dust, or a fiber. Further classification is based on whether or not the hazardous condition is likely to persist over time; if it might be there only intermittently; or if it might be there only as the result of an abnormal set of circumstances like overpressure valves opening. It is also important to determine the geographical location where the devices will be installed, because different regulatory bodies control what set of rules are going to apply.

In the US, the regulations are primarily laid out by the National Fire Protection Association (NFPA) and administered by UL. You submit your design to UL to determine whether it meets the standards. Although Canada has their own set of regulations, there's a reciprocity agreement with the US, so UL can certify to both Canadian and US standards. In Europe the regulatory body is called CENELEC and their regulations are referred to as ATEX, which is an abbreviation for ATmosphères EXplosives. Again, there's a reciprocity agreement with UL, so they can certify a design to the ATEX regulations. Finally, the International Electrotechnical Commission (IEC) covers much of the rest of the world.

The US regulations are descriptive, while ATEX applies numerical values to determine what category the equipment falls under. Under ATEX, if you expect that the hazardous condition will exist 20 out of 24 hours, then that's defined as persistent. If you expect a hundred hours a week, that would be considered intermittent, but a hundred hours a month might be considered abnormal.

Under the NFPA standards, the top-level classification is the Class: whether the hazardous environment is a gas, a powder, a cloth, or a fiber. The Division is determined by whether or not it's persistent, or intermittent, or abnormal. A slight difference among the US, the IEC, and European approaches, is regarding the US Division 1 classification. In Europe and throughout the rest of the world they use what are called Zones: Zone 0, Zone 1, and Zone 2. When you're mapping the Zone system into the Division system, Zone 0 and Zone 1 both map into Division 1. The only circumstance where that creates a significant difference between the two approaches is in the case of an explosion-proof installation. In the US you can use the term “explosion-proof” for a Zone 0 or a Zone 1 application — the European version of this category is “flame-proof.” However, although Division 1 “explosion-proof” will work in Zone 0, the European “flameproof” will not work in Division 1.

Designing an Encoder for Hazardous Duty Operation

Scott Orlosky, Product Marketing Manager for Position Sensors at Sensata Technologies (Attleboro, MA), discussed the ways in which Sensata encoders are designed for hazardous environments. “We primarily use one of two methods to manufacture our encoders: explosion proof or intrinsically safe. Once equipment is rated at the highest class and division level, it can be used in any other installation that is of a lesser requirement,” said Orlosky.

Explosion Proof Encoders

Explosion proof design assumes that the explosive mixture in the operating environment can enter the encoder, but if an explosion is touched off, it will be contained within the housing. The gaps between interfaces in the encoder are designed so once the exploding gas gets pressurized and leaks out, it will have cooled enough that it will not propagate an explosion outside its containment vessel. The key element here is that you have to design what is called the “flame path” — the path that the explosion will take — so that it's narrow enough to restrict the flow.

Intrinsically Safe Encoders

For intrinsically safe devices, there has to be a current-limiting electronic barrier between the device and the outside environment, which when combined with low operating voltage, limits the electrical energy into the device to a safe level even in the event of an open or short circuit.

In addition to the current limiting rule there are temperature restrictions. Sensata uses a two-fault policy: if there are two successive faults, none of the surface temperatures of any of the products will exceed the lower explosive limit of the gas that they’re protecting against.

Explosion Proof vs. Intrinsically Safe

“Explosion proof designs have very high installation and maintenance costs. They require a heavy-duty conduit connection on the encoder and a continuous conduit run from the hazardous zone all the way to the safe zone,” said Orlosky. “For maintenance, you have to shut off the power, break into the interconnections, do rewiring, etc.”

According to Orlosky, Sensata uses both techniques and the user chooses which they prefer. At this time, he said, it’s about 50-50.

Some people prefer explosion-proof because they have fairly fixed installations without a long cable, or they don't have any room in their cabinets for any additional equipment. With many intrinsically safe installations, there could be six or more barriers, along with extra wiring. However, Sensata/BEI designed an all-in-one IS solution with only one barrier per encoder. (Figure 1)

“Intrinsically safe makes for a quicker and easier installation and it can be better in offshore installations. It is critical for both intrinsically safe and explosion-proof installations to maintain a high integrity earth ground, which is hard to do offshore. Since the Sensata/BEI intrinsically safe barrier is a galvanically isolated circuit, the earth grounding requirements are not relevant,” said Orlosky.

Summing Up

Use the NFPA, ATEX, and IEC regulations to determine whether your operating environment requires designing for hazardous duty. Sensata Technologies designs all of their hazardous duty devices to meet the most stringent requirements of all three main agencies. They can satisfy these standards by providing explosion/flame-proof or intrinsically safe encoders, as well as products for less hazardous designs, depending on their customers’ requirements.

This article was written by Ed Brown, Editor of Sensor Technology. For more information, visit here .

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