Encoders Go Barrier-less In Hazardous Locations

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Encoders Go Barrier-less in Hazardous Locations By Allen Chasey

With today's widespread demand for energy, oil exploration and production are critical to the global economy. Drilling companies and equipment manufacturers are continually looking for ways to improve efficiency and save cost while improving production. In most drilling applications encoders are used to provide position and speed feedback for proper control of equipment like top drives, draw works, and pipe racking systems. However, encoders are electronic devices so they require special attention when used in hazardous environments such as a drilling platform. The energy contained Encapsulated hollowshaft encoder from Dynapar within a typical industrial encoder circuit must not be allowed to ignite vapors and gases present in the drilling process, and there are several methods used to accomplish this. In this paper we will discuss some of the current options, their drawbacks, and a potential solution. Regulation Before investigating the different ways to make encoders suitable for hazardous duty, we must first take a quick look at how they're classified. ATEX, an acronym for ATmospehere Explosible (French for "explosive atmosphere"), is a commonly recognized European Union directive that establishes a detailed code for products used in hazardous locations. Encoders we'll discuss in this paper generally fall into Zone 0, where flammable materials are continuously present, or Zone 1 where flammable materials are intermittently present. Further clarification is required for the ignition energy of flammable material, commonly referenced by typical gases such as acetylene, ethylene, or propane, for example. These are classified as groups, IIC, IIB, and IIA, respectively. Special Enclosures One method used to make an encoder suitable for hazardous duty is to provide a special housing for the electronics. This special housing can take on several configurations, each with a specific ATEX code. The first method is to immerse the energized circuit in an oil bath. In this case the surrounding oil prevents contact with the hazardous atmosphere (ATEX protection code EEx o). Another method used is filling the housing with powder. In this example the circuit is surrounded by a powdered material, preventing arcing (ATEX code EEx q). A third method commonly employed is the use of a pressurized housing. In this case the encoder electronics are sealed inside a tough vessel. If an ignition does take place here, the ignition and possible subsequent explosion is contained within the housing (ATEX code EEx d). While each of these methods makes for a viable solution, the housing required usually turns a small encoder into an unwieldy device that takes up more space and places more loads on coupled components due to increased weight. There is also the consideration of the extra cost involved with the special housings and/or fillings. Intrinsically Safe Designs Another common type of protection class for encoders is Intrinsic Safety. Encoders classified in this area can be approved for use in either Zone 0 or 1. Again, the ATEX protection code makes the distinction here: Zone 0 (EEx ia) or Zone 1 (EEx ib). As the name implies, intrinsically safe designs are inherently "safe" through limitation of the energy allowed in the circuit, preventing unwanted effects of arcing. Intrinsically safe designs are commonly used with electrical barriers, external devices that effectively isolate the electrical connection between the encoder, controls, and power supply. This design however, does have its drawbacks. Most obvious is the added cost of the barrier itself. Typically two barriers are needed for each encoder; one to isolate power, the other to isolate the encoder signal. At a rough "street" price of $200 per barrier, this can quickly add cost to the system, especially when multiple encoders are used. Another drawback is in the complexity of wiring. The encoder must be wired through the barrier, and not directly wired to the power or controls. Additionally, there is a voltage limitation within the circuit. The output voltage from the encoder is relatively low, commonly 5VDC. This decreased signal-to-noise ratio can be a problem when used in electrically noisy environments. This also limits allowable cable length, as a low power signal can degrade quickly over long distances. A B... [download for more]