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By Paul Miller
As old timers say, “new” is not always “better.” Consider for a moment, the transition from pneumatics to analog electronic to digital technology. Sure, we’ve realized important benefits from digital automation technology, which – among other things – provides the basis for today’s plant asset management strategies. However, when it comes to preventing explosions in hazardous classified plant areas, analog electronics represented a step backwards from pneumatics and digital technology introduces further cost and complexity.
Pneumatic instrumentation used compressed air to transmit the process variable measurements from field devices and then translate them into the appropriate control action. No electricity means no sparks or arcs. This eliminates one of the legs of the fuel/oxygen/ignition triangle, all three of which are required to initiate a dangerous explosive situation. That’s one of the reasons why, even in the “digital age,” if you look hard enough, you’ll still see pneumatic instrumentation puffing away in plants around the world.
Intrinsic Safety (IS) is one of the concepts developed to enable electronic instrumentation to be safely used in plant areas classified as “hazardous” due either to actual or potential presence of explosive gases or dust.
While pneumatics avoided the issue of electricity altogether, IS concepts limit the amount of electrical and other energy allowed to enter hazardous plant areas, thus eliminating the possibility of igniting explosions. This is typically done by using either zener diode or galvanic isolated barriers. Zener barriers are passive devices that require appropriate IS grounding. Isolated barriers do not require additional grounding.
With the straightforward, point-to-point wiring used with conventional 4-20 mA analog field communications, this doesn’t present any huge problems (although it does add cost and complexity, and the barriers themselves tend to take up a lot of valuable plant real estate). However, one of the major benefits of digital field instrumentation and communications is the ability to drop multiple instruments on the same wire. By limiting the amount of electrical current you can deliver to a fieldbus segment for intrinsic safety reasons, you also restrict the number of devices you can install on that segment. In many cases, this has made fieldbus cost-prohibitive.
“Intrinsic safety is an excellent technique for protecting electronic instrumentation in hazardous locations, and intrinsic safety provides the highest level of explosion protection for electronics,” says Mike O’Neil, director of the Moore-Hawke Division of Moore Industries. “As the uptake of fieldbus becomes the norm rather than the exception, there are many users who are demanding similar protection for their fieldbus networks. However, there are significant conflicts between the technique of intrinsic safety and multiple-device, multi-drop networks.”
One of the reasons for the slower than anticipated acceptance of fieldbus is general confusion over how hazardous plant areas are classified and specific confusion over how you implement and verify digital devices and communications networks in these types of environments.
According to Chuck Carter, director of the fieldbus training center at Lee College in Baytown, Texas, “The whole topic of what is happening in the IS world vis-à-vis fieldbus appears to be in a state of flux, with confusion reigning supreme. On top of that, everyone seems to have a unique set of circumstances regarding their facilities’ IS requirements and applications. Clearly, the industry must settle on one method for classifying and using IS, or this issue will remain in a state of flux for the foreseeable future, and that serves no one’s interest in the long run.”
The Entity approach for fieldbus in hazardous plant areas was used successfully for the ethylene dichloride cracking furnace process at the Shin-Etsu manufacturing facility in Rotterdam, The Netherlands.
In North America, hazardous areas are identified by “classes,” “divisions” and “groups.” These indicate the type of flammable material (gases, dust or fibers), whether the presence of the flammable material is normal or abnormal, and the specific type of gas or dust, respectively.
In Europe and most other countries outside of North America, hazardous plant areas are classified using “zones,” “groups” and “protection types.” Zones define the probability that the flammable material will be present; groups classify the flammable nature of the material; and protection types indicate the level of safety required for the device.
For a variety of reasons – some logical, some not so logical – the major approaches used to help ensure safe operation in hazardous environments also evolved differently in North America and Europe.
In North America, the “explosion proof” approach initially gained wide acceptance in refineries, petrochemical plants and other industrial facilities with hazardous environments. This approach uses specially designed and constructed, NEMA-rated electrical conduit and equipment enclosures to isolate electronics from the hazardous gases or dust and physically contain any initial explosions that might inadvertently occur. This prevents secondary explosions from occurring outside of the enclosures. Not only are these heavy duty, explosion proof enclosures very expensive to purchase and install, but also they preclude the possibility of doing any “live work” on the equipment contained within.
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