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By Rich Merritt
Maybe one answer is blowin’ in the wind. North American chemical process plants tend to be wide open, outdoor facilities that enjoy a relatively constant wind, and European facilities tend to be inside buildings. With a higher air speed, explosive dust and vapors can disperse, making the site safer and limiting the number of hazardous areas where safe systems are needed. In Europe, because the buildings are sealed, the entire building is a hazardous area, thus requiring intrinsically safe equipment throughout.
That’s as good a reason as any to explain why Europeans have adopted intrinsically safe I/O and North American plants cling to the age-old practice of putting I/O in purged or explosion-proof cabinets. There are other reasons as well.
In 2003, VDC Research did a study of user preferences in hazardous environments, and explosion-proof and purged cabinets came in first and second, with intrinsic safety coming in fourth. The survey was for users in North America and Europe, and VDC reported that Europeans picked intrinsic safety first, while North America picked explosion-proof cabinets first.
One of the study’s key finding was, “Based on the survey data, it appears much more likely that users will switch to intrinsic safety in the construction of new facilities and in major updating projects in existing facilities, rather than for replacements and minor projects, where the perceived benefits of switching are less.”
However, things may be changing. A recent market study by ARC Advisory Group says that increased emphasis on process safety will cause a 12% growth in safety systems. “The safety system market has experienced unprecedented growth for the last two years,” according to Asish Ghosh, author of the study.
Essentially, there are two basic ways to keep I/O from sparking and causing an explosion in a hazardous area: The first is to put all the I/O in a purged or explosion-proof cabinet, so any sparks that result inside the cabinet cannot cause an explosion outside the cabinet.
The second method is to use barriers to limit the amount of current going to instrumentation in a hazardous area, so that it can’t cause a spark if a cable is cut or a connection comes loose (Figure 1). This second method is called intrinsic safety.
Figure 1. An intrinsic safety system uses Zener diode safety barriers to limit the amount of current to instrumentation in a hazardous area.
Explosion-proof enclosures can be put anywhere in a plant, including in the middle of hazardous areas. However, there is a major drawback—servicing anything inside or outside the cabinet requires shutting off all electrical power before opening the door. This can also shut down an entire process unit.
Explosion-proof cabinets are marvels of engineering, with shaped doors and seals that prevent heat or sparks from an internal explosion from getting outside. Unfortunately, such cabinets often have several interlocks and many bolts holding the seals closed. So, maintenance people may forget—or skip the process in a time crunch—to seal the cabinet properly, thus rendering it unsafe.
Intrinsically safe systems have other advantages. Joseph Kaulfersch, market analyst with Pepperl+Fuchs explains, “When one is making $20,000 an hour on a product, and an instrument can be fixed ‘hot’ or while running, it doesn’t pay to stop production. In the case of explosion-proof, users need to get a hot permit which takes time, plus the process needs to be shut down. An intrinsically safe barrier has a 30-sec return on investment.”
Figure 2. An intrinsically safe installation using Turck IM intrinsically safe barriers.
Intrinsically safe I/O, such as I/O cards, fieldbus interfaces and other devices, are kept in a non-hazardous area, with Zener safety barriers or isolation transformers limiting power to instrumentation cables going into hazardous areas (Figure 2). This method eliminates the need for explosion-proof cabinets and makes instrumentation in the hazardous area easier to service, because disconnecting wires to service instruments can’t cause a spark.
However, since IS limits the amount of current going into a hazardous area, you have to use IS instrumentation, switches, monitors and analyzers that can operate on the limited amount of current available. Fortunately, IS-rated instrumentation is becoming more widely available every day.
The answer depends on whom you speak to. “I really haven’t seen many requirements for intrinsic safety in North America,” says Bruce Jensen, manager, systems marketing and sales support at Yokogawa. “Those requirements and installations we do have in North America are generally in the pharmaceutical industry, where there is a combination of general-purpose, Class 1 Div II and Class 1 Div 1 areas. Here we have used barriers from different suppliers. Yokogawa does make I/O modules for its I/O subsystem in the Centum series with built-in barriers. However, the market for those appears to be mainly the Middle East and Europe.”
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