More Input on Intrinsic Safety

April 3, 2009
Safety Differences between North American Processing Plants and European Processing Plants
This article was printed in CONTROL's April 2009 edition.

Readers' Feedback

I enjoyed reading Rich Merritt’s recent article “Intrinsic Safety: A Foreign Concept” (Control, Feb. ’09, p.56). I found it to be a very well-written and balanced article, fairly presenting the various aspects of this issue. One reason noted is that North American plants tend to be wide open, outdoor facilities that enjoy a constant wind, whereas European facilities tend to be inside buildings. Rich goes on to note “There are other reasons as well.”  I would like to present some of those “other reasons.”

Many North American plants have hazardous areas inside buildings, where those hazardous locations are defined on the hazardous area maps per appropriate standards, such as NFPA 497’s “Classification of Class I Hazardous Locations for Electrical Installations in Chemical Plants.”

Most of our plant buildings are combinations of classified and non-classified areas. In such cases, it’s often not required to make the I/O cabinets and control panels explosion-proof. They only have to be relocated outside of the hazardous area. Moving a panel 20 feet one way or another is often sufficient to remove it from the hazardous area. Doing this allows the use of standard hardware and standard maintenance practices for the equipment installed in the non-hazardous location. Of course, all of the wiring systems and equipment installed in the hazardous area need to comply with the applicable regulations for the hazardous area.

Intrinsic safety (IS) systems have advantages and disadvantages. Advantages include the fact that IS systems can be worked on while energized when the area is hazardous and that lower-cost wiring methods are acceptable. However, the IS concept does have limitations. For example, high-power equipment, such as 460 VAC motors and pipeline heat tracing, can’t be made IS, so working on motors and other power equipment always requires de-energization in hazardous areas.

A disadvantage for IS systems is that they generally require more complex engineering to be done, costing more from an engineering standpoint than explosion-proof systems. Article 504 of the National Electrical Code includes various requirements that tend to increase engineering costs for IS systems. (I know others will disagree with this, and it’s somewhat a function of what type of system the engineering contractor is most experienced with.) For example, per Article 504, all IS wiring must be segregated from non-IS circuit conductors. In plants consisting of a combination of IS and non-IS wiring, this requires installation of two wiring systems for instrumentation, and means that care must be taken as the system is modified to maintain this required segregation.

Costs for wiring methods for IS systems are lower than for explosion-proof systems. However, the customer must make the calculations to make sure the wiring system cost savings aren’t eaten by any increase in engineering costs. Another real disadvantage of IS systems is the installed base and existing knowledge about explosion-proof systems in plants in North America versus general lack of knowledge of IS systems. In addition to the required training, maintaining a plant that’s a combination of explosion-proof and IS systems can be confusing for plant E&I people.

For all of these reasons, the comments of Andre Dicaire, of EmersonProcess Management, are understandable. “We do a lot of upgrades and expansions in North America, and typically we just connect to the existing field instrumentation. Customers do not rip out their functioning field instrumentation to reinstall it using IS barriers.”

Generally, if a plant is IS, it makes sense for its expansions to be IS. If it’s explosion- proof, it makes sense to keep its expansions that way. For new greenfield sites, a thorough cost analysis would be appropriate.

Jim Reizner
Corporate Engineering
The Procter & Gamble Company

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