Foundation fieldbus in hazardous areas

Implementing Foundation fieldbus can seem like a daunting task, and when coupled with hazardous-area considerations, may approach information overload; however, it doesn't have to be an explosive concept.

By C. Bruce Bradley

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By C. Bruce Bradley, PE

Some portion, if not the majority of most chemical plants today are electrically classified as a hazardous area. This classification adds a layer of complexity to the design and implementation of the control system.  Implementing control networks such as Foundation fieldbus into the system may make the task seem overwhelming.

The National Electrical Code (NEC) defines classified or hazardous locations as those areas “where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings.” The NEC recognizes class and zone as two methods for electrically classifying an area as hazardous. The class method is the primary method use in the U.S.

The Class System
The hazardous area classification has three components. The class separates fuels into families. The three classes are 1, 2 and 3, which signify an environment of flammable gases, vapor or liquids, combustible dusts, or ignitable fibers and flyings, respectively.

The division term separates the area into two parts based on the probability that an explosive fuel and air mixture will be present. In Division 1 areas, ignitable concentrations of fuel can exist all or some of the time under normal operating conditions. In Division 2 areas, under normal operating conditions, ignitable concentrations of fuels are not likely to exist.

The group term categorizes materials with similar explosive properties. The group categories are A, B, C, D, E, F and G, which signify acetylene, hydrogen, ethylene, propane, metal dust, coal dust and grain dust, respectively.

A chemical plant with a classification of Class 1, Division 2, Group C/D would have an environment of flammable gases, vapor or liquids with similar explosive properties to ethylene and propane in ignitable concentrations not likely to exist under normal operating conditions. For more information regarding hazardous area classifications reference NEC Chapter 5, “Special Occupancies” or “Intrinsically Safe Foundation Fieldbus H1 Networks” in the January 2007 issue of Control.

ISS Fieldbus Implementation Options

 

 

Explosion-Proof

Intrinsically Safe (Entity)

Intrinsically Safe (FISCO)

Intrinsically Safe (Routemaster)

Nonincendive (FNICO)

Hybrid (HPT)

Control Drawing

 

Not
Required

Required

 

List of devices only

Required

List of
devices only

Required

 

Entity
Calculations

 

Not
Required

 

Required

 

Not required.
Cable meets
FISCO specs

 

Required, but only done once for the worst-case scenario of the longest spur and the type of cable specified

Not required. Cable meets FNICO specs

 

Required, but per spur only, since each spur is a separate circuit.

Maximum
Current

None

80 mA

120mA IIC 265 maIIB

350 mA

180mA IIC 320 mA IIB

>500 mA

Allowable Area Classification Implementation

 

Division 1 and 2

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