Automation in any hazardous area has its challenges, regardless of the predominantly North American class and division or zone classification system of that hazardous area.
Machine builders rarely have to deal with the constraints of hazardous zones since most machine operations would be assembled in a safe environmental space. However, process control systems are more likely to have to deal with the various zones for protection.
The main issue at hand is the fact that electrical devices create sparks. That spark can and has ignited hazardous gases and dust/fibers that were in the environment of the devices.
This explosion risk is different from a high current arc flash type of explosion, which can be created by a line fault. An environmental explosion is created by three requirements: energy, fuel and oxygen.
The energy can be referred to as the spark that lights the fuse. The fuel is the hazardous gas/dust/fiber, and the air in which this spark exists supplies the oxygen. It’s akin to lighting a match or striking the flint starter on a propane grill.
There is a need to understand the environmental classifications to gain some insight into what constitutes a hazardous location.
Class I, Div. 1, defines an area of combustible gases, liquids or vapors such as ammonia or hydrogen. Zone 0 and Zone 1 classified locations further define the hazard by using phrases such as continuously present or most likely to exist.
Class I, Div. 2, suggests that the hazardous material (gas/fluid/vapor) would only be present under abnormal conditions.
Class II, Div. 1, suggests that the environment can support combustible dust under normal conditions, while Class II, Div. 2, suggests that combustible materials are unlikely to exist normally. Carbon would be considered a combustible dust. The group rating further defines the type of combustible material present.
There are many resources available to define certain areas for class/division and zone identifications.
There are four different containment technologies to learn to gain some insight into how automation can be implemented in this environment.
Potential explosion devices are spark-generating and are enclosed in explosion-proof, NEMA-rated enclosures when installed in a hazardous location. The main characteristic of the enclosure is that there is a wide flange that has multiple bolts to torque down the cover. The premise for this design is that, should an explosion occur within the enclosure, the ionized gas that would be created inside the enclosure would be cool by the time it drifted through the flange into the hazardous area, so an external explosion would not occur. Motor starters/contactors and relays would be examples of these devices.
Intrinsically safe (IS) devices limit the amount of energy that is delivered to the device in the hazardous area. This energy level is defined as the level that would be below the level required to ignite the hazardous material where the device is located.
A relatively new specification is identified as non-incendive. These devices are similar to intrinsically safe but are not suitable for Class I, Div. 1, Zone 0. These devices include many automation products from all major manufacturers including remote I/O, PLCs/PACs and drives, among the most popular.
ATEX comprises two EU directives concerning explosive atmospheres. There are similar certifications in various other countries, as well. In reviewing a brochure from Rockwell Automation, I was surprised to see the number of hazardous-location devices available, including power supplies and proximity sensors.
I was also introduced to NAMUR sensors. These are devices that adjust current levels to identify the presence of the detected material instead of taking the output of the sensor from zero to some voltage. These devices can be installed directly in the field, as can intrinsically safe components.
Lastly explosion containment can be attained by having a purged enclosure. These enclosures are supplied with either clean air or an inert gas, which will not ignite based on the devices that are in the enclosure itself.
Purging can be a huge cost-saving approach to controlling a process or machine that is deep in a hazardous area. Standard components can be implemented in the solution. The enclosure can also be selected for installation in certain environmental considerations such as NEMA 4X for corrosion protection.
The main issue of hazardous location controls is cost. The explosion-proof enclosures are not cheap and require proper maintenance, such as tight closure, while IS devices are more expensive and limited to how much energy they can provide, so they’re not suitable for all applications, although IS costs are becoming less of an issue.
There are many options available for automation in hazardous locations now. Design well.
