A better way to install automation in classified areas
Classified areas are often found in industrial plants due to the presence of hazardous gases and dust. When automation components are installed in these areas, certain rules, regulations, and design standards must be followed to ensure safety.
Hazardous areas are classified according to either National Electrical Code (NEC) or the International Electrotechnical Commission (IEC). Table 1 shows a simplified version of this classification system, and much greater detail can be found within the detailed regulations, specifically NEC Article 500 and IEC 60079.
The NEC standards are useful for reference. As shown in Table 1, Division 1 is more hazardous than Division 2 because hazards are more likely to be present. Therefore, automation components installed in Division 1 require a higher level of protection than those installed in Division 2. Three main methods are used to implement levels of protection as required in classified areas: explosion proof, purging, and intrinsic safety. All these methods of protection require careful design-and often expensive components, installation, and maintenance.
It’s useful to look at these three protection methods before examining an alternative way for installing automation components in classified areas.
Explosion-proof enclosures and conduit systems protect automation components in hazardous areas through two methods. First, they are sealed to limit the amount of gas or dust that can enter the enclosure or conduit system. Second, they must have the structural integrity to contain an explosion so it doesn’t propagate throughout the classified area. These enclosures are typically used in Division 1 areas.
Table 1: NEC Divisions and IEC Zones
|Frequency of Hazard Occurrence||NEC Classification||IEC Classification|
|Continuous hazard||Division 1||Zone 0|
|Intermittent or periodic hazard||Division 1||Zone 1|
|Abnormal condition hazard||Division 2||Zone 2|
|Table courtesy: Hoffman|
This method of protection has been in use for many decades and is thus well understood by many engineers, designers, and plant maintenance personnel. Many suppliers provide explosion-proof enclosures and conduit systems, and the design standards for such systems are readily available.
But compared to the standard National Electrical Manufacturers Association (NEMA) 4 enclosures, explosion-proof enclosures are very expensive, quite large, and very heavy (see Figure 1). Because these enclosures are sealed, the only practical method to dissipate heat is through the enclosure itself, limiting the type of components the enclosure can house, or requiring the enclosure to be oversized which further adds to costs.
Any operator interface components must be rated for use in the area, which precludes the use of any type of graphical interface in a Division 1 area. Graphical interfaces can be mounted inside the enclosure and viewed through a window, but this limits visibility.
Great care must be taken during installation of the enclosures and conduit systems to make sure proper sealing is preserved, and conduits must often be potted at transitions from Division 1 to Division 2 areas. Hot-work permits and accompanying precautionary measures are required to open explosion-proof enclosures, making servicing of the components inside the enclosure very expensive and difficult.
Finally, regular inspections of explosion-proof enclosures and conduit systems are required in some classified areas and strongly recommended in others. These inspections are necessary to verify the integrity of the sealing systems and routinely reveal the need for maintenance.
Another method for installing automation components in hazardous areas is to purge enclosures with compressed air or an inert gas. Purge systems don’t allow hazardous gases or dusts to enter an enclosure because the interior is always under positive pressure.
Depending on the type of purge system, this method allows standard enclosures and unrated automation components to be used in Division 1 and Division 2 areas. But purge systems can be quite expensive, particularly when they are used in Division 1 areas (see Figure 2).
The cost doesn’t stop with installation, as purge systems require a constant supply of compressed air or an inert gas. This approach makes maintenance of the components within the enclosure problematic because the enclosure cannot be opened during normal operation.
Limitations of IS options
Intrinsically safe (IS) systems can be safely installed in Division 1 and 2 hazardous areas because the components and wiring systems cannot release sufficient energy to ignite gas or dust in the area. Unlike explosion-proof or purge systems, this method of protection permits many types of maintenance during normal operation.
Because each component must be rated for use in the area, the range of items available for use is limited. Many common automation components, such as programmable logic controllers (PLCs) and motor drives, aren’t available with an IS rating. And even when IS-rated components are available, there is often an added charge from the supplier.
The design of IS systems requires a high level of engineering expertise, adding to implementation time and expense. Transitions from one classified area to the next often require IS barriers, further adding to the complexity of these systems.
Avoid the hazard
As related in the above sections, all three of the leading protection methods require detailed design and often considerable expense for system purchase and installation. And the expense doesn’t stop with installation, because IEC 60079 requires not only initial inspections of components installed in classified areas, but also ongoing periodic inspections, typically done annually. The inspections often reveal areas requiring maintenance, further adding to expense.
Realizing this, many suppliers began to introduce components within the past few years which can be safely installed and operated in Class 1, Division 2 (or Class I, Zone 1) classified areas. There is a significant difference between those two divisions even though they are in the same class, and the requirements for applicable devices and components are also significant.
Learn more about automation components rated for Division 2 use as well as information about component selection and application.
Location, location, location
Like real estate, when it comes to hazards, it’s all about location. And often the best approach for dealing with hazardous locations is staying out of them. Install as much of your automation equipment as possible outside of your Class 1, Division 1 areas. Even moving to a Division 2 area makes a major difference. This isn’t as difficult as it may sound. In many industrial plants and facilities, there are relatively few areas classified as Division 1, and these areas are often quite small. Even when they have a large footprint, improvements can often be made to limit the amount of gases released to the atmosphere, thus reducing the footprint of areas classified as Division 1.
Once Division 1 areas are reduced to a practical minimum, then all new automation components should be located in an adjacent Division 2, or even better, in unclassified areas unless there’s a compelling reason to do otherwise. And in some cases, it may make sense to relocate existing automation components from Division 1 to Division 2 areas to avoid the related inspections and maintenance.
For those few automation components that must be located in Division 1 areas, it’s often possible to use IS-rated components and connect to those components from Division 2 areas by way of IS barriers.
Once all Division 1 areas have been minimized, and Division 2 and unclassified areas consequently increased, careful automation component selection can be made to use only those items rated for use in Division 2.
Component selection and application
As recently as 20 years ago, there were very few automation components rated for use in Class I, Division 2 areas—and many of these were related to PLCs and relays. This situation has changed dramatically in the past few years, as the automation components listed in Table 2 indicate. And the list is steadily growing as suppliers respond to end-user demand.
|Table 2: Automation components rated for use in Division 2|
|Operator interface devices|
|PLCs and other controllers|
|Variable frequency drives (VFDs) rated up to 100 hp|
|LED flood lamps|
If a component is rated for use in Division 2, it can be installed in a standard enclosure and safely used without requiring any additional protection. Due to the wide availability of rated components, it’s now possible to assemble a complete control system with a human-machine interface (HMI), a PLC, an input/output (I/O) device, a power supply, motor drives, and other required components—all rated for Class I, Division 2 use.
This approach requires much less detailed design than the three main protection methods and costs less. No initial or periodic inspections are required, and maintenance is greatly reduced.
When automation components suitable for Division 2 use were first introduced, they often carried a substantial price premium over their nonrated counterparts. But this price premium has been greatly reduced in recent years, often eliminated completely (see Figure 3).
In many cases the product parts and materials have not been changed, just the inspection requirements for compliance. For example, in the case of an electromechanical switching compartment, the spark-producing components must be plastic sealed. The Division 2 rating assures that the sealed compartment will not leak over the component’s life. As a result, there is now a wide variety of electromechanical and solid-state relays rated for use in Class I, Division 2 areas.
As LED lighting has become commonplace, new Class I, Division 1 and 2 lamps have become available and are often retrofitted in oil refineries and other facilities as they modernize their lighting systems.
Typical applications for automation components in Class I, Division 2 areas include but aren’t limited to:
- Upstream oil and gas facilities for exploration and production
- Midstream oil and gas pipelines and storage facilities
- Oil refineries
- Petrochemical plants
- Paint booths and paint mixing facilities
- Water and wastewater treatment plants
- Mining facilities
- Pharmaceutical plants
- Agriculture facilities such as grain silos
- Food processing plants.
Protection vs. location
The three main methods for protecting automation components installed in classified areas are explosion-proof enclosures, purged enclosures, and intrinsically safe systems. Depending on the specific application and automation components, each of these methods can be effective. But each entails considerable upfront cost and ongoing care in terms of service and maintenance.
A viable alternative in many cases is to install automation components and systems in Division 2 instead of Division 1 areas and to then use components rated for Division 2 use. This simplifies design, cuts costs, and reduces operating and maintenance expenses.
In response to user demands, suppliers have introduced many automation components suitable for use in hazardous areas, with more becoming available on a regular basis. With careful components selection, most types of automation systems can thus be safely installed in Division 2 areas.
– Steve Massie is a product manager responsible for relays, lighting, circuit breakers, terminal blocks, and hazardous location products for IDEC. Edited by Peter Welander, Control Engineering contributing content specialist, email@example.com.
"Inspecting Instruments Installed In Hazardous Locations." In Control Engineering, Mar. 2015.
- Automation equipment used in hazardous areas requires special protection to prevent fires and explosions.
- Protective measures necessary can be expensive or problematic.
- Locating equipment outside of the most strictly regulated areas can be far simpler.
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