- By Jim Dunn
- Automation Basics
By Jim Dunn
Certain areas in industrial settings are classified as hazardous due to the presence of flammable gases, vapors, dusts, or fibers. This article describes various ways to design these panels so they do not become potential ignition sources, using the International Electrotechnical Commission (IEC) zone hazardous area classification system for this purpose.
Figure 1 depicts how the IEC zone system classifies areas based on the ignitable concentrations of flammable gases or vapors. Zone 0 is the most hazardous area, followed by Zone 1, and then Zone 2. It is much more expensive, complex, and time consuming to design, fabricate, and maintain control panels to use in Zone 0 rather than Zone 1 or 2. So, the first step is to locate control panels outside of Zone 0 areas wherever possible. This can often be accomplished by moving panels just a short distance, often as little as a few feet.
Figure 2 shows a typical zone classification scheme for a process plant. Zone 0 areas are only those inside or right next to vessels or pipes, with Zone 1 areas in relatively close proximity to Zone 0, and Zone 2 areas a bit farther away.
Once all panels have been located to the least restrictive zone possible, the three main design methods for compliance can be considered:
- rated enclosures and components
- air-purge systems
- intrinsic safety practices
Figure 1. The IEC zone system classifies areas according to the expected presence of hazardous atmospheres.
Figure 2. Control stations and panels should be located outside of Zone 0 whenever possible, which often just requires relocation to a few feet away.
Use Zone 1 and 2 components and enclosures
Using properly rated components and enclosures is the simplest method for compliance in Zone 1 and 2, although it is not suitable for Zone 0 in most cases due to the lack of availability of Zone 0-rated components. Any control panel suitable for installation in Zone 1 will also be suitable for Zone 2, so the focus of the rest of this article will be on Zone 1. The enclosure and the components should all be specified for use in Zone 1, and the panel design should also meet Zone 1 requirements.
This method works well for smaller and simpler control panels-often referred to as control stations-populated with push button, switch, and indicator light devices. However, it often does not work well for enclosures populated with more complex components\, such as programmable logic controllers (PLCs), motor drive, and human-machine interfaces (HMIs), because many of these components are not available with proper ratings. For example, your company's preferred make and model PLC might not be certified for use in Zone 1.
Preassembled Zone 1 standard control stations populated by an assortment of rated push buttons, switches, and lights are available from some suppliers (figure 3). Some suppliers will build customized control stations, so end users and integrators can tailor the panel to their exact requirements, saving time and reducing cost. Buying preassembled Zone 1 control stations, either standard or custom, eliminates design expense.
Once Zone 1 control stations or control panels are specified or designed-and then installed-maintenance is minimal, and years of trouble-free service can be expected. The cost of designing with this method of protection, as well as the cost of the components, is considerably less than the next two methods of protection for most control panels.
Figure 3. Preassembled control stations, such as this IDEC unit\, eliminate design expense for Zone 1 and 2 installations.
Figure 4. An air-purge system can allow installation of general-purpose automation devices and components in a Zone 1 area. Source: P+F
Another popular protection method is an air-purge system (figure 4), which is suitable for Zone 1, but not Zone 0. These systems supply air or an inert gas to the enclosure to maintain a positive internal pressure with respect to the environment, thereby preventing flammable gases or vapors from entering the enclosure. Because the enclosure is under positive pressure, general-purpose enclosures, devices, and components can be used.
Designing these systems is a bit more complex than simply specifying Zone 1-rated enclosures, devices, and components. Different zones require different types of purge systems, with costs increasing as the zone becomes more hazardous (e.g., a purge system that allows the use of general-purpose components in Zone 1 is more expensive than one that allows the use of Zone 2 components in Zone 1).
These systems are not practical for small, simple control stations. The cost of the purge system does not decrease in linear proportion to the enclosure size, but instead has a relatively high minimum price. On the other hand, purge systems do work well for larger and more complex control panels, particularly those populated by more complex components, such as PLCs, motor drives, and HMIs. This is because they allow nonrated, general-purpose components to be installed in Zone 1, a feature not available with the other two methods of protection.
Maintenance consists of making sure the air-purge system is operating as designed, a task made simpler by systems with a pressure gauge or transmitter to ensure positive pressure is maintained. A gauge must be manually monitored to ensure correct pressure. A transmitter can send a signal proportional to pressure to a remote monitoring and control system, easing maintenance.
If a panel needs to be opened for any reason, such as to repair or replace a component, pressure and protection are lost. Consequently, the entire area must first be made safe with respect to the presence of flammable gases or vapors. This can be problematic, because it often requires a partial or full shutdown of the plant area.
This method of protection limits the amount of electrical energy that can be released to a level insufficient to ignite flammable gases or vapors. This is the only method of protection suitable for Zone 0, and of course also works in any Zone 1 or 2 area.
The design of these systems is much more complex than the previous two protection methods, because every component must be carefully selected to make sure it is intrinsically safe. The electrical energy delivered by wiring to these components from outside Zone 0 must be limited to levels insufficient to ignite flammable gases or vapors. Intrinsic safety barriers are commonly used for this purpose.
With proper precautions regarding tools and work methods, maintenance can be performed on any component without having to ensure the area is free of flammable gases or vapors. This is possible because each component in the system cannot release electrical energy sufficient to ignite flammable gases or vapors.
Zone 1 components
In the past, most Zone 1 installations used either air-purge systems or intrinsic safety protection methods. This has changed in recent years due to the more widespread availability of devices, components, and enclosures rated for use in Zone 1.
Push buttons, switches, lights, panel meters, and other simple devices are widely available with Zone 1 ratings-and the variety of devices available for use in these areas has grown rapidly over the past few years.
The Zone 1 product offerings continue to grow as some suppliers now offer more complex components, such as PLCs, HMIs, and power supplies with this rating. It is now possible to design a complex control panel for use in Zone 1 by simply selecting the right devices and components and by following simple design guidelines.
Preassembled Zone 1 standard and custom control stations populated by an assortment of push buttons, switches, and lights are also available. This expanded array of options makes it simpler to design control stations and panels for Zone 1, while reducing upfront and maintenance costs.
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