InTech

MAY-JUN 2017

Issue link: http://intechdigitalxp.isa.org/i/833037

Contents of this Issue

Navigation

Page 42 of 53

INTECH MAY/JUNE 2017 43 AUTOMATION BASICS grators can tailor the panel to their exact re quirements, saving time and reducing cost. Buying preassembled Zone 1 con- trol 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 mini- mal, 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 protec- tion for most control panels. Air-purge systems 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. Be- cause the enclosure is under positive pressure, general-purpose enclosures, de- vices, and components can be used. Designing these systems is a bit more complex than simply specifying Zone 1–rated enclosures, devices, and compo - nents. Different zones require different types of purge systems, with costs increas - ing 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 lin- ear 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 popu- lated 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 de- signed, 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 transmit- ter 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 com- ponent, 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. Intrinsic safety 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 pro- Figure 4. An air-purge system can allow installa- tion of general-purpose automation devices and components in a Zone 1 area. Source: P+F tection methods, because every compo- nent must be carefully selected to make sure it is intrinsically safe. The electrical energy delivered by wiring to these com- ponents from outside Zone 0 must be limited to levels insufficient to ignite flam- mable gases or vapors. Intrinsic safety bar- riers 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 flam - mable 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 con- tinue 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 ex - panded array of options makes it simpler to design control stations and panels for Zone 1, while reducing upfront and maintenance costs. n ABOUT THE AUTHOR Jim Dunn (jimd@idec.com), product manag- er at IDEC (www.idec.com) is an experienced industrial automation professional. Dunn has held multiple product marketing/manage- ment positions with Japanese and European industrial automation companies responsible for various sensor and safety products. Figure 3. Preassembled control stations, such as this IDEC unit, eliminate design expense for Zone 1 and 2 installations.

Articles in this issue

Links on this page

Archives of this issue

view archives of InTech - MAY-JUN 2017