Thomas Stevic is a controls engineer at Star Manufacturing, an engineering and production company in Cincinnati. Contact him at [email protected].
Putting an operator interface in an atmospherically hazardous location requires an understanding of the hazards involved. If the interface can be located outside of the hazardous area, that would be the best solution. However, that is not always possible or practicable. The goal is to minimize the possibility of atmosphere ignition. Regulatory bodies such as OSHA, NEC and IEC often differ in the exact wording of the different classes, divisions and zones used to describe an atmospherically hazardous location. Refer to the applicable standards for the location and environment. An understanding of the risks is paramount to selecting correct equipment to be deployed.
The abundance of information that is available from modern control systems and the ease in which we can display this information have made many of us comfortable with providing much more information than required for machine operation in the form of colorful animated graphics. A review of what information is required to operate a machine may reduce the informational requirement for any specific project.
For many years, designers have used indicator lamps to indicate certain conditions. Quickly looking at a control panel and seeing a row of green lamps indicating all is well in the process may be all that is required. Lamps are much easier and much less expensive to incorporate into a control system.
In day-to-day operation, machine operators rarely use the total amount of information available on a graphical HMI. Using one or two status screens is often standard operating procedure until a malfunction or deviance occurs. A data historian will allow process corrections that do not require interaction with an operator.
If an HMI is used for informational purposes only, perhaps the simplest way to provide the required protection is to place the device inside an explosion-proof enclosure rated for the applicable hazard level. The enclosure can have a glass instrument cover, but these covers are somewhat limited in size. A 9-inch (225 mm) window will allow the use of a small screen to display important information to the operator. The use of a purge/pressure unit will keep the hazardous gas, dust or fibers from entering the enclosure.
When a full graphical interface is required, several manufacturers offer solutions for all types of hazardous locations. These can range from specially built versions of the manufacturer’s standard HMI hardware to ruggedized versions of an industrial PC. Hazardous-location hardware is designed with additional enclosure protection and low-power electronics. A standards body, such as UL in the United States or a notified body for the ATEX Directive in Europe, will test and certify equipment that may be used in hazardous locations.
If the hazardous-location HMI is a different model than the standard used in the rest of the plant, it may require additional programming to maintain the factory’s standard HMI look and feel. Most HMI manufacturers offer a runtime version of software that can be installed on any computer. Using a runtime version from the same manufacturer that supplied other HMIs can reduce programming costs. Often the design environment is the same when programming a standard HMI or programming a project that will be installed on a PC runtime.
An approved intrinsically safe thin client will reduce the complexity of electronic components exposed to the hazardous environment. Operating a client-server architecture offers benefits beyond the reduction of electronic complexity. Often the thin client will provide higher resistance to vibration and shock than a PC. A wider operational temperature range and lower maintenance requirements are often desirable qualities, especially for equipment installed in hazardous locations. When the server machine is located outside the hazardous area, the application and operating system can be modified and updated without entering the restricted area.
At first look, smartphones and tablets would seem to be a viable solution to the problem of putting an HMI in a hazardous area. Increasing numbers of HMI software developers are building Web-page interfaces that can be displayed on a smartphone or tablet. Under normal operational conditions, the phones and tablets are intrinsically safe devices. Several companies offer devices that carry approvals for use in a hazardous area. There is an excellent IEEE whitepaper at www.controldesign.com/cellphones that explores the use of wireless devices in hazardous areas. If a wireless HMI solution is chosen, some attention must be paid to the security issues exposed. At the very least, a VPN application will reduce the exposure to unwanted intrusions into your network.
When the goals of a particular project are to provide necessary information, to allow required interaction with an operator and to avoid causing an explosion, simple is always better. Over-engineering any project is a risk we always face. Over-engineering a project that will be installed in a hazardous area tends to drive up costs rapidly.
Also read:Â Machines for extreme environments
Homepage image courtesy of Stuart Miles at FreeDigitalPhotos.net
