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Douglas L. Keech, Industrial PC Product Manager, HMW Enterprises, Inc.
Explosive vapors and dusts occur during normal operations in a surprising variety of industries. These range from the obvious-for example, petrochemical processing-to lesser known ones like grain storage and include unexpected cases such methane production during wastewater treatment.
Introducing an electrically powered device like a computer into a classified hazardous area carries with it inherent risks of explosion. Modern methods of measurement and control of industrial processes make it almost mandatory to use software-based solutions, and PCs have become a standard form of operator interface. However, the potential loss of life, equipment and productivity in areas where explosive fumes or powders occur makes it an expensive proposition to utilize the power of NT-based PCs.
The Thin Client model suggests a solution to some of the problems inherent in hazardous area computing.
The US National Electric Code and the Canadian Electric Code classify hazardous areas by two parameters: type of flammable material present and likelihood of its presence. Flammable materials are divided into three classes (I through III): gases, dusts and fibers. Gases and dusts are further categorized in groups (A through G) according to their explosive potential. The chances of these materials being present is indicated by specifying Division 1 (hazardous materials present during normal operation) or Division 2 (present as the result of a mishap such as a leak, ventilation failure or unexpected breakdown). The probability of its presence must be less than one percent for an area designated Div 2.
There are two recognized methods of designing electrically powered instruments to go into hazardous areas. One is the intrinsically safe (IS) method, which uses extremely lower power-so low that it's not practical for computing. The other method is to use explosion proof enclosures that - in the cases of Class I, Div 1 computers - must also be purged with an external supply of air. These enclosures must have sealed and securely fastened access doors and must protect the normal power circuits from coming into contact with an explosive atmosphere. Even in fault conditions the small amount of air for combustion inside the enclosure must limit that chance of fire or explosion, and the housing must be strong enough to contain any internal explosion.
Because of their bulk, weight, purging system, seals and gaskets-and because servicing requires that they be powered down and purged before restarting-PCs installed in hazardous areas are difficult to maintain and upgrade.
It's important to understand what we mean by "Thin Client." Thin Clients operate on a mainframe paradigm, and all their instructions and sessions come directly from a central, secure server. Each Thin Client realizes its own Windows NT session and operates independently from the other Thin Clients in the system. A Thin Client is a computer with no hard drive or other moving parts that can lead to machine failure. Because they have no hard drives, there is no loss of data if a local Thin Client gets damaged or has a local power failure.
The major reason to use the Thin Client model rather than networked PCs is lower Total Cost of Ownership (TCO). By reducing maintenance and install time, Thin Clients keep downtime to a minimum. For example, updating the HMI software on a server and five PCs requires the configuration of all five PCs - one at a time. With Thin Clients, the update is made once, on the server and all five Thin Clients realize the update immediately.
Thin Clients can be used anywhere to replace an NT workstation. The operator never knows the difference, and the system is much easier to maintain. The engineering department is able to keep all systems current with the latest software, without having to spend nights and weekends going from system to system.
To be able to provide the ideal industrial Thin Client for hazardous areas, HMW partnered with Automation Control Products (ACP), the recognized leader in industrial Thin Client technology. This partnership allows HMW to offer Thin Clients designed specifically for industrial installations that can be managed with ACP's management software, ThinManager.
HMW Enterprises has thirty years experience in designing computers and operator interfaces for hazardous area. In working on the design of Thin Clients for hazardous areas, we have found that the hardware generally offers several advantages over conventional NT workstations for hazardous area applications:
Although there are no formal standards that currently govern it, application security must be a critical concern in any hazardous area.
By its very nature, putting an NT-based PC into a hazardous area always provides excess flexibility-right where it should not be located. The plant floor is an area where there are PCs to do primarily one thing-monitor and control a manufacturing process. The operator is not switching applications, not compiling any source code, not reading files from floppy disks. The PC has actually become just a really powerful operator interface panel, with its sole purpose being to facilitate the process. The only reason that PCs are used instead of OI panels is that it's impossible to find a panel that can run the current crop of control and monitoring software.
In these environments operators should never start other applications, should never need access to any of the disk drives, and should not be allowed to update software or introduce viruses.
Major security concerns with NT-based PCs include the possibility of operator modification of configuration or other tampering, introduction of recreational software products, and the addition of viruses, Trojan horses and other security problems.
There are no such worries with Thin Clients, because the client is no more than a terminal. There is no point during power-up when the operator can gain access to DOS, and no disk drive to be used as a boot disk or virus entry portal. Security features are updated and enforced back on the server, monitored by the computer management staff.
Thin Clients used for data collection also simplify data consolidation, as all data retrieved through any of the clients is immediately available on the server. While all of the different applications for each client run on the server (or servers), they can all access the same database, even over a local network if desired. This keeps the overhead of potentially large data queries and updates off of the entire network.
When there are operator stations scattered all around the factory floor, upgrading NT machines can become a major project. The equipment often is required to be running 24 hours a day, 7 days a week. Imagine the plight of the engineer who has to install 20 or 30 upgrades of new HMI software-and shut down multiple production lines to do it. The Thin Client approach means that this can be done from a central server location and downtime is momentary at worst.
If a Thin Client is damaged, a new one can be brought in and configured with a button-click to replace the previous client. This requires no interaction at the server-it's all done from the floor. Adding a new client is even easier. Once the system administrator has set up a default configuration, any new client that is plugged in and turned on will power up with that configuration.
Most office installations don't use the serial port-there just aren't that many serial devices around the workplace. But in the factory, much of the data received from industrial equipment comes through a serial cable. A properly designed Thin Client needs at least two serial ports, so you can use one to communicate with your equipment and one for a serial interface to a touch screen. The ideal Thin Client will also have two separately addressed Ethernet ports to allow connection to (a) the central server and through it to the enterprise network and (b) to the instrumentation network, where TCP/IP is becoming the communications medium of choice.
Thin Clients are gaining widespread acceptance on the factory floor. They offer major advantages for use in hazardous areas. However, the user must still make sure that units specified for installation in classified areas meet necessary specifications for volume, electrical connections, enclosure strength and purging. This is where HMW, applying its extensive experience in hazardous area computing, is able to work with ACP to provide the perfect Thin Client system for hazardous locations.
For more information on ACP Industrial Thin Client computers, please visit our web site at http://www.thinmanager.com
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