"We supply HMI with our machines in hazardous environments. We have an opportunity to design smaller machines for cleanroom environments that still have an explosive atmosphere risk. What are key differences we should look for to supply OI for this environment? Can we standardize on one for both types of install?"
—From September '12 Control Design
What to Consider
There are two design issues here — the first dealing with the location and the second dealing with component selection. Much of the design will depend on area classification from an environmental and explosion hazard perspective, and the local authority having jurisdiction. Just as important are selecting reliable and approved components for use in those areas, particularly when the desire is to have a common design for both types of environments. Installation time and labor, and long-term serviceability, also contribute to total cost of ownership, and could influence customer perception of value.
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• Are the HMI components certified for use in hazardous locations?
• Will the HMI be mounted in a Class I, Div. 1 environment?
• Is the touchscreen glove-friendly? A resistive touchscreen is glove-friendly, even with a triple latex glove.
• Will it require an operational touchscreen in Div. 1? If so, look for an intrinsically safe touchscreen. These are available in 12–22 in. sizes from a few vendors.
• Will the HMI be in Class I, Div. 2? In that case, the possibilities are broader, including non-purged options.
• What is the interface to the computer: KVM, thin client or internal PC?
• Will the HMI be integrated into a control panel or on the machine, or as a standalone packaged solution?
• If panel-mounted, is the panel purged/pressurized?
• What's the rating of the enclosed equipment? If it's Div. 2, it might be possible to eliminate a Z purge. If not, a Z purge will be required for Div. 2, X purge for Div. 1.
• If it is a standalone package, how will it be mounted? A flush-mount solution minimizes intrusion into the room, but could be difficult to install due to space behind the wall. A wall-mount solution will require a sealer between the HMI enclosure and the wall to prevent bacterial growth behind the unit.
With regard to use of HMI in cleanrooms specifically, there are more key considerations:
• For cleanrooms in the life sciences, is gasket material FDA grade and compatible with standard operating procedure (SOP) instructions?
• Is the system sealed to prevent internal particles from entering the room, and is it washdown/spray and wipe compatible?
• For cleanroom environments, there are requirements for positive pressure in the room, minimal leak rates. Is the equipment designed to withstand this positive pressure and hold a purge/positive pressure associated with the purge system?
• Are the touchscreen and keyboard tested for anti-microbial properties and compatible with SOP instructions?
• Is the HMI solution easy to install? The physical installation should take about 1 hr with a flush-mount solution, and similar or less time for other styles.
• Is the HMI solution easy to maintain over the life of the plant? An RTV-based door seal will require scraping if the enclosure requires any internal maintenance, plus resealing and vulcanization time of 24 hr. An FDA gasket approach will have a 10 min MTTR.
To answer your question about whether you can standardize on one for both types of install, the simple answer is yes — provided you plan well. You'll need to ask some questions first, including determining if the application is a semiconductor-type cleanroom or a pharma/biotech cleanroom.
Biotech might not require as clean a room (Class 100 or Class 1, for instance), but gasketing materials and cleanability can create contamination at the microbial level. Cleanability includes the type of stainless steel (316L vs. 304), and the finish of the metal, including weld seams, polish and surface passivation.
Gasketing must not be porous, but be able to withstand cleaning according to SOPs. If a keyboard is provided, it needs anti-microbial properties tested against a standard and needs to be sealed so no particles are expelled into the cleanroom.
If this is a North American design, you likely have UL as a requirement. UL has an OEM panel program for industrial controls for both general-purpose (UL508A) and hazardous locations. The OEM can certify the system for use in this hazardous location, subject to agency review, and shorten the development and delivery time. Under this program, all components must be UL-listed or UL-recognized and suitable for the environment.
Non-UL approved components, Class II installation or Y-purge systems still require physical inspection and approval that will increase costs and extend development and/or delivery time.
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