Purging a Class I/ Div. 2 HMI for Div. 1 applications is practicable
We get many questions about whether it is acceptable to put a Class I/ Div. 2 rated HMI into a purged enclosure for Class I/Div. 1 installation. It is certainly possible, but there are a few requirements.
First of all, the HMI device must be able to be purged. You must be able to verify that you can exchange enough air volume out of the HMI to ensure there is no build-up of explosive gases. NFPA standards require a minimum of four volume exchanges. So if you have an IP65 rated HMI that is completely sealed, then it cannot be purged.
Another consideration is how does the operator need to interface with the HMI device? Is it for visual purposes only, are there pushbuttons, is there a touchscreen? If the operator only needs to see the HMI screen, without needing to interface with it, then the HMI device can be put inside the purged enclosure behind a window.
In most cases, this is not practical, because most HMI installations require the operator to have some type of interface with the display. The preferred way is to install the HMI in the door or wall of the purged enclosure, where the rear side of the HMI is being purged, and the display side is exposed to the hazardous environment. When mounted in this fashion, it is critical to maintain a Type 4/4X seal for the enclosure. This is important for proper ingress protection and maintaining an air seal that allows the purge system to function properly.
The next hurdle when attempting to install a Class I/ Div. 2 HMI into a purged enclosure is addressing any pushbuttons or touchscreen interface. Typically, the pushbuttons on an HMI are part of a membrane keypad on the front face of the display. If the membrane keypad is sealed, then you must be able to verify that the internal volume of each pushbutton is less than 1.22 cubic inches (NFPA 496; 2008 edition, sec. 220.127.116.11). In some cases, the membrane keypad is just an overlay, and there is an actual contact behind each button. If this is the case, and you verify that the purge system is exchanging the correct air volume around the contact, this would be acceptable.
Touchscreens can be even more complicated. There are many types of technologies used today such as resistive, projected capacitive, and infrared to name a few. Many of these technologies are not suitable for Class I/Div. 1 use unless a second method of protection is implemented.
The most common way is to make the touchscreen intrinsically safe. Intrinsic safety, which implements energy limitation to the device, is a suitable protection method for Div. 1 use. The issue here is that most HMI manufacturers do not offer an option for an intrinsically safe touchscreen. So, if a touchscreen interface is a requirement, make sure you partner with a manufacturer that is able to provide this. Pepperl+Fuchs is a supplier of Class I/ Div. 2 HMI equipment that has the option for an intrinsically safe touchscreen.