Specifying enclosures for hazardous area applications
The function of an enclosure is to protect the internal components from excess heat, humidity, dirt, dust or water. Whether sourcing an enclosure from a hazardous environment such as an oil and gas platform, or for a dusty environment such as a flour mill or sugar production plant, vital components inside need to be sheltered from the conditions in which they operate.
Engineers can often fail to fully appreciate the difference between Ingress Protection (IP) ratings and other requirements such as being fully certified for use in hazardous gas or dust environments. Enclosures for these environments are normally manufactured from stainless steel or glass-reinforced plastic (GRP), but there is an increasing number of materials from which they can be manufactured.
"It is generally accepted in the oil and gas industry that hazardous area enclosures need to be manufactured from 316L stainless steel as a minimum," says Gary Johnson, technical manager at Cooper Crouse Hinds UK. "In reality, however, most of our hazardous area enclosures are made from electro-chemical, polished 316L high-grade stainless steel as standard, as this offers enhanced corrosion resistance over other finishes."
GRP enclosures also offer technical advantages. The material can be graphite-loaded in order to avoid a build up of electrostatic charge. This is obviously of particular importance in hazardous areas.
Moulded, non-metallic enclosures enable the manufacturer to construct very modular or flexible design features for the customer, including quick fitting of components such as pushbuttons, switches and lamps. This type of enclosure also enables it to be designed with 'low-sided' enclosures for easier wiring.
Moulded enclosures have one specific shortcoming; no inherent attenuation to the passage of electric or magnetic fields. In many applications, this deficiency is of no consequence, but if radiation emitted by the electronics or their susceptibility to external fields is a potential problem, the lack of screening could be an issue.
Enclosure manufacturers have typically addressed this issue by adding conductive coatings to the inside of the enclosures, although some products are moulded from a conductive plastic. Whichever approach is used, the design of the mouldings can have a significant impact on the screening effectiveness of the conductive coating.
Suppliers of conductive coatings have developed several different main materials for spraying on the inside of enclosures to achieve different levels of attenuation versus cost. Vero Technologies has worked closely with its partners RF Solutions and Polymer Coatings to offer three coatings that will suit more than 95% of possible applications, with other coatings available for highly specialised uses.
Hazardous area enclosures are subjected to a series of mechanical impact tests, in places that are considered to be the weakest points of the enclosure. If the material is considered to have a reduction in resistance to impact at lower temperatures, the box is also impact tested at the lower end of its operating temperature range.
"These tests are important," says Johnson, "because once installed in a harsh hazardous area, it is often difficult to service an enclosure due to the hazardous area or it could get inadvertently knocked or bashed in a working environment. This is why it is important that the enclosure must remain intact."
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