By Loren Shaum, contributing editor
Before installing machines in hazardous areas, all sorts of horrific possibilities become part of the design consideration to ensure a safe operation. Certain machines are inherently volatile because of the materials incorporated into what the machine produces. Gluing and printing machines come to mind as those that require adherence to certain hazard standards. However, any machine installation in hazardous environments must be adapted specifically to that classified environment to assure inherent safety.
In most North American quarters, there are two schools of thought.
- Buy explosion-proof equipment, but risk much higher control costs than might be necessary.
- Buy more-conventional equipment and purge to keep a hazardous atmosphere away from the control devices.
To make the right choice, the normal functioning of the apparatus and the eventual malfunctioning of the apparatus due to faulty components must be considered, says Chris Romano, product manager at Pepperl+Fuchs (P+F). Moreover, all those conditions that can occur accidentally, such as a short circuit, open circuit, grounding and erroneous wiring of the connecting cables, must be evaluated.
Explosion-proof enclosures usually are heavy, cast enclosures that use a multitude of bolts to secure the cover. They are designed to contain an explosion inside the enclosure and prevent flame transmission to the area outside the enclosure, says Matthew Piecuch, Adalet sales and marketing manager.
One advantage of explosion-proof enclosures is they can be used in almost any hazardous location application, says Greg Quick, product manager, Hoffman. They also are low maintenance and typically do not involve additional electronics or moving parts, he adds. Some disadvantages, however, include their excessive weight and bulkiness, all those bolts to tighten and loosen that limit accessibility, and cost. Explosion-proof enclosures cost much more than a similarly sized general-purpose enclosure.
Adalet recommends explosion-proof enclosures because it views them as a simple, rugged solution for electrical components in hazardous locations.
An alternative protection method, acceptable in many of the same applications, involves purging or pressurizing the enclosure. The words purge and pressurization commonly are used interchangeably, says Quick. Purging actually describes multiple exchanges of enclosure volumes of air required in Zone 1 and Class 1 applications to expel gases and vapors. Pressurization is the subsequent action of supplying a positive pressure of an inert gas or instrument-quality air to keep combustible substances from entering the enclosure.
Purge/pressurization systems permit the use of general-purpose enclosures in hazardous locations where this protection method is allowed. By purging, says Quick, the area classification inside the enclosure is reduced (Div. 1 to Div. 2, Div. 2 to non-hazardous), which might allow use of lower-cost components. In addition, pressurized airflow reduces moisture and corrosion on internal components, the enclosures are much more accessible, and multiple enclosures can be connected in line with a single purge/pressurization system.
The enclosure must have a certain degree of tightness. However, there are no particular mechanical requirements because the pressure supported is not very high. In installations requiring large electrical apparatuses or control panels where the dimensions and high-energy levels make it impractical to use an explosion-proof enclosure, purging often is the only answer.
The methods disadvantages include the requirement for an inert-gas supply, some moving parts, alarm and monitoring requirements that might require electronics and electrical power, and the purge system typically is mounted to the outside of an enclosure, says Quick.
Purged or pressurized enclosures require constant pressure from inert gases, which could leak or become over-pressurized, cautions Piecuch. Proper ventilation and airflow also needs to be considered with purge or pressurization, and this can also fail or become compromised.