This technique excludes a potentially hazardous atmosphere through the use of positively pressurized enclosures or rooms. It is particularly useful for large installations such as control rooms, rotating equipment such as motors and generators, and for computer systems. The obvious disadvantage is that the equipment must be tethered to a source of clean purge gas at all times.
This technique is relatively straightforward once it is fully understood, but there are several important requirements that must be observed. For example, it is often a requirement that fail-safe mechanisms be in place so that if the positive pressure fails, the protected equipment is automatically shut down. This may include situations such as opening access panels and doors, which makes equipment access less convenient.
Another hidden meaning is contained in the very name of the protection technique. Specifically, "purge" and "pressurized" are often taken to be synonyms for each other, but they actually refer to the two different stages of establishing the purge protection on the equipment.
Finally, purge/pressurized equipment is subject to purge failures or shutdown. In order to re-activate the system, it is necessary to have some equipment operating in the absence of purge/pressurization protection. Therefore, most purged systems contain at least a few pieces of equipment that are protected by an alternative technique, such as explosion-proof or intrinsic safety.
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