Nitrogen backup systems

A maintenance supervisor decided to temporarily use instrument air as a source of breathing air. For the safety of the workers, he disconnected the automatic nitrogen backup system as a well-intended one-minute modification. ... 

The nitrogen backup system was not reconnected. Later, when the instrument air compressor tripped, the auxiliary pneumatic power (nitrogen) was unavailable. Without the... 

Backup supplies are always useful when a utility is lost Spare equipment or multiple-train equipment may be used in order to keep the utility on line even if one part of the system shuts down. An alternative or supplement is to provide backup from a different utility system. The number of different air systems found in a plant provides an opportunity to do this. Moreover, nitrogen can be used to back up some of the air systems. All these tie-ins require careful review during hazard analyses. Usually, the backup supply is activated when the primary supply loses a certain amount of line pressure. If this is done by means of a single pressure regulator and the pressure is also low on the backup system, the flow may be opposite to that intended. It is essential to keep nitrogen out of an air supply to a confined, inhabited space (e.g., the instrument air supply to a control room), and it is essential to keep air out of nitrogen when it is used as an inert gas. 

A well-meaning engineer modified the original system when he chose to use the plant nitrogen supply as a backup for this single air compressor. Several years after the new... 

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