Purpose of inerting
Inerting means changing an atmosphere’s composition in a controlled way so that it cannot support the combustion under consideration. A common application uses nitrogen to reduce oxygen in a tank’s vapour space. Effectiveness depends on the fuel, gas used and process conditions. Introducing gas for an arbitrary period does not demonstrate that a safe condition has been reached.
Related concept: Explosive atmospheres.
Variables to control
The installation needs defined target concentrations, measurement points, flow rates, pressure and responses to failure. Air ingress, opening access points, emptying, filling and loss of supply must be assessed. A sensor reading represents only its location and measurement conditions; it does not replace investigation of poorly purged areas or stratification.
A fire risk can become an asphyxiation risk
An atmosphere that is inert for a process can be fatal to a person. The gas displaces oxygen and may reach neighbouring spaces through leaks or discharges. Oxygen deficiency cannot be identified by smell. Ventilation, signage, access control and detection suited to the scenario are needed, including for contractor work and maintenance.
Before opening or entering
Opening and entry require specific isolation, atmospheric-checking and authorisation procedures where applicable. Contaminants and the possibility of a hazardous atmosphere being re-established must also be assessed. A respiratory filter does not supply oxygen and cannot resolve an oxygen-deficient atmosphere. Rescue must be prepared, without improvised entry by colleagues.
Example during a shutdown
During a hypothetical shutdown, a nitrogen line is disconnected and a tank is prepared for maintenance. Isolation, ventilation and the atmosphere at relevant points are verified before handing over the work. If a connection later changes or purging restarts, the previous authorisation no longer describes actual conditions.
Limitations and maintenance
Sensors, alarms and interlocks require testing and maintenance. No single oxygen concentration can be specified as universally suitable for every product. Process safety conditions and conditions for human occupancy differ and must be documented separately. Inerting does not, by itself, remove pressure, temperature or chemical-incompatibility hazards.
