During a plant shutdown, what is the primary concern when purging the system?

During a plant shutdown, the primary concern when purging the system is to safely remove all flammable and hazardous materials, such as hydrogen and hydrocarbons, and prevent the formation of explosive mixtures with air. Purging involves displacing the process gases with an inert gas, such as nitrogen, to reduce the concentration of flammable materials below their lower explosive limit (LEL). The LEL is the minimum concentration of a flammable gas in air that will support combustion. It's crucial to maintain a sufficient flow rate of the inert gas to ensure complete displacement of the process gases and to prevent pockets of flammable gas from remaining in the system. Inadequate purging can lead to the formation of explosive mixtures, which could be ignited by static electricity, hot surfaces, or other ignition sources, resulting in a fire or explosion. Proper grounding and bonding of equipment are also essential to prevent static electricity buildup during purging. The purging process must be carefully monitored to verify that the concentration of flammable materials is consistently below the LEL before any maintenance activities are performed or the system is opened to the atmosphere. Therefore, complete removal of flammables and avoidance of explosive mixtures are the main objectives during purging.