Purging refers to a controlled process of removing or displacing hazardous substances (such as toxic gases, flammable vapors, or contaminants) from a confined space, pipeline, vessel, equipment or system by introducing an inert gas (such as nitrogen, carbon dioxide, or argon) before maintenance, inspection, or entry. This is done to ensure a safe working environment by eliminating potential risks of fire, explosion, or chemical exposure.
Brief Explanation of the Term “Purging”
Purging is a critical safety procedure in industries such as oil and gas, chemical manufacturing, and wastewater treatment ensuring a safe working environment before performing tasks like hot work, welding, or opening confined spaces. The goal is to eliminate the risk of ignition or harmful exposure by displacing dangerous substances using inert gases (like nitrogen or carbon dioxide), air, or steam, depending on the system and risk level.
Purging is essential when working with systems that handle flammable or toxic substances. It ensures the internal atmosphere of the system is rendered non-hazardous—particularly below LEL (Lower Explosive Limit) or free from toxic concentrations—prior to:
- Hot work (e.g., welding, cutting)
- Confined space entry
- Equipment startup or shutdown
- Product changeovers
The method depends on the type of hazard:
- Flammable substances may be purged using inert gas to prevent ignition.
- Toxic chemicals may require ventilation or displacement with fresh air.
- Oxygen-deficient or enriched atmospheres are adjusted to safe oxygen levels (typically 19.5%–23.5%).
Purging prevents:
- Accumulation of flammable mixtures within explosive limits (LEL/UEL)
- Exposure to toxic gases
- Uncontrolled chemical reactions
Proper purging is often followed by gas testing to confirm the environment is safe for workers. Purging ensures that the residual gases are diluted or displaced to a concentration below hazardous thresholds such as the Lower Explosive Limit (LEL) or Occupational Exposure Limits (OELs).
The purging medium is often:
- Inert gas (e.g., Nitrogen, Argon) N₂ is widely used in petrochemical and pharmaceutical sectors.
- Steam (Useful for removing volatile or condensable liquids)
- Water or Air (May be used in less hazardous scenarios with caution, depending on flammability risk)
There are three common purging methods:
- Displacement purging – uses a heavier-than-air gas to push out the hazardous gas.
- Dilution purging – mixes an inert gas with the hazardous gas to lower its concentration.
- Vacuum purging – removes air/gas under vacuum and then fills the space with inert gas.
Example
A maintenance crew needs to open a vessel that contained propane (a flammable gas). To eliminate the explosion risk, the vessel is purged with nitrogen gas until gas detectors confirm that the propane concentration is below 10% of its LEL. Only then is the vessel deemed safe to open.
This chart shows that the concentration of a flammable gas (e.g., methane, propane) decreases over time when an inert gas (like nitrogen) is used to purge a system (like a pipeline or vessel).
The curve starts at 100% (indicating the system is fully filled with flammable gas) and exponentially drops as purging continues — simulating how gas is gradually displaced or diluted.
The rate of concentration reduction follows an exponential decay, meaning gas is removed faster at the beginning and more slowly as the process continues. This is typical for gas displacement processes.
Standards and Guidelines
- OSHA 1910.146 – Permit-required confined spaces (emphasizes atmospheric testing and inerting/purging requirements)
- API Standard 521 – Pressure-Relieving and Depressuring Systems – covers purging of flammable gases.
- NFPA 56 – Standard for Fire and Explosion Prevention During Cleaning and Purging of Flammable Gas Piping Systems
- ISO 10691 – Natural gas — Guidelines for the installation of pipeline systems using inert gas purging