--- title: Inerting and Purging: How They Work and How to Prove Them url: https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html description: Inerting, blanketing and purging explained: pressure, vacuum, sweep-through and dilution methods, the maths of a dilution purge. updated: 2026-11-03 source: html --- Learn · Safety and hazardous areas # Inerting and purging: how they work and how to prove them Everything depends on the last step: measurement. A purge that was meant to reach 1 % oxygen is only a purge once an analyser says it did. Intermediate 3 min read Lesson 3 of 8[Safety and hazardous areas](https://www.sarveshanalytics.com/learn/track-safety.html) Helpful first: [What is limiting oxygen concentration (LOC)?](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html) Quick answer Inerting keeps a vessel's oxygen below the limiting oxygen concentration by filling it with nitrogen or another inert gas. Purging replaces the vessel's atmosphere before start-up or opening. Common methods are sweep-through, pressure cycling, vacuum cycling and dilution; with good mixing about 3 vessel volumes take air from 21 to 1 % oxygen. An oxygen analyser on the vent or headspace proves the result. Updated 3 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Compare the main purging methods - Estimate the inert gas needed for a dilution purge - Place an oxygen analyser to prove inerting - Plan margins and alarms ## Inerting versus purging versus blanketing - **Purging:** replace the atmosphere in a vessel or pipe before start-up, after maintenance or before opening. - **Inerting:** keep oxygen below a safe level during operation. - **Blanketing:** maintain a slight positive pressure of inert gas above a liquid so that air cannot enter. ## Methods | Method | How | Good for | |---|---|---| | Sweep-through (flow) | Inert gas flows in at one point and the old atmosphere leaves at another | Pipes and vessels with good flow paths | | Pressure cycling | Pressurise with inert gas, vent, repeat | Vessels that can take pressure; fewer volumes of gas than dilution | | Vacuum cycling | Evacuate, break the vacuum with inert gas, repeat | Vessels rated for vacuum | | Siphon or liquid displacement | Fill with liquid, then displace with inert gas as it is drained | Tanks where liquid is available | | Dilution | Continuous inert gas flow through a mixed vessel | Mixed vessels; the simplest to calculate | ## A dilution purge in numbers In a well-mixed vessel, oxygen falls exponentially as inert gas is added. The number of vessel volumes is the natural log of the starting over the target oxygen: about 3.0 volumes from 20.9 to 1 %, and about 5.3 to reach 0.1 %. A 10 m3 vessel needs about 30 m3 of nitrogen to reach 1 %.[**Try the calculator** Inerting purge time and volume calculator](https://www.sarveshanalytics.com/calculators/purge-time-calculator.html) **Common mistake** Trusting the arithmetic without measuring. Dead legs, poor mixing and leaks make real vessels take more gas than the ideal curve. ## Where to measure - On the **vent** during a sweep or dilution purge, since the vent shows the last place to clear. - In the **headspace** for blanketed tanks, away from the inert gas inlet: [What is limiting oxygen concentration (LOC)](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html). - At the **farthest point** of a pipe network from the inlet. - With a **suitable analyser:** ACCURRA-S for percent oxygen in safe areas; ACCURRA-Hz or INFINITY-Tx in hazardous areas, since flammable vapour may be present. ## Margins and alarms Set the target oxygen from the limiting oxygen concentration of the worst-case flammable material, minus a margin set by your safety assessment. Codes such as NFPA 69 give minimum margins for monitored systems; apply the one that governs your site. Alarm on oxygen rising and link the alarm to a response, such as increased inert gas flow or a trip: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html). ## Where this is used Pharmaceutical dryers and reactors, solvent tanks, powder handling and storage, and pipe commissioning use inerting: [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html). Dilution purge with perfect mixing. Oxygen falls to 2 % after about 2.35 volume changes; dead corners and poor mixing need more, which is why the end point is measured, not assumed. **In short** - Inerting keeps oxygen below the LOC; purging replaces an atmosphere; blanketing keeps air out. - Common methods: sweep-through, pressure and vacuum cycling, siphon, dilution. - Dilution needs about 3 vessel volumes to reach 1 % oxygen, with good mixing. - Prove it with an oxygen analyser at the vent or headspace. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **How many vessel volumes does a perfectly mixed dilution purge need to go from 20.9 to 1 % oxygen?** About 3.0, since ln(20.9) is 3.04. **Where should a purge be verified?** At the vent or the point farthest from the inert gas inlet, where the old atmosphere clears last. **Why use a hazardous-area analyser on a solvent tank headspace?** Flammable vapour may be present, so equipment must be certified for the zone. ## Key terms - [Inerting](https://www.sarveshanalytics.com/learn/glossary.html#t-inerting) - [Purge](https://www.sarveshanalytics.com/learn/glossary.html#t-purge) - [Limiting oxygen concentration (LOC)](https://www.sarveshanalytics.com/learn/glossary.html#t-limiting-oxygen-concentration-loc) - [Vent](https://www.sarveshanalytics.com/learn/glossary.html#t-vent) ## Frequently asked questions **How do you purge a tank with nitrogen?** By sweep-through or dilution to a target oxygen, then confirm with an analyser. **How much nitrogen do I need to purge a vessel?** About three vessel volumes for 1 % oxygen if well mixed, more in practice. **Which standard sets inerting margins?** Standards such as NFPA 69 give guidance; follow the one that applies to your site. **What oxygen alarm should I set?** Below the LOC by the margin your safety assessment requires. ## Related products [ ### ACCURRA-S Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample. View ACCURRA-S ](https://www.sarveshanalytics.com/safe-gas-analyser.html)[ ### ACCURRA-Hz Process gas analyser in an enclosure certified for Gas Group IIC, Zone 1 and 2, T6. View ACCURRA-Hz ](https://www.sarveshanalytics.com/hazardous-gas-analysers.html)[ ### INFINITY-Tx Loop-powered two-wire transmitter for percent O2 and trace CO in hazardous areas. View INFINITY-Tx ](https://www.sarveshanalytics.com/oxygen-transmitter.html) ## Where this applies - [Nitrogen blanketing and inerting](https://www.sarveshanalytics.com/applications/nitrogen-blanketing-and-inerting.html) - [Closed-loop dryers and spray dryers](https://www.sarveshanalytics.com/applications/closed-loop-and-spray-dryer-oxygen-safety.html) ## Where to go next - [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html)