Installation
Nitrogen purge volume and time
- where
- pipe internal volume (litres)
- pipe internal diameter (mm)
- pipe length (m)
- nitrogen volume to purge (litres)
- number of pipe volumes to purge
- purge time (min)
- nitrogen flow rate (L/min)
Explain this equation
Works out the internal volume of a pipe run and how long a nitrogen purge takes at a set flow. Use it to plan purging before brazing or to know how much nitrogen to bring.
The bore area is π/4 times diameter squared, with the diameter converted from millimetres to metres. Area times length gives cubic metres, and the final 1000 turns that into litres. Purging aims to replace the pipe contents N times. Dividing that volume by the flow rate on the regulator gives the time.
Volume of a cylinder, then time equals volume divided by flow.
Sense check: 152 m of pipe with a 50 mm bore holds about 304 litres. At about 14 L/min a single volume change takes around 21 minutes.
Watch out: a purge to displace air takes more than one volume to get the oxygen down. While brazing, a low steady flow is enough; too much flow blows the joint.
Works out the internal volume of a pipe run, the nitrogen needed to displace it a chosen number of times, and how long that takes at your regulator flow.
Enter your values to see the result.
Good to know
Purge flow on common flow regulators is about 25–35 SCFH (12–17 L/min); flow while brazing should be very low, about 2–5 SCFH (1–2.4 L/min). 1 SCFH ≈ 0.472 L/min. Start the flow before heating and keep it going until the joint has cooled. One pipe volume does not remove all the air; use the number of volumes in your procedure. Check the purge outlet with an oxygen meter where available.
Common mistake
Using the outside diameter, or assuming a brazing-rate trickle will purge a long run quickly (500 ft of 2-1/8 in takes over 3 hours at 3 SCFH).
Limits
Plug-flow estimate; mixing at tees and dead legs needs more gas. Volumes are at regulator (near-atmospheric) conditions.
Sources
- HVAC School: Flowing nitrogen – purge first (pipe volume, purge time = volume ÷ flow; 25–35 SCFH purge, 2–5 SCFH brazing; 500 ft of 2-1/8 in example)
- HVAC School: How and why to flow nitrogen while brazing (very low flow, 2–5 SCFH)
- Harris Products Group: Nitrogen purge and brazing (start flow before heating, keep flowing until cool)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.