Commercial Gas Purging Calculator

What the purging calculator works out

Purging replaces the air (or nitrogen) in a commercial gas installation with fuel gas so appliances can be lit safely, or replaces fuel gas with air or nitrogen when an installation is taken out of service. Before a purge starts, the engineer needs to know how much gas has to pass through the pipework, how quickly it must flow, and how long that will take. This calculator gives a planning estimate for each of those figures.

Installation volume is the starting point. The calculator multiplies each run length by the internal volume per metre for its material and nominal size, adds a ten per cent allowance for fittings, and adds the internal volume of the meter. The purge volume is then taken as one and a half times the installation volume, which is the usual minimum used for natural gas installations under IGEM/UP/1.

The largest pipe in the installation sets the minimum purge flow rate. Gas has to move fast enough through the biggest bore to sweep air out rather than layer over it, so a 100 mm main needs a far higher flow than a 25 mm branch. The calculator looks up a typical minimum flow and a suitable purge point size for the largest pipe you enter, then divides purge volume by flow rate to give a minimum purge time in minutes.

How to use the results on site

Treat the purge time as a minimum, not a target. A purge is complete when gas detection at the purge point shows the required concentration, not when the clock runs out. On natural gas, engineers typically look for a stable reading of at least 90 per cent gas in air on a suitable detector before closing the purge point, then prove the appliance lights cleanly.

Before any purge, the installation must have passed a tightness test, the purge point and vent hose must discharge to a safe place outdoors well away from openings and sources of ignition, no-smoking and no-ignition notices must be in place, and a fire extinguisher should be at hand. Where the purge volume is large or the vent cannot be routed to a safe area, a flare stack or indirect (nitrogen) purge may be needed instead of a direct purge.

The calculator assumes a single purge point at the end of the longest run. Branches must each be purged in turn, and dead legs that the main purge flow does not sweep need their own purge points. Record the purge volume, flow, duration and final gas reading on the CP16 testing and purging certificate.

Worked example

A restaurant installation has 20 m of 50 mm steel, 10 m of 25 mm steel and a U16 meter. The pipe volume is 20 x 0.0024 plus 10 x 0.00064, which is 0.0544 m³. Adding ten per cent for fittings gives 0.0598 m³, and adding 0.025 m³ for the meter gives an installation volume of about 0.085 m³. The purge volume is 1.5 times that, roughly 0.127 m³.

The largest pipe is 50 mm, so the minimum purge flow is about 3.5 m³/h through a 20 mm purge point. 0.127 m³ at 3.5 m³/h takes a little over two minutes. In practice the engineer would run the purge for at least that long, then confirm the gas reading at the purge point before closing it.

Limits of this calculator

This tool covers natural gas installations at low pressure using the general method in IGEM/UP/1. It does not cover LPG, medium pressure systems, installations above 150 mm, nitrogen purging calculations, or the special requirements of large or complex installations where a written purge procedure and risk assessment are required. Pipe and meter volumes are typical published values; the actual meter data plate and current edition of the standard always take precedence.

Purging is restricted work. It must only be carried out by a Gas Safe registered engineer holding the relevant commercial competence for testing and purging (such as TPCP1 or TPCP1A). If you need a purge planned or carried out in London, our engineers can attend, calculate the purge from a site survey and issue the certificate on completion.