CCN1 Purging Procedures 1 — Questions and Answers
Question 1: What is 'purging' in the context of gas pipework?
- The process of displacing air or inert gas from pipework with gas (commissioning purge) or displacing gas with air or inert gas (decommissioning purge) (Correct answer)
- The cleaning of gas pipework using high-pressure water
- The removal of debris from a gas main before commissioning
- The testing of a flue system for blockages
Correct answer: The process of displacing air or inert gas from pipework with gas (commissioning purge) or displacing gas with air or inert gas (decommissioning purge)
Purging involves displacing one gas with another. Commissioning purge replaces air in new or repaired pipework with gas. Decommissioning purge replaces gas in pipework with air or inert gas before it is worked on or abandoned.
Question 2: Why is it dangerous to simply turn on the gas supply without purging new pipework?
- Air trapped in the pipework mixes with incoming gas, creating a potentially explosive air-gas mixture that could ignite when the appliance attempts to light (Correct answer)
- Air is harmless and purging is an optional precaution
- Air in the pipework only causes the appliance to fail to light — no explosion risk
- New pipework contains only clean air so purging is not necessary
Correct answer: Air trapped in the pipework mixes with incoming gas, creating a potentially explosive air-gas mixture that could ignite when the appliance attempts to light
Air remaining in new or repaired pipework mixes with incoming gas to create an air-gas mixture within the explosive range. If an appliance attempts to ignite this mixture, an explosion can result. Purging eliminates this risk.
Question 3: According to IGE/UP/1, how must a purge of domestic gas pipework be carried out safely?
- Into the outside atmosphere via an appropriate vent point — never into a confined room or near ignition sources (Correct answer)
- Into the kitchen via the gas cooker burners
- Into the boiler flue
- Purging into a room is acceptable if windows are open
Correct answer: Into the outside atmosphere via an appropriate vent point — never into a confined room or near ignition sources
IGE/UP/1 requires that purges of domestic gas pipework are discharged to the outside atmosphere via an appropriate vent point, away from ignition sources. Purging into a room creates an explosion risk even with windows open.
Question 4: When must a decommissioning purge be carried out?
- Before any gas pipework is cut, disconnected, or opened for work — to displace gas with air or inert gas and prevent ignition during work (Correct answer)
- Only before major renovation work
- Only for commercial gas installations
- Only when pipework is being permanently abandoned
Correct answer: Before any gas pipework is cut, disconnected, or opened for work — to displace gas with air or inert gas and prevent ignition during work
A decommissioning purge must be carried out before cutting, disconnecting, or opening gas pipework for any purpose. Displacing gas with air or inert gas prevents ignition of residual gas during hot work or other activities.
Question 5: What instrument is used to verify that gas has fully purged air from new pipework and reached the appliance?
- A gas detector or combustible gas indicator at the appliance connection point (Correct answer)
- A pressure gauge at the meter
- A manometer at the appliance
- Purging is verified by smell alone
Correct answer: A gas detector or combustible gas indicator at the appliance connection point
A combustible gas indicator (or gas detector) is used at the appliance connection to confirm that gas has displaced all air and reached the point of use. Smell alone is unreliable and insufficient.
Question 6: What is the correct approach for purging a very long gas pipe run in a domestic installation?
- Purge in sections, venting each section to the outside atmosphere, confirming gas presence at each stage before moving to the next (Correct answer)
- Purge all at once from the meter with a very large gas volume
- Leave the gas on for 15 minutes and allow natural purging to occur
- Purge by lighting appliances in sequence from the furthest to the nearest
Correct answer: Purge in sections, venting each section to the outside atmosphere, confirming gas presence at each stage before moving to the next
Long pipe runs should be purged in sections, venting each section to the outside and confirming gas presence before proceeding. This ensures complete purging throughout and minimises the volume of gas discharged at any one point.
CCN1 Purging Procedures 1
What is 'purging' in the context of gas pipework?