Tightness Testing, Purging, Commissioning and Decommissioning of Non-Domestic Gas Systems
This domain tests the sequence that turns new pipework into a safe, live installation - and the sequence that takes it out of service.
Tightness testing
After strength testing, isolate the section, fit a suitable gauge (water gauge or digital with adequate resolution), pressurise to the specified test pressure for the MOP, allow stabilisation, record the start, wait the specified duration and compare any drop with the allowable figure. Temperature change, leaking test equipment and trapped liquid cause false results. A test on a large medium-pressure section is not the same as a 20 mbar domestic test.
Purging
Purging displaces air with gas, or gas with inert gas. Use an inert purge where the standard requires it, discharge to a safe outdoor location away from openings and ignition sources, and never purge into a confined space. Commission only when the gas concentration at the purge point meets the acceptance criterion and all appliance isolation valves are closed.
Commissioning and decommissioning
Commissioning includes gas rate, operating pressure, flame picture, combustion readings, controls and safety devices, flueing and ventilation checks. Decommissioning means isolating, purging, physically disconnecting or capping, labelling the pipe, and recording the change.
Worked example
A 40 m length of 50 mm steel at 2 bar has an internal volume of about 0.098 m³. Purging to the standard's criterion may need several pipe volumes of inert gas, so the discharge point, gas volume and monitoring must be planned before starting. A 5 m³ cylinder of nitrogen is not automatically enough; calculate first.
Traps
Testing at the wrong pressure or for too short a time; accepting gauge movement caused by temperature; purging gas into a plant room; commissioning with appliance valves open; leaving a decommissioned pipe unlabelled.
Exam focus
Test pressure selection, stabilisation, allowable drop, purge criteria and the commissioning checklist.