Short answer
Large installations are tested under IGEM/UP/1 and are usually broken into sections so that each has a manageable volume and test duration. Electronic gauges with fine resolution, longer stabilisation periods, close attention to temperature change and careful let-by testing of every sectioning valve are what make a large test reliable.
Sectioning
Dividing the installation at isolation valves keeps test durations practical and makes a leak easier to locate. Each section is tested, then the joints between sections are checked when the whole installation is brought together.
Temperature
A one-degree change in gas temperature changes pressure by roughly 0.3% — on a large volume this can look like a leak or hide one. Avoid testing while sunlight, heating or ventilation is changing pipe temperatures, extend stabilisation, and note conditions on the record.
Gauges and duration
Higher volumes produce longer test durations for a given gauge. Using an electronic gauge with 0.1 mbar resolution, or a higher-pressure gauge for medium pressure sections, keeps durations reasonable while maintaining sensitivity.
Coordination
Large tests often mean shutting down plant, kitchens or processes. Agree a programme with the client, have the purge and recommissioning plan ready, and have extra engineers available to check every appliance afterwards.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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