Short answer
A strength test proves that new or modified pipework and joints can safely withstand pressure well above normal operation. A tightness test proves the pipework does not leak at around operating pressure. Strength testing comes first, is usually done with air or nitrogen, and is required on new and substantially altered commercial installations; tightness testing is required every time.
Purpose
The strength test is a mechanical integrity check — it finds badly made joints, unsupported runs and defective fittings before they are ever exposed to gas. The tightness test is a leak-rate check, measuring whether the installation loses pressure faster than the permitted rate.
Pressures
The strength test pressure is derived from the maximum operating pressure using the tables in IGEM/UP/1 or UP/1A and is significantly higher than working pressure. Components not rated for that pressure — regulators, meters, appliance valves, some pressure switches — must be isolated or removed first. The tightness test is carried out at the tightness test pressure, which relates to operating pressure.
Test medium and safety
Strength tests are carried out with air or inert gas, never fuel gas. Because a pneumatic test stores a lot of energy, the area should be cleared and the pressure raised in stages. A failed strength test can mean a joint separates, so exclusion zones matter on larger diameters.
When a strength test is not needed
Minor alterations to existing installations may only need a tightness test, depending on the standard and the extent of the work. A new installation, extension or replacement section normally needs both.
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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