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The Specification applies to NRVs that are used in gas supply installations and gas fired plants to prevent the admission of any extraneous gases into the gas supply specified under the requirements of the Gas Act 1986 (as amended 1995), Schedule 4, para. 18. This applies to NRVs for the use on first and second family gases for installations of maximum operating pressure not exceeding 7 bar. It does not apply to valves used in oxygen or oxygen-enriched air supplies. However, they may be used for protection of the gas supply in oxy-gas systems provided the oxygen pressure not exceeding 2 bar (29 lb/in2).

It also applies to both general purpose NRVs for normal low pressure duty and also special purpose NRVs when higher operating pressure differentials are available. The requirements for these are identical in all respects except for the flow characteristics.

This Specification applies to NRVs in the size range DN15 (nominal diameter) to DN300 (½ in NB (nominal bore) to 12 in NB) nominal pipe size.

This Specification does not apply to NRVs using external power sources to open or close the valve. However, the general performance requirements may be considered relevant.

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