Station design

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Ventilation of below-ground structures

AS 2885.1 Clause 6.5.3 (Below-ground structures) says “Each equipment structure that has an internal volume of not more than 6 m3 and located so that no part of the equipment structure is above the surface of the ground shall be ventilated or sealed. Where a structure is ventilated it shall conform with the requirements of Clause 6.5.2.”

Questions:

1 – What is the basis for the requirement that internal volume of not more than 6 m3 be either ventilated or sealed?
2 - What requirements apply to buried structures with internal volume of more than 6 m3?

Answers:

The 6 m3 volume limit appears in the first edition of AS 2885 in 1987, and the basis for it is not known with any certainty. Speculative comment: It seems likely that it was copied from AS 1697 (Australian gas pipeline standard superseded by AS 2885) which in turn was largely based on 1970s versions of ASME/ANSI B31.8 (US gas pipeline standard). It may not be coincidence that 6.0 m3 is almost exactly the volume of a 6 foot cube.

The exact words from the 1987 edition are:

5.3 Ventilation and sealing.
5.3.1 General. A structure having an internal volume not greater than 6 m3, which is nowhere above the surface of the ground, shall be ventilated or sealed. Any other structure shall be ventilated.

The last sentence does not appear in any later edition of the Standards. The 1987 edition included further clauses on ventilation and sealing respectively, very similar to the current wording and not distinguishing between below- and above-ground structures. This whole section was rewritten in the 1997 edition and has not changed since. It appears that the editorial changes for 1997 unintentionally omitted that last sentence about "any other structure".

It is recommended that buried structures larger than 6 m3 are ventilated in accordance with the same requirements that apply to buildings, consistent with the intent of the 1987 Standard.