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3.57 The discharge pipe (D2) should be made of:
a. metal; or
b. other material that has been demonstrated to be capable of safely withstanding temperatures
of the water discharged and is clearly and permanently marked to identify the product and
performance standard (e.g. as specified in the relevant part of BS 7291- 1:2006 Thermostatic
pipes and fittings for hot and cold water for domestic purposes and heating installations in
buildings. General requirements).
3.58 The discharge pipe D2 should be at least one pipe size larger than the nominal outlet size
of the safety device unless its total equivalent hydraulic resistance exceeds that of a straight pipe
9m long, i.e. for discharge pipes between 9m and 18m the equivalent resistance length should
be at least two sizes larger than the nominal outlet size of the safety device; between 18 and 27m
at least 3 sizes larger, and so on; bends must be taken into account in calculating the flow
resistance. See Diagram 1, Table 3.1 and the worked example.
Note: An alternative approach for sizing discharge pipes would be to follow Annex D, section D.2
of BS 6700:2006 + A1:2009 Specification for design, installation, testing and maintenance of
services supplying water for domestic use within buildings and their curtilages.
Diagram 1 Typical discharge pipe arrangement
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