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Worked Example
Extract from Table 3.1 Building Regulations G3
Note:
Data provided for G½” outlet size and copper pipework only. Other outlet sizes and pipe materials should
be calculated using data prepared for the size and type of pipe being used.
Table 6
: Sizing of copper discharge pipe (D2) for G½” valve outlet
The example below is fo
r a G½” temperature relief valve with a discharge pipe (D2) having 4 no.
elbows and a length of 7m from the tundish to the point of discharge. The calculation shows that
22mm pipe would be
unacceptable .
UNACCEPTABLE
Discharge pipe (D2) run in 22mm copper:
Length of straight pipe = 7.0m
Resistance created by bends (0.8m x 4) = 3.2m
Total resistance of discharge pipe
= 10.2m
Maximum resistance allowed for a 22mm copper discharge pipe (D2) from
a G½” temperature relief valve is 9.0m, which is less than 10.2m.
Therefore, installation unacceptable :
Discharge pipe (D2) needs to be larger than 22mm.
ACCEPTABLE
Discharge pipe (D2) run in 28mm copper:
Length of straight pipe = 7.0m
Resistance created by bends (1.0m x 4) = 4.0m
Total resistance of discharge pipe
= 11.0m
Maximum resistance allowed for a 28mm copper discharge pipe (D2) from
a G½” temperature relief valve is 18.0m, which is more than 11.0m.
Therefore, installation acceptable :
Discharge pipe (D2) can be run in 28mm.
Valve
outlet size
Minimum size of
discharge pipe
D1
Minimum size of
discharge pipe D2
from tundish
Maximum resistance allowed,
expressed as a length of straight
pipe (i.e. no elbows or bends)
Resistance
created by each
elbow or bend
G½”
15mm
22mm Up to 9m 0.8m
28mm Up to 18m 1.0m
35mm Up to 27m 1.4m