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7
We strongly recommends the use of metal pipe-
work only and GDC Group Ltd does not take
responsibility for any damage caused from
discharges.
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 than the nominal outlet
size of the safety device; between 18 and 27m at
least 3 sizes larger, and so on; bends must be
takenintoaccountincalculatingtheow
resistance. See Figure 1, Table 2 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).
Trapped
gully
Fixed
grating
Discharge below
fixed grating
tundish
Metal discharge pipe (D1) from
temperature relief valve to tundish
Metal discharge pipe (D2) from tundish,
with continuous fall. See Table and
worked example
300mm
minimum
600mm maximum
Safety device
(eg. temperature
relief valve)
Figure 1: Typical Discharge Arrangement
Valve Outlet
Size [-]
Minimum size of discharge
pipe before tundish [mm]
Minimum size of discharge
pipe aer tundish [mm]
Maximum allowed length of
pipe aer tundish [mm]
Length to be substracted for
each elbow or bend [m]
22 9 0.8
28 18 1.0
35 27 1.4
28 9 1.0
35 18 1.4
42 27 1.7
35 9 1.4
42 18 1.7
54 27 2.3
15
22
28
G1/2
G3/4
G1/2
Table 2: Sizing of copper discharge pipe “D2”
for common temperature relief valve outlet sizes
Pre-Installation
This example is for a G½ temperature relief valve
withadischargepipe(D2)(asttedon125Lto
300L cylinders) having 4 No. 22mm elbows and
length of 7m from the tundish to the point of
discharge.
From Table 2, the maximum resistance allowed for
a straight length of 22mm copper discharge pipe
(D2) from a G½ temperature relief valve is 9.0m.
Subtract the resistance for 4 No. 22mm elbows at
0.8m each = 3.2m.
Therefore the maximum permitted length equates
to 5.8m, which is less than the actual length of
7m, therefore calculate the next largest size.
Maximum resistance allowed for a straight length
of 28mm copper discharge pipe (D2) from a G½
temperature relief valve equates to 18m.
Subtract the resistance for 4 No. 28m.m elbows at
1.0m each = 4m. Therefore the maximum
permitted length equates to 14m.
As the actual length is 7m, a 28mm (D2) copper
pipe will be satisfactory.
Notes:
1.) Where a single common discharge pipe
serves more than one system, it should
be at least one pipe size larger than the
largest individual discharge pipe (D2) to
be connected.
2.) The discharge pipe should not be
connected to a soil discharge stack
unless the soil discharge stack is
capable of safely withstanding
temperatures of the water discharged,
in which case, it should:
- contain a mechanical seal, which
allows water into the branch pipe
without allowing foul air from the
drain to be ventilated through the
tundish.
- there should be a separate branch
pipe with no sanitary appliances
connected to it.
- if plastic pipes are used as branch
pipes carrying discharge from a
safety device, they should be either
polybutalene (PB) or cross-linked
polyethylene (PE-X) complying with
national standards.
- be continuously marked with a
warning that no sanitary appliances
should be connected to the pipe.
5.4.2 Worked Example
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