14
Sizing of copper discharge pipe “D2” for common temperature relief valve outlet sizes
Valve outlet Min size of
Min size of
Max resistance
Resistance
size, diameter discharge pipe
discharge pipe
allowed, expressed
created by
in ‘inches’
D1 in ‘mm’
D2 from tundish
as a length of
each elbow
in ‘mm’
straight pipe, i.e.
or bend in
no elbows or
‘m’
bends.
22
up to 9
0.8
1/2
15
28
up to 18
1.0
35
up to 27
1.4
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28
up to 9
----
3/4
22
35
up to 18
----
42
up to 27
1.7
35
up to 9
1.4
1
28
42
up to 18
1.7
54
up to 27
2.3
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Worked example
The example below is for a ½”diameter temperature relief valve with a discharge pipe (D2)
having 4 No elbows and a length of 7 m from the tundish to the point of discharge.
From
Maximum resistance allowed for a straight length of 22mm copper discharge pipe (D2) from a
½”diameter temperature relief valve is: 9.0 m.
Subtract the resistance for 4 No 22mm elbows at 0.8 m each = 3.2 m.
Therefore, the maximum permitted length equates to: 5.8 m.
5.8 m is less than the actual length of 7 m, therefore, calculate the next largest size.
Maximum resistance allowed for a straight length of 28mm pipe (D2) from a ½”diameter
temperature relief valve equates to: 18 m.
Subtract the resistance for 4 No 28mm elbows at 1.0 each = 4 m.
Therefore the maximum permitted length equates to 14 m.
As the actual length is 7 m, a 28mm diameter copper pipe will be satisfactory.
