Grant HPMONO/IND125, Installation, Servicing And User Instructions Manual

Page: 20 / 40

Page 20 Section 4: Secondary Circuit Installation
4.9 TUNDISH
A suitable tundish is supplied loose with the cylinder for fitting in
the common discharge pipe from the T&P and Expansion relief
valves.
The tundish should be vertical, located in the same space as
the unvented hot water cylinder and be fitted as close to, and
lower than, the T&P valve with no more than 600mm of pipe (D1)
between the valve outlet and the tundish.
! WARNING !
The tundish must NOT be positioned above or in close
proximity of any electrical current carrying devices or
wiring.
A discharge pipe must be fitted to the outlet of the tundish. This
must conform to the requirements as given in Sections 4.10 to
4.13 of these Installation and User Instructions.
4.10 DISCHARGE PIPE
1. The discharge pipe (D2) from the tundish should terminate in
a safe place where there is no risk to persons in the vicinity
of the discharge.
a) It should be made of metal or other material that has
been demonstrated to be capable of withstanding
temperatures of the water discharged.
b) Be at least one pipe size larger than the normal 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 long
the equivalent resistance length should be at least two
sizes larger than the normal outlet size of the safety
device, between 18m and 27m at least three sizes larger
and so on.
Bends must be taken into account in calculating the flow
resistance. Refer to Sections 4.11, 4.12 and 4.13.
c) Have a vertical section of pipe at least 300 mm long,
below the tundish before any elbows or bends in the
pipe work.
d) Be installed with a continuous fall of 1:200 (0.286°).
e) Have discharges visible at both the tundish and the
final point of discharge but where this is not possible or
practically difficult there should be clear visibility at one
or other of these locations.
2. Examples of acceptable discharge arrangements are:
a) ideally below a fixed grating and above the water seal in
a trapped gully.
b) downward discharges at a low level; i.e. up to 100mm
above external surfaces such as car parks, hard
standings, grassed areas etc. are acceptable providing
that where children may play or otherwise come in to
contact with discharges, a wire cage or similar guard
is positioned to prevent contact, whilst maintaining
visibility.
c) discharges at high level; e.g. into a metal hopper and
metal down pipe with the end of the discharge pipe
clearly visible (tundish visible or not) or onto a roof
capable of withstanding high temperature discharges of
water and 3m from any plastics guttering systems that
would collect such discharges (tundish visible).
3. Where a single pipe serves a number of discharges, such as
in blocks of flats, the number served should be limited to not
more than 6 systems so that any installation can be traced
reasonably easily.
The single common discharge pipe should be at least one
pipe size larger than the largest individual discharge pipe to
be connected.
If unvented hot water storage systems are installed where
discharges from safety devices may not be apparent i.e.
in dwellings occupied by blind, infirm or disabled people,
consideration should be given to the installation of an
electronically operated device to warn when a discharge
takes place.
! NOTE !
The discharge will consist of scalding water and steam.
Asphalt, roofing felt and non-metallic rainwater goods may
be damaged by such discharges.
4.11 DISCHARGE PIPE SIZING
Refer to Table 4-1 (discharge pipe sizing).
Table 4-1: Discharge pipe sizing
Valve outlet
size
Diameter
(inches)
Minimum
size of
discharge
pipe D1
(mm)
Minimum
size of
discharge
pipe D2
from
tundish
(mm)
Maximum
resistance
allowed,
expressed
as a length
of straight
pipe, i.e. no
elbows or
bends (m)
Resistance
created by
each elbow
or bend (m)
½ 15
22
28
35 Up to 9
Up to 18
Up to 27
0.8
1.0
1.4
¾ 22
28
35
42 Up to 9
Up to 18
Up to 27
1.0
1.4
1.7
1 28
35
42
54 Up to 9
Up to 18
Up to 27
1.4
1.7
2.3
! NOTE !
The above table is based on copper tube. Plastic pipes
may be of a different bore and resistance.
Sizes and maximum lengths of plastic pipe should be
calculated using data for the type of pipe being used.
4.12 WORKED EXAMPLE
The example below is for a 1/2” diameter temperature relief valve
with a discharge pipe (D2) having 4 x 22mm elbows and a length
of 7 m from the tundish to the point of discharge.
From Table 4-1:
Maximum resistance allowed for a straight length of 22mm copper
discharge pipe (D2) from a 1/2” diameter temperature relief valve
is 9.0m.
Subtract the resistance for quantity of 4 x 22mm elbows at 0.8m
each = 3.2m.
Therefore, the maximum permitted length is 9.0 - 3.2 = 5.8m.
5.8m 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 1/2” diameter temperature relief valve
is 18m.
Subtract the resistance for a quantity of 4 x 28mm elbows at 1.0m
each = 4m.
Therefore, the maximum permitted length is 18 - 4 = 14m.
As the actual length is 7m, a 28mm diameter copper pipe will be
satisfactory in this case.