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INSTRUCTIONS FOR USE
Protection against backflow by fluids containing contaminants up to
fluid category five can be provided by using a type AB air gap in
conjunction with a storage cistern. The type AB air gap
overflow is a simple device complicated by the requirements of the
validation criteria. It is a requirement that the dimensions
of the weir overflow slot in the cistern wall comply with the
Regulators’ Performance Specification this can be determined either
by test or calculation.
The Water Industry has provided this Type AB Air Gap Calculator
to ensure the Regulators’ requirements for the weir overflow of
the Type AB air gap are met. The calculator has been derived
from the British Standard BS EN13077: 2008, which supersedes all
previous versions. The calculator shows the minimum distances
and values required to satisfy these requirements. This Note is to
assist designers or installers in interpreting the calculator and in
ensuring a satisfactory installation.
There are two types of overflow. In one, the overflow weir
slot has no screens to obstruct the water overflowing during fault
conditions. The second has the addition of a mesh screen to
prevent the ingress of insects and other creatures. A screen
will be required where the use of the water in the storage cistern
requires it to be wholesome and it may be needed in other
circumstances where maintaining water quality is important (e.g. for
some industrial purposes). The calculator takes into account
the effect that a screen might have on the overflow
The Air gap calculator is able to accommodate a number of variables
including multiple inlets and screen options. All validation
criteria must be met to ensure the overflow will function as
designed when fault conditions occur; these are listed separately
for each option. Only one option should be selected and the relevant
validation criteria for that option are used depending on the
design.
Interpretation
(N. B. The weir calculator
includes a diagram to illustrate these dimensions.)
W
– The minimum overall width of the slot in the storage cistern.
Ow
– The minimum height of the slot; this measurement is taken from the
lowest point of discharge of the inlet device to the bottom of the
overflow slot.
Cw
– The maximum distance allowed from the inside face of the cistern
wall where the water will start to overflow to where the water will
fall vertically downwards outside the cistern.
Uw – This is the minimum vertical
distance from where water will spill out of the overflow slot to the
point where water will reach the first horizontal surface outside
the cistern. Where the overflow is installed in a raised
housing the measurement is taken to the lid of the storage cistern.
Where the overflow is in the side of a cistern then the measurement
is taken to either the floor or to a point where the overflowing
water is directed away from the side of the cistern.
A
–
Minimum air gap required, which shall be twice the bore on the
inlet.
h
–
Depth of water above the base of the weir slot that will build up
during fault conditions, when there is a maximum inlet flow and
outlet flow is only via weir.
It is the installer’s responsibility to ensure that the air gap and
overflow weir comply with the Regulations when first installed. It
is the user’s responsibility to ensure that whether or not a
screened overflow is used, suitable maintenance is carried out when
necessary, to enable the overflow to work as designed when it is
required
To download a printable PDF version of these instructions, please
click here.
To make use
of the Type AB Air Gap - Weir Calculator you will need
a spreadsheet application. This calculator has only been tested
in Microsoft Excel 97 and later. The spreadsheet contains
password protected Macros and so you will need to have Macros enabled
otherwise this calculator will not function. Before
downloading, please open Microsoft Excel on your computer and on
the tools menu under Macros and Security, please ensure
the security level is set to either medium or low. Please note
that you should adjust this setting back to the previous
security level after using the Air gap calculator to ensure
Microsoft Excel is adequately protected against potential
security threats from other sources.
You will
then need to download and save the
file onto your computer before opening to be able to operate the
calculator fully. To download the file to your computer, right
click on the link below and select "save target as" from the drop down
menu..
You can then open the file from the location
you saved it. The file will open and either automatically run
the startup splash screen (if you set the security level to low
previously) or ask you if you want to enable macros (if you set
the security level to medium previously. If you are asked
to enable the macros, please select enable macros from the
choices. Once the startup screen has finished, please select
Continue on the menu and then read the instructions on the next
screen before closing the instructions window. After you have
read and closed the instructions window, please select Data Form
and then Enter New Data on the menu to bring up the data entry
form.
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