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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.
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: 2003, which supersedes BS6281-1: 1992. 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 weir calculator
has several options. Where no screen is used on the overflow weir,
dimensions for option Op1 are to be used. When a screen is
to be used, one of two methods must be employed to ensure the air
gap is not compromised. To maintain the air gap, in option Op2
the base width is increased and in option Op3 the overall
height is increased. 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 wier.
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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