Type AB Air Gap - Weir Calculator

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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.