Civil Designer Tech Tips

Air and Scour Valve Placement and Sizing

Adding the Valves

Select the pipe you want to add the air and scour valves by left clicking in the pipe.

Water Mode, Tools Menu > Vertical Alignment

Click on the toolbar icon to automatically arrange the air and scour valves on the longitudinal section.

The following dialog will open.

All existing air and scour valves on the active longitudinal section will be removed before placing new ones. Air and scour valves will be placed:

Where unconditionally required: On high (apex) points and on points where negative pressure occurs during scouring or where there is excessive flow for any other reason. This option cannot be deactivated.

On slope change: Air valves will be placed where the slope change, by percentage, exceeds the set value. This is recommended but not compulsory.

Observe maximum distance: Distance between the air valves will not exceed the set value.

Click 'Sizing Criteria' to set the air valve parameters.

Scour valve nominal size: Scour valves are not sized during the procedure. Enter the desired size.

Removing Excess Valves

The procedure for the automatic placement of air and scour valve can create a large number of valves or valves which are closely spaced. Unwanted valves could be manually removed using the toolbar commands, but that would also violate the setup rules.

It is therefore preferable to reduce the number of valves by altering the longitudinal section, thus making valves redundant.

The longitudinal section example below results in 3 scour and 2 air valves on a stretch of around 300 metres.

Lowering 2 high points by 500 mm will eliminate four valves and create a more economical and reliable solution.

Air Valve Sizing Criteria

In the Vertical Alignment window, click on the Air Valve Sizing button. You can change the defaults if necessary.

Permissible differential pressures:
  • During air intake - differential pressure during air intake should not exceed 0.4 bar in general. For thin wall pipes (steel and PVC) the limit should be even lower due to the danger of collapsing.
  • During air exhaust - differential pressure should not exceed 0.2 bar.
System filling up

Initial filling up should be done in a controlled way, with velocities not exceeding 0.6 m/s.


Combination air valves can be set to either a two (large and small orifice) or three stage (large, intermediate and small orifice) type. The type of combination valve can be altered later on in the editing window.

Ratio large orifice / nominal diameter: The diameter of the large orifice is often significantly smaller than the nominal diameter. For air valves with spherical floats it is typically between 0.6 and 0.7 of the nominal diameter, while for valves with disk floats or of non-return type it is usually equal to the nominal diameter. The actual orifice diameters should be checked with the manufacturer prior to ordering.

Editing Air and Scour Valves

The air and scour valves on your pipeline may be edited by clicking on the Edit Air and Scour Values button in the Vertical Alignment window.

  • Chainage: Valve chainage on the active longitudinal section
  • Category: Either Air or Scour Valve
  • Air Valve Type: One of either Small Orifice, Large Orifice, Two-Stage or Three-Stage
  • Diameter: Nominal valve diameter
  • Air intake: Design inflow used to determine required size:
    • For air valves placed on apex points: higher of possible gravity flows in front and after the valve, as the possibility of simultaneous scouring or pipe breakage on both sides of the valve is neglible.
    • For valves placed on change of slope: difference between possible gravity flows out of the node and into the node
  • Requires: Minimum orifice diameter based on the designed inflow and selected permissible differential pressure
  • Air exhaust: Design outflow based on the filling up velocity
  • Requires: Minimum orifice diameter base on the design outflow and selected permissible differential pressure

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