Air and Scour Valves

Air Valves

All water contains dissolved air. Normally this would be about 2% but it can vary largely depending on temperature and pressure. Air trapped in the line in pockets is continually moving in and out of solution.

Air in the line not only reduces the flow by causing a restriction but amplifies the effects of pressure surges. Air valves should be placed in the line at sufficient intervals so that air can be evacuated, or, if the line is drained, air can enter the line.

Air valves should be placed along the pipeline at all high points or significant changes in grade. On long rising grades or flat runs where there are no significant high points or grade changes, air valves should be placed at least every 500 – 1,000 metres at the engineer’s discretion.

Typical Air Valve Size
Up to 100 25 single
100 – 200 50 double
200 – 450 80 double
Size DN Air Valve Size

Scour Valves

Scour valves are located at low points or between valved sections of the pipeline. Their function is to allow periodic flushing of the lines to remove sediment and to allow the line to be drained for maintenance and repair work.

The scour valve should be sized to allow a minimum scour velocity of 0.6 m/s to be achieved in the main pipe. Scour tees over nominal size 100 should be offset tees to allow the debris to be taken from the invert of the pipe. In the absence of specific design criteria, the following sizes are generally acceptable.

Typical Scour Valve Size
Up to 100 80
100 – 200 100
200 – 450 150
Size DN Scour Valve Size


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