Air valves are needed to release built up air pressure in the system. Most aqueduct systems have high and low points throughout the line due to the topography of the region. In these high points, air can sometimes build up. The air will become compressed. If there is sufficient pressure, the air block will be pushed through the system. If there is insufficient pressure, the air will become compressed to the point of creating a block in the system where water cannot pass through the tubes. An air valve releases this air so that water can pass uninhibited through the system.
There is a good primer on air blocks in the appendix of A Handbook of Gravity-Flow Water Systems, Thomas Jordan Jr.
The design of where to put air blocks is outside of the scope of this document. Air valves can be purchased in most hardware stores. NeatWork software will point out locations along the line where air valves should be installed. 
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