Air in Water Distribution Systems—Challenging the Status Quo

Air in water distribution systems and feedermains (water mains) may lead to serious operational issues. Trapped air pockets can cause a reduction in capacity, induce transients, or even give rise to sudden failures with potentially significant consequences. Air release valves are typically installed to remove air from water distribution systems; however, guidance on air valve placement is typically limited to placement at high points, spacing at mostly regular intervals, and changes in grade. Most guidance does not provide sufficient background to fully assess air valve placement and sizing. Henry’s law describes the relationship between dissolved air and pressure, and in conjunction with the temperature dependence of equilibrium constants described with the Van't Hoff equation, this paper applies Henry’s law to establish the potential for dissolved air within water and then to determine where air may remain dissolved or may come out of solution at any given location in a water system. Finally, practical considerations for air release valve placement, including a recommended air valve configuration, are proposed to capture and remove entrapped air from within water mains.