Scour Velocities for Optimal Force Main Design: Rethinking the Need for Automatic Air Release Valves

Most automatic air release valve (ARV) manufacturers recommend the installation of automatic ARVs at all high points, both absolute and local. Too often, engineers include ARVs based on this criterion alone without further consideration. The four purposes that automatic ARVs serve must each be considered and evaluated relative to the force main system when determining if an automatic ARV is necessary: removing gas pockets, filling/draining, preventing negative pressures, and dampening transient waves. It is important to remove pockets of gas in the pressure pipe to improve pumping efficiency and minimize corrosion of ferrous pipe. There are several ways to design systems so that automatic ARVs are not necessary to remove gas pockets. Designing force mains so that they do not have local high points is an ideal solution; however, this is not typically practical due to conflicting utilities and slopes that would require deep force mains. Alternately, the force main can be designed so that typical flow velocities will sweep air bubbles from the high points and carry air down the slope of the force main. These design velocities are available in published tables, such as the one in Pumping Station Design. For example, a flow rate of 3.6 fps will sweep air down a 6-in. pipe with a 5% downward slope. Using these tables and available empirical equations, the force main can be designed to avoid the need for air release valves. AWWA Manual of Water Supply Practices M51 provides a discussion of the scouring velocities required to sweep air pockets down slope. Since there is no universally accepted empirical equation specifically for determining the air pocket scouring velocity in force mains, the authors will compare and summarize the sweeping velocity results from different empirical experiments and sources and recommend some design guidelines.