DWR_2801666 - Laserfiche WebLink

Home Search

12 Design Considerations for Outlet Works Air Vents Introduction Outlet works air vent design is often a difficult, misunderstood, or even unknown subject for many design engineers. This article introduces the subject of air demand and air vent sizing, and discusses possible consequences of inadequate air vent design. Important design criteria and guidelines are summarized, and a conservative, generalized approach for estimating air demand and sizing air vents is provided. Several references containing alternate design methodologies are presented in this article; however, it is cautioned that there are limitations associated with each design method. Designers should check these limitations to ensure the specifics of their projects are consistent with the methods being employed. Why Air Vents? An important consideration in any closed conduit design for an outlet works is the proper use of air venting. An air vent simply allows air under atmospheric pressure to flow into an outlet works conduit, introducing (or entraining) air into the flow. Specifically, a properly designed air vent serves the following purposes:  Reduces potential for formation of low pressures within the flowing water;  Reduces potential for unstable flow in the conduit; and  Allows bleeding of air from a conduit prior to operation. Air vents are typically installed downstream from a control gate or valve, where formation of low flow pressures can occur. In the absence of adequate air venting, low flow pressures can lead to cavitation, air blow back, pipe collapse, excessive vibration, and excessive noise. Each of these possible consequences is discussed below. Consequences of Inadequate Air Vent Design Cavitation, or the formation of vapor cavities (bubbles) in low pressure areas just downstream from the control gate/valve, is the most common consequence of inadequate air vent design. As cavitation bubbles are carried downstream from the gate into higher pressure flow areas, they rapidly collapse (implode), sending out high-pressure shock waves that can damage a conduit wall near the implosion. Cavitation damage generally occurs downstream of the gate slots in the outlet works, but can also occur on the invert downstream of the control gate. Figure 1 shows typical cavitation damage on an outlet gate and conduit walls. Air blowback can occur as air collects on the crown of the conduit downstream of a control gate and forms a large pocket of air that can violently “blow back” toward the control gate and intake structure, causing damage to those structures. Pipe collapse downstream from a gate can also occur if low pressure flow is extreme enough, as illustrated in Figure 2. Excessive vibration in low pressure or unstable flow areas downstream of a control gate can lead to structural damage of the conduit and gate, if severe enough. Excessive noise can occur at the air vent opening if the air vent is designed too small. The noise can even be so loud that it is damaging to hearing. At one dam, nearby residents complained of a popping noise coming from the air vent that was keeping them up at night. Figure 1: Typical cavitation damage on gate and conduit