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<br /> <br /> <br /> <br />9 <br />Photo 2: Trash racks surrounding an intake structure <br />Conduit Operating Conditions – The conduit that <br />discharges water from the intake structure is called the <br />outlet conduit. The outlet conduit should operate <br />under one of two conditions throughout its length: (1) <br />fully pressurized pipe flow or (2) non-pressurized, <br />open-channel flow conditions. Mixed flow conditions <br />in which only a portion of the conduit is pressurized <br />are undesirable because air trapped within the pipe <br />may lead to burping, surging, cavitation, and vibration. <br />As a result, venting becomes an important component <br />for both types of systems (see discussion of cavitation <br />and venting below). <br />Fully pressurized systems require a flow control <br />mechanism (typically a valve) at the downstream end, <br />which is used to regulate the flow while maintaining <br />full pipe flow throughout the conduit. <br />Non-pressurized conduit systems require the outlet <br />conduit to be large enough that open channel flow <br />conditions are sustained throughout the length of the <br />conduit over the entire range of operating flows. Flow <br />is regulated at the upstream end with a slide gate or <br />valve, and the flow discharges freely at the <br />downstream end. <br />Control Mechanisms – Various control mechanisms <br />are used for gated intakes and conduits. Common flow <br />control mechanisms include gates and valves, which <br />can be controlled manually or hydraulically. The <br />control mechanism can be positioned at various <br />locations including at the upstream intake, along the <br />conduit within the embankment, and at the <br />downstream end of the conduit. <br />Gates are one type of control mechanism often used. <br />Gates are suitable for flow regulation under low head <br />conditions. However, under high head, partially open <br />gates may vibrate and cavitate. For these reasons, the <br />gates should only be operated to be fully open or fully <br />closed. <br />Valves are another common type of control <br />mechanism used along the conduit. Common types of <br />valves include knife gate, butterfly, fixed cone, and <br />pivot. Butterfly valves are generally cost-effective, but <br />are susceptible to severe vibration and cavitation when <br />partially open. Therefore, they are typically used for <br />full-open or full-closed operation rather than for flow <br />regulation. Flow regulation valves are typically placed <br />at or near the downstream end of the outlet conduit, <br />which allows them to discharge freely to the <br />atmosphere, eliminating most of the potential for <br />cavitation. However, placing the control mechanism at <br />the downstream end causes the conduit to be <br />pressurized through the embankment, which induces <br />additional risk if cracking or rupture of the conduit <br />occurs due to deterioration, differential settlement, <br />joint separation, or other structural failure. <br /> <br />Photo 3: Angled intake gate parallel with upstream face of dam <br />Conduits – A variety of options are available for outlet <br />conduits ranging from small pipes to large tunnels. <br />Common conduit materials historically are reinforced <br />concrete (cast-in-place or precast), metal pipe (steel, <br />corrugated metal pipe [CMP], ductile iron, cast iron), <br />and high density polyethylene (HDPE). A summary of <br />common conduit materials used today is presented in <br />Table 1. Cast-iron, ductile-iron, and CMP are no longer