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13 It is not recommended that ball valves be used for throttling as demonstrated on Figure 4 below, showing the exponential curve relating the ball position and percent valve opening. Figure 4: Typical relationship between percentage opening and ball position for a ball valve Photo 6: Ball Valve with Actuator Howell Bunger Valve (fixed cone or jet flow valves): These are generally used for discharge of pressurized flow from a closed conduit to free atmosphere. Fixed cone valves provide accurate control of the flow rate during discharge and their symmetric geometry minimizes the vibration associated with the release of the water. The discharge pool should be secured from the public around the jet flow area because of the danger of high pressure flow release. A hood can also be added to limit the spray area. Cold weather discharge from these valves can cause ice to form on downstream structures. This can create safety concerns if access in this area is necessary. Photo 7: Howell Bunger Valve In Line Gate Valve: These valves have a low friction coefficient when fully open. They are not recommended for flow regulation or throttling. Manual operation is usually slow, minimizing the risk of water hammer effect. They are sometimes divided into seating-head gates upstream and unseating-head gates downstream. Table 1: Comparison Table for Control Valves in Pressure Conduits Butterfly Valve Ball Valve Gate Valve External space required Low Low High Friction (open position) High Low Low Vibration during opening under high pressure High Average Average Vibration while open under high pressure Average Low Low Hydraulic Control Valve Discharge Valves are characterized by a coefficient, CV, which varies with the degree of opening. This coefficient, usually given by the manufacturer, can be used to calculate the flow as follows: