Engineering Manual o.. - HVAC.Amickracing

VALVE SELECTION AND SIZINGValves must be compatible with system mediacomposition, maximum and minimum temperature, andmaximum pressure. The temperature and pressure of themedium being controlled should not exceed themaximum temperature and pressure ratings of the valve.For applications such as chlorinated water or brine, selectvalve materials to avoid corrosion.— What is the pressure drop across the valve? Is the pressuredrop high enough?The full open pressure drop across the valve must behigh enough to allow the valve to exercise control overits portion of the hydronic system. However, the full openpressure drop must not exceed the valves rating for quietservice and normal life. Closed pressure drop must notexceed valve and actuator close-off rating.GLOBE VALVEGlobe valves are popular for HVAC applications. They areavailable in pipe sizes from 1/2 in. to 12 in. and in a large varietyof capacities, flow characteristics, and temperature and pressurecapabilities. They provide wide rangeability and tight shutofffor excellent control over a broad range of conditions. Globevalves are made in two-way, straight or angle configurationsand three-way mixing and diverting designs. Globe valves closeagainst the flow and have arrows on the body indicating correctflow direction. Incorrect piping can result in stem oscillations,noise, and high wear.A two-way globe valve has one inlet port and one outlet port(Fig. 5) in either a straight through or angle pattern. The valvecan be either push-down-to-close or push-down-to-open.Pneumatic and electric actuators with linear motion to operateglobe valves are available for operation with many controlsignals.BALL VALVEBall valves are available for two-position applications eithermanual (hand) or power operated or for modulating applicationswith direct coupled electric actuators. Ball valves are relativelylow cost and provide tight close off and available in two-wayand three-way configurations. As with all other valves, ballvalves must be properly sized to provide good flow control.When used in modulating service, ball valves must bespecifically designed for modulating service as compared totwo-position service. Packing must provide leak-free sealingthrough thousands of cycles to ensure trouble-free HVACservice. The ball and stem should be made of stainless steel orsimilar material that minimizes sticking to the seat.Two-way ball valves have equal percentage flow controlcharacteristics and flow can be in either directionThree-way ball valves can be used in either mixing ordiverting service. They have linear flow control characteristicsfor constant total flow.BUTTERFLY VALVEButterfly valves (Fig. 6) control the flow of hot, chilled, orcondenser water in two-position or proportional applications.Butterfly valves are available in two-way or three-wayconfigurations. Tight shut-off may be achieved by properselection of actuator force and body lining. The three-way valvecan be used in mixing or diverting applications with the flowin any direction. The three-way valve consists of two butterflyvalves that mount on a flanged cast iron tee and are linked toan actuator which opens one valve as it closes the other.Minimum combined capacity of both valves occurs at the halfopenposition.ININPUSH-DOWN-TO-CLOSEPUSH-DOWN-TO-OPENC2328Fig. 5. Two-Way Globe Valves.M10403Fig. 6. Butterfly Valve.ENGINEERING MANUAL OF AUTOMATIC CONTROL407