Engineering Manual o.. - HVAC.Amickracing

VALVE SELECTION AND SIZINGTable 1. Corrosive Elements in Hydronic Systems.Brass or Bronze ComponentCorrosive Substance Corrosion ColorChlorideAmmoniaCarbonatesMagnesium or CalciumOxidesSulphide (Hydrogen)IronLight Blue-GreenBlue or Dark BlueDark Blue-GreenWhiteBlack (water)Black (Gas)RustIron or Steel ComponentCorrosive Substance Corrosion ColorMagnesium or CalciumIronWhiteRustGlycol solutions may be used to prevent hydronic systemsfreezing. Glycol solutions should be formulated for HVACsystems. Some available glycol solutions formulated for otheruses contain additives that are injurious to some system seals.In addition, hydronic seals react differently to water and glycolsuch that when a new system is started up with water or glycolthe seals are effective. The hydronic seals are likely to leak ifthe system is later restarted with media changed from to waterto glycol or glycol to water. To prevent leakage part of theprocess of media changeover should include replacing sealssuch as, pump and valve packing.VALVE SELECTIONProper valve selection matches a valve to the control andhydronic system physical requirements. First consider theapplication requirements and then consider the valvecharacteristics necessary to meet those requirements. Thefollowing questions provide a guide to correct valve selection.— What is the piping arrangement and size?The piping arrangement indicates whether a two-wayor three-way mixing or diverting valve is needed. Thepiping size gives some indication of whether the valverequires a screwed end or a flanged end connection.— Does the application require two-position control orproportional control? Does the application require anormally open or normally closed valve? Should theactuator be direct acting or reverse acting?In its state of rest, the valve is normally open or closeddepending on the load being controlled, the fluid beingcontrolled, and the system configuration.For chilled water coils, it is usually preferable to closethe valve on fan shutdown to prevent excessivecondensation around the duct and coil, and to savepumping energy. This may be accomplished with eithernormally closed valves or a variety of other controlschemes. Lower cost and more powerful normally openvalve assemblies may be used with the close-onshutdownfeature and allow, in the case of pneumaticsystems, the capability to provide heating or cooling inthe event of air compressor failure.Converter control valves should be normally closed andoutdoor air preheat valves should be normally open.— Is tight shut-off necessary? What differential pressuredoes the valve have to close against? How much actuatorclose-off force is required?Single-seated valves provide tight shut-off, while doubleseatedvalves do not. Double seated valves are acceptablefor use in pressure bypass or in-line throttlingapplications.The design and flow capacity of a valve determine whomuch actuator force is required for a given close-off.Therefore, the valve must first be sized, then, the valveand actuator selected to provide the required close-off.— What type of medium is being controlled? What are thetemperature and pressure ranges of the medium?Valves 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.432ENGINEERING MANUAL OF AUTOMATIC CONTROL