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Characteristic CurvesThe inherent flow characteristic of a control valve is therelationship between the flow and the lift of the plug atconstant pressure drop. The characteristics normallyavailable ar....

Characteristic CurvesThe inherent flow characteristic of a control valve is therelationship between the flow and the lift of the plug atconstant pressure drop. The characteristics normallyavailable are shown on Figure 7.Maximum Leakage RatesLeakage rates are normally measured in accordance withthe ANSI / FCI70.2 specification using the class designation.The following Table defines the achievable leakage class foreach available plug design.Figure 8. Characteristic curves tableTable 8. Maximum leakage ratesPlug DesignSeating StyleAchievableLeakage ClassUnbalanced(SP)Metal/Metal (std)IVUnbalanced(SP)Metal/Metal (spl)VUnbalanced(SP)Metal/PTFEVINote - (std)or (spl) refer to the amount seat/plug lappingcarried out at final assemble.Balanced (BP)Metal/Metal (std)IVBalanced (BP)Metal/Metal (spl)VBalanced (BP)Metal/PTFEVIBalanced (DP)Metal/Metal (std)I IBalanced (DP)Metal/Metal (spl)I I IDefinitions:?LinearFlow is directly proportional to valve lift.? Equal %Flow changes by a constant percentage of itsinstantaneous value for each unit of valve lift.?Quick OpeningFlow increases rapidly with initial travel reaching nearits maximum at a low lift.Modified ParabolicProvides fine throttling action at low valve lift andApproximately a linear characteristic for upper portionsof travel.7