What are ANSI/FCI valve leakage classifications?

ANSI/FCI 70-2 defines allowable seat leakage: Class I - no test required (worst), Class II - 0.5% of rated capacity (metal seat, general purpose), Class III - 0.1% of rated capacity (improved), Class IV - 0.01% of rated capacity (metal-to-metal, tight shutoff), Class V - 0.0005 mL/min per inch of seat diameter per psi differential (very tight), Class VI - bubble-tight per defined table (soft seat, tightest). Selection depends on application: Class II-III for general control, Class IV-V for isolation, Class VI for critical shutoff. Tighter classes require better trim design and higher cost.