What special considerations apply to high-temperature control valve design?

High-temperature valve design (above 500 degrees F): material selection per ASME B31.3 (high-temperature allowable stress, creep consideration), extended bonnet design (keeps packing at acceptable temperature), special packing (flexible graphite rated for temperature), thermal expansion (clearances, anti-binding features), actuator protection (mounting orientation, heat shields, fins), and positioner location (away from heat source or high-temperature rated). Additional: consider thermal cycling (startup/shutdown) effects on bolting, gaskets, and trim. Calibration: account for stem expansion in position feedback. Materials: commonly A217 WC6 or WC9 bodies, Stellited trim. Verify fire-safe rating if required (API 607).