Double Eccentric Butterfly Valve

Double Eccentric Butterfly Valve – Cam-Action Sealing for Extended Service Life & Frequent Operation

Double-eccentric (double-offset) butterfly valves incorporate two geometric offsets that fundamentally change the disc sealing mechanism from continuous friction (concentric designs) to cam-action contact only at final closure. The first offset positions the shaft axis behind the disc sealing surface; the second offset locates the shaft centerline away from the pipe centerline. This dual-offset geometry enables the disc to swing clear of the seat during opening, eliminating friction throughout the 90° rotation except during the final 10-15° of closure when cam action generates seating force.

Operational advantages transform valve performance in frequent-cycling applications: concentric butterfly valves experience continuous seat contact during rotation, causing wear, high operating torque, and limited cycle life (50,000-100,000 cycles typical). Double-eccentric designs achieve 1-5 million cycle life through friction elimination, making them essential for African water treatment plants requiring multiple daily operations, automated process control systems cycling continuously, and power plant applications with frequent modulating duty throughout the operational envelope.

The cam-action sealing mechanism generates higher seating stress than concentric designs, enabling bubble-tight shutoff (ISO 5208 Rate A) with resilient seats or metal-to-metal sealing with specialized seat configurations. For African oil & gas applications requiring tight shutoff in hydrocarbon service, double-eccentric butterfly valves bridge the gap between standard concentric designs limited to soft-seat materials and triple-offset configurations engineered specifically for severe service at significantly higher cost.

Temperature and pressure capabilities extend beyond concentric limits: reduced seat friction enables PTFE, graphite, or metal seat materials unsuitable for high-friction concentric designs. Common applications span -40°C to +400°C with PTFE or graphite seats, addressing African petrochemical processes, power generation steam service, and industrial applications where concentric soft-seat temperature limits (typically -20°C to +120°C with EPDM/NBR) prove inadequate. Pressure ratings to PN 40 (ANSI 300) in standard configurations suit most African industrial requirements while high-performance variants reach PN 100+ for specialized applications.