Cryogenic Valve Extended Bonnet Design and Qualification

ted wang
Jun 4
2 min read

Why Cryogenic Valves Need Extended Bonnets

At cryogenic temperatures (below -46°C / -51°F), conventional packing materials lose flexibility and sealing capability. Extended bonnets, also called cold box bonnets or top-entry extensions, keep the packing chamber at or near ambient temperature by providing sufficient thermal insulation length between the cold media and the packing zone.

Design Standards and Requirements

BS 6364: primary standard for cryogenic valve design and testing
MSS SP-134: valves for cryogenic service including extended body requirements
API 6D: covers pipeline ball and check valves used in LNG systems
ASME B16.34: pressure-temperature ratings for valve body and bonnet flanges
ISO 21011: valves for cryogenic service in LNG terminals

Materials of Construction

Body and bonnet materials must maintain ductility at cryogenic temperatures. Suitable alloys include ASTM A351 CF8M (316 SS), A352 LC3 (3.5% Ni steel), and A522 Type I (9% Ni steel). Austenitic stainless steels are preferred for LNG service. Carbon steel becomes brittle at low temperatures and must not be used without low-temperature impact testing qualification.

Extended Bonnet Sizing

Minimum bonnet extension length depends on pipe insulation thickness and valve size
Typical extensions range from 150 mm to 600 mm for LNG and liquid nitrogen service
Calculate packing temperature rise to ensure packing remains above minimum operating temperature
Account for thermal contraction of extension tube in design and bolt torque calculations
Provide venting holes to prevent gas pockets forming inside the extended bonnet

Qualification Testing

Qualification testing per BS 6364 includes pressure testing at ambient temperature, cold cycle testing with liquid nitrogen or LNG media, and seat leakage verification after thermal cycling. Valves must demonstrate leak-tight performance after repeated cycles from ambient to service temperature, confirming packing integrity and body seal reliability throughout the product service life.