Cryogenic Valve Extended Bonnet Design

Purpose of the Extended Bonnet

In cryogenic service, an extended bonnet (or extended stem) positions the packing gland and stem seal well above the cold zone. This keeps the packing at near-ambient temperature, ensuring reliable sealing and preventing packing material from becoming brittle.

Design Standards

• BS 6364: UK standard for valves for cryogenic service

• MSS SP-134: Valve Packing Gland Requirements for Cryogenic Service

• ISO 28921: Industrial valves for cryogenic service

• LNG plant specifications typically call for BS 6364 compliance

Extended Bonnet Length Calculation

Minimum bonnet extension is calculated to maintain packing temperature above the freezing point of the packing material under worst-case process temperatures. For liquid nitrogen service (−196°C), extensions of 150–300 mm are common.

Materials for Cryogenic Valves

• Body: ASTM A351 CF8M (316 SS casting) or CF3M for low carbon

• Trim: 316 SS or Monel for LNG

• Packing: PTFE or PTFE-graphite composite

• Bolting: B8M (316 SS) for cryogenic temperatures

• Avoid carbon steel—brittle below −29°C

Testing Requirements

Cryogenic valves undergo cold testing by immersion in liquid nitrogen to verify seat leakage and operational torque at temperature. Testing confirms the bonnet extension is adequate to maintain packing sealing under design conditions.