In LNG storage and transport, media are maintained at -162°C for long periods. This cryogenic condition demands specialized valve designs. For cryogenic ball valves, extended bonnets are not redundant but a necessary technical solution to ensure sealing reliability, thermal stability, and operational safety. This design is mandated by international standards like API 624 and BS 6364 as a core requirement for safe LNG system operation. The specific reasons are as follows.
I. Isolate Low Temperatures to Ensure Sealing Performance
The packing box is the core of a valve's dynamic sealing system. Conventional packing loses elasticity at -162°C, failing to seal the stem tightly and causing leaks. Meanwhile, moisture in the air condenses and freezes in the packing box. Ice crystals can score the packing and seize the stem, blocking valve operation.
Extended bonnets solve this by lengthening the neck to lift the packing box away from the cryogenic media. The neck acts as a thermal barrier, reducing upward heat transfer and keeping the packing box above 0°C. This preserves packing elasticity, prevents icing, ensures smooth stem rotation, and maintains dynamic sealing performance in line with low-emission standards.
II. Provide Space for Insulation to Minimize Heat Ingress
LNG has a low heat of vaporization. Even small heat ingress can cause it to vaporize, expanding ~600 times and spiking pipeline pressure. Conventional valves lack space for insulation, leading to heat loss through the valve body. Extended bonnets offer ample space for thick insulation layers around the neck, forming a continuous thermal barrier.
This barrier blocks external heat, reduces LNG vaporization, and stabilizes pipeline pressure. The insulation's outer surface stays near ambient temperature, preventing valve icing that could cause operator frostbite and avoiding damage from falling ice.
III. Enable Physical Isolation to Enhance Operational Safety
LNG is flammable and explosive. Leaks pose major risks. Conventional valves place operating mechanisms near cryogenic zones, allowing vaporized gas to reach operators quickly if leaks occur. Extended bonnets create physical separation, keeping handles and actuators at a safe distance from hazardous areas. Even with minor leaks, gas diffuses and dilutes in the extended neck, lowering flammable gas levels at the operator's position.
For maintenance, LNG valves are often in confined spaces like cold boxes. Extended bonnets place packing followers and grease fittings outside the insulation, allowing routine maintenance without removing insulation-cutting operational difficulty and costs. This design also meets BS 6364's operational height requirements for safe valve operation.
IV. Comply with Standards and Design Specifications
Extended bonnet length must follow strict standards. Per BS 6364, neck length is ≥250mm for valves ≤DN100 and ≥300mm for larger valves. These dimensions are thermally validated to keep the packing box above 5°C at -196°C. Neck wall thickness also requires cryogenic strength checks to prevent thermal stress cracks.
Materials must match the valve body's cryogenic toughness, such as ASTM A350 LF2 carbon steel or ASTM A182 F304 stainless steel, to avoid brittle fracture. Neck-to-bonnet connections use integral forging or full-penetration welding to eliminate thermal bridges from flanges, boosting insulation performance.
Industry standards mandate extended bonnets for LNG cryogenic ball valves. API 624 requires them and -196°C cryogenic sealing tests. GB/T 24924-2020 also sets rules for neck length and packing box temperature. These standards ensure technical consistency and safety across the LNG supply chain.
Extended bonnets on cryogenic ball valves are a systematic solution for LNG's cryogenic, flammable, and vaporizable nature. They ensure sealing via thermal isolation, enable insulation for heat retention, and enhance safety through physical separation-meeting strict industry standards. This design is essential for valve function in cryogenic service and a must for safe LNG system operation.
