Why are trunnion ball valves used in cryogenic applications?

Cryogenic conditions put forward strict requirements for valve performance, which also meet the requirements of sealing, low temperature resistance, impact resistance and operating stability. Because of its structural design, cylindrical Trunnion ball valves (also known as fixed ball valves) has obvious advantages in cryogenic applications, and has become the preferred valve type ofLNG, ethylene, liquid oxygen and other cryogenic media transportation systems.

1.Dual Sealing Guarantee against Cryogenic Leakage
     low viscosity and high permeability of low Cryogenic media require extremely high sealing performance. Trunnion ball valve is designed with fixed ball plus bidirectional sealing design. The sphere is fixed by upper and lower trunnions, which reduces the effect of media flow on the sealing surface. At the same time, a spring-preloaded valve seat realizes bidirectional sealing. Even if one party breaks down, the other must maintain the seal closed to avoid safety risks caused by the leakage of the cryogenic medium. In addition, some designs employ a combination of metal hard seal and soft seal, balancing abrasion resistance and cryogenic adaptability, ensuring no leakage even at -196°C (e.g. for LNG applications).

2.Low-Temperature Material Adaptability and Resisting Brittle Fracture
     In low temperature environment, the brittle fracture of common metal materials is easy to occur because of uneven shrinkage. Trunnion-type ball valves typically use austenitic stainless steel (e.g. 304L, 316L), nickel-based alloys (e.g. Inconel) or low temperature carbon steel to make spheres, seats and valves. These materials maintain good toughness and strength at low temperatures. The trunnion structure disperses stress, reduces the local stress concentration caused by thermal expansion and shrinkage, and further reduces the risk of brittle fracture.

3.Resistant to impact, vibration and complex working environment.
     The valve may be exposed to water hammer, pressure fluctuations or external vibration during the transport of a cryogenic medium. The axial ball valve adopts a fixed ball structure, supported by the trunnion ball valve, which separates the ball from the seat and reduces the direct influence of media on the sealing surface. At the same time, the shaft neck itself has anti-vibration properties, preventing seal failure caused by vibration. In addition, its compact design reduces the overall weight of the valve, reduces impact loads caused by inertia, and makes it suitable for dynamic working conditions such as offshore platforms and shakers.

4. Long-necked bonnet design to prevent packaging freezing
     Cryogenic media can easily lead to ice in the stuffing box and affect valve operation. Trunnion-type ball valves generally adopts the long-necked bonnet structure. By extending the length of the cover, the stuffing box is kept away from the cryogenic medium, maintaining the temperature of the packing area above 0°C and preventing icing. Some designs also include drain groove at long necks to guide condensation, further preventing packaging from freezing. This feature makes it excellent for scenes like cryogenic storage tanks and cryogenic pump outlets.

5. Two-way flow and double vents to ensure system safety
     Cryogenic media may vaporize due to higher temperature, resulting in abnormal increase in pressure in the valve cavity (abnormal increase in pressure). Trunnion-type ball valves support bidirectional flow, and can be closed to achieve dual-block venting. This means that residual media in the valve cavity is expelled through the vents on the valve body, preventing pressure from accumulating. This function is essential to prevent valve body rupture or seal failure during LNG pipeline maintenance or emergency shutdowns, and to ensure the safety of personnel and equipment.

6. Modular Design for online maintenance
     Prolonged downtime is critical for the maintenance of cryogenic valve maintenance because it can cause medium vaporization or system temperature fluctuations. Trunnion-type ball valves features anentry structure at the top, with the body and cover bolted together, allowing internal components such as valve seat and spheres to be replaced online without the need to remove piping. Some designs also support real-time maintenance, allowing for adjusting the sealing preload or replacing seals when the system is running with specialized tools, thus significantly reducing downtime and maintenance costs.

Question 1: What is the difference between a cylindrical ball valve and a floating ball valve in cryogenic applications?
A: In a floating ball valve, the ball is free floating and sealed by pressing the valve seat with dielectric pressure. At low temperature, the change of medium viscosity will lead to unstable sealing. In trunnion-type ball valves, the ball is fixed and the valve seat is pressurised by springs, making the more reliable sealing performance, more impact resistance and more suitable for cryogenic conditions.

Q2: How to select the material for cryogenic trunnion-type ball valves?

A2: Cryogenic valve seat materials requires a balance of abrasion resistance, corrosion resistance and low temperature toughness. Commonly used materials include modified polytetrafluoroethylene (PTFE) materials,such as graphite-filled PTFE, and metallic graphite. Hard metal seals (such as Stellite alloys) or flexible graphite seals seals are preferred at ultra-low temperatures (e.g. -196 ° C) to avoid the brittleness soft sealing materials.

Q3: Can trunnion-type ball valves be used under alternating conditions of high and low temperature?
A3: Yes, but choose materials and designs that are temperature adaptable. For example, the valve body should be made of low temperature carbon steel or stainless steel, while the shaft neck and seal should be made of high temperature resistant materials (e.g. nickel-based alloys). At the same time, residual stress should be eliminated by heat treatment to avoid deformation or leakage caused by temperature change.

Q4: What are the special requirements for installing cryogenic trunnion-type ball valves?
A4: During installation, ensure that the flow of the valve is in line with the flow of the medium to avoid reverse pressure affecting the sealing cover; long-necked valve covers must be installed vertically to prevent condensation buildup; slow pre-cooling is required before initial use to prevent thermal stress from cracking valve body; and bolts are tightened diagonally to prevent uneven stress on the sealing cover.

Q5: How to determine if the cryo-volley valve needs repair?
A5: If operating torque increases, sealing leakages, abnormal frosting on the valve body or leakage of outlet media, it may indicate seal wear, ageing of packing or abnormal valve cavity pressure. Timely inspection and replacement of parts is required. Regular online testing,such as ultrasonic testing, can detect potential problems early and extend valve life.