Frost formation and opening/closing jamming,leakage risks of cryogenic globe valves

Frosting and jamming often occur simultaneously

In the LNG storage and transportation system, frost formation on the outer surface of cryogenic globe valves is a common phenomenon, but when frost formation is accompanied by increased opening and closing resistance of the handwheel. Frost formation indicates that cold energy conducts outward through the valve stem path; jamming points to the fitting state between the valve stem and the packing area.

Causes of valve frosting and stuck opening/closing

1. The stainless steel valve stem is directly connected to the cryogenic valve body and an external handwheel. Without sufficient extension, cold flow will rise along the stem, causing the temperature in the packing gland area to drop below the ambient dew point, resulting in frost formation.
2. The valve packing is made of PCTFE, with a temperature range suitable from -196°C to +80°C. The elasticity and rebound capability of the packing at low temperatures determine its compensating ability.
3. The medium flows downward from the valve disc, and vertical installation with the valve stem oriented upward is recommended. Deviations in installation orientation can alter the direction of force on the valve disc, indirectly increasing the opening and closing resistance.

Causes of leakage in low-temperature shut-off valves

When deep-cold pipeline cooling reaches the liquid nitrogen or liquid oxygen temperature zone, the shrinkage differences between the steel pipe and the valve end occur. In butt weld connections, the valve and pipeline form a continuous metal path, causing stress concentration to first appear as cracking in the weld seam and heat-affected zone, leading to leakage of low-temperature fluid.

Causes of the risk of low-temperature welding cracking

1. The valve body, valve cover, valve disc, and valve stem are all made of stainless steel, with a temperature range suitable from -196°C to +80°C. Stainless steel retains toughness even in deep cryogenic conditions; however, when the shrinkage differs between the material and the pipeline, constraint stresses may develop at the joints.

2. The wall thickness at the valve end is typically greater than that of the pipeline, and the difference in stiffness causes the weld to become a stress transition zone.
3. The wall thickness at the valve end is typically greater than that of the pipeline, and the difference in stiffness causes the weld to become a stress transition zone.

Freezing doesn't mean failure; stalling is the sign

Frosting on the outer wall is a predictable physical phenomenon in deep-cold pipelines. What truly needs assessment is whether the opening and closing torque exceeds the operating range and whether there are visible leaks in the packing. When selecting valves, one should not rely on "zero leakage" or "maintenance-free" as criteria, but instead use four key parameter:"temperature range, packing and seat material, medium flow direction, and installation orientation",as verification benchmarks.

Welding is simplified, not exempt from inspection

The butt-weld connection eliminates the flanges and gaskets on both sides, shifting potential leakage points from the "flange-gasket" path to a continuous metal path—an engineering simplification. However, it also shifts the responsibility for verification to the welding process and post-weld inspection: groove shape, welding method, interpass temperature, and whether penetration or radiographic testing is performed after welding—all of which determine the joint's performance under deep cryogenic cycles.