What problems may occur with LNG stainless steel pipes in a low-temperature environment

LNG stainless steel pipes exhibit significant differences in performance when exposed to a low temperature of -162℃ compared to normal temperature conditions. The main issues revolve around the deterioration of mechanical properties, structural deformation failure, decreased sealing reliability, and system compatibility faults. The specific details are as follows: 

1. Degradation of mechanical properties: Insufficient toughness leads to brittle fracture

Low temperatures can alter the microstructure and mechanical properties of stainless steel, which is the core risk causing pipeline accidents: 

Low-temperature embrittlement: If the low-temperature toughness (impact energy) of the pipe material or weld seam does not meet the standards (such as being lower than the requirement of ≥27J as stipulated in GB/T 24593), the material will transition from the "tough state" to the "brittle state" at low temperatures. Under slight external forces (such as vibration, impact), a sudden brittle fracture may occur without any warning, especially in the heat-affected zone (HAZ) of the weld seam due to the coarse grain structure, where the risk of embrittlement is even higher. 

Strength and plasticity imbalance: Low temperatures cause the yield strength and tensile strength of stainless steel to increase, but the elongation and reduction of area significantly decrease (for example, the elongation of 316L stainless steel at -162℃ is about 30% lower than at room temperature), resulting in weakened resistance to deformation of the pipe. If there are residual stresses in the system (such as the stress generated by forced butt welding during installation), it is prone to cause local cracking. 

2. Structural deformation and stress damage: Cold contraction and vibration combined damage

Under LNG conditions, "sudden temperature drop" and "system vibration" can cause irreversible structural damage to the pipe materials: 

Low-temperature contraction and stress concentration: The expansion coefficient of stainless steel is relatively high (for example, 316L is approximately 16.5×10⁻⁶/℃). When the temperature drops from 25°C to -162°C, the contraction amount for every 10 meters of pipe length can reach 29mm. If there is not enough compensation (such as not setting a compensator or a natural compensation section), the contraction force will concentrate at the flange, valve interface or weld seam, causing the interface to warp, the weld seam to crack, and even pulling apart the interface of equipment such as storage tanks and pump bodies. 

Increased wear and fatigue due to vibration: The operation of pumps and compressors in the LNG system generates continuous vibration. At low temperatures, the rigidity of pipes increases and the damping decreases, making the vibration energy more likely to be transmitted to the support points or connection areas. Long-term vibration can cause the insulation gaskets (such as polytetrafluoroethylene) between pipe clamps and pipes to wear, resulting in direct metal contact (causing cold bridges); at the same time, micro cracks will form at welds and bends due to "vibration fatigue", gradually expanding into macroscopic leaks. 

3. Sealing reliability failure: Failure of sealing structure at low temperatures

The sealing system is the core protective component of the LNG pipeline. Low temperatures can cause failure of sealing parts or connection structures: 

Sealing components cracking and shrinking: Ordinary rubber sealing components (such as nitrile rubber) will lose elasticity and undergo cracking at low temperatures. Even for low-temperature-resistant sealing components (such as EPDM, FKM), they will shrink due to low temperatures, resulting in insufficient compression - if no shrinkage allowance is reserved during normal temperature installation, gaps will occur at the sealing surface, causing LNG leakage (with the leakage volume reaching the explosion limit, there is a risk of combustion and explosion). 

Flange connection loosening: If the bolt material does not match the pipe material (for example, carbon steel bolts are used with stainless steel flanges), the difference in contraction amounts between the two at low temperatures (the linear expansion coefficient of carbon steel is approximately 13×10⁻⁶/℃, which is lower than that of stainless steel) will cause the pre-tightening force of the bolt to decrease, and the sealing surface pressure of the flange to drop; moreover, the bolt's toughness is insufficient at low temperatures, and after repeated cold contraction and hot expansion, it may "brittle fracture", directly damaging the sealing structure. 

4. System compatibility issues: Material-interaction failure with the medium

In low-temperature environments, the interaction between the pipe material and the internal medium of the system, as well as other components, will be exacerbated, leading to functional failures: 

Electrochemical Corrosion and Cold Bridge Corrosion: If the support components and fasteners are made of carbon steel (instead of low-temperature resistant stainless steel), an "electrochemical couple" will form when in contact with the stainless steel pipe. At low temperatures, the trace moisture in the LNG (even after drying treatment may still remain) acts as an electrolyte, accelerating the corrosion of the carbon steel. At the same time, direct contact between the metal causes a "cold bridge", resulting in the transfer of cold from the pipe to the support structure. Water in the air condenses into ice on the surface of the support components, further exacerbating corrosion and structural freezing. 

Impurity freezing and blockage: If the pipe materials were not thoroughly degreased and dried before installation (with residual oil, iron filings, and moisture), these impurities will freeze into hard particles at low temperatures. On one hand, these particles will wear down the pump impeller and valve sealing surfaces as they flow with the LNG, reducing the equipment's lifespan. On the other hand, fine particles may clog the flow meters and safety valve's throttling holes, causing the system pressure to become out of control or the metering to fail.