How to solve the problem of sealing failure of LNG stainless steel pipes in low-temperature environm

To solve the sealing failure problem of LNG stainless steel pipes in low-temperature environments, it is necessary to start from four core dimensions: selection of sealing components, optimization of connection structures, control of installation processes, and matching of materials. A complete protection plan should be formulated. The specific measures are as follows: 

1. Precisely select low-temperature resistant sealing components: Matching the elasticity and weather resistance for low-temperature conditions

Sealing components are the first line of defense against leakage. Priority should be given to selecting materials that maintain elasticity and resist brittleness at low temperatures. The key selection criteria and recommended types are as follows: 

Core performance requirements: Must meet - a permanent deformation rate of ≤ 20% at -162℃ (LNG boiling point) (in accordance with GB/T 7759 standard), a retention rate of tensile strength of ≥ 70%, and resistance to LNG medium swelling (swelling rate ≤ 5%).

Recommended seal types:

Static seals (flanges, valve interfaces): Preferentially select fluorosilicone rubber (FFKM) or modified polyether propylene (EPDM) gaskets, as the former has the best low-temperature and chemical stability (able to withstand -200℃ to 260℃), suitable for high-pressure, high-purity LNG systems; the latter has a lower cost and is suitable for medium-low pressure conventional conditions. Do not use nitrile rubber (NBR) and other low-temperature brittle materials.

Dynamic seals (valve stems, pump shafts): Use a combination of filled polytetrafluoroethylene (PTFE) and metal springs for sealing. PTFE has low-temperature resistance (-200℃) and low friction coefficient, and the spring can compensate for the contraction of the sealing element under low-temperature conditions to ensure continuous tightness under pressure. 

2. Optimize the flange connection structure: Mitigate low-temperature contraction and stress

Flange connections are a common area where sealing failures occur. Therefore, through structural design, the sealing surface gap and the reduction in bolt preload caused by low-temperature contraction should be counteracted: 

Using mortise-slot face / convex-concave face flanges: Compared to flat flanges, the sealing surface of mortise-slot flanges adopts a "mortise - slot" complementary structure, which can limit the displacement of the sealing element under low temperatures and prevent the misalignment of the sealing surface due to contraction; at the same time, the slot can store sealing grease (such as low-temperature silicone-based sealing grease), further enhancing the sealing performance.

Setting a bolt preload compensation structure:

Selecting low-temperature ductile bolts: The bolt material must match the pipe material (such as 316L stainless steel bolts paired with 316L flanges), avoiding the loss of preload due to differences in linear expansion coefficients (such as different contraction amounts between carbon steel bolts and stainless steel flanges); key parts can use low-temperature alloy bolts (such as Inconel 625), which have better low-temperature ductility and anti-relaxation performance.

Using disc-shaped spring washers: Installing low-temperature resistant disc-shaped springs (made of 17-7PH stainless steel) between the bolt and the flange, using the elastic deformation of the spring to compensate for the contraction of the bolt under low temperatures, maintaining the constant adhesion pressure of the sealing surface and preventing the loss of preload. 

3. Strictly control the installation process: Eliminate installation risks

Operational deviations during the installation process are a significant cause of seal failure. The following process requirements must be followed: 

Sealing surface pre-treatment: The sealing surface of the flange needs to be thoroughly degreased and dried with alcohol, and impurities such as oil stains and iron filings should be removed (to prevent damage to the sealing surface after low-temperature freezing); the surface roughness should be controlled within Ra 1.6-3.2 μm (detected with a roughness tester), and no scratches, depressions, or other defects are allowed.

Screw tightening process:

Use the diagonal step-by-step tightening method (in a "crossing" sequence, tighten 3-4 times) to ensure uniform force on the flange surface and avoid local warping;

Use a torque wrench to tighten according to the designed torque (for example, the torque of a M20 bolt is approximately 180-220 N·m, which needs to be determined based on the bolt material and specification), and never over-tighten (causing the bolt to break) or under-tighten (insufficient pre-tightening force);

Reserve sealing material shrinkage allowance: During normal temperature installation, the compression amount of the sealing material needs to be increased by 5%-10% compared to the design value (for example, if the designed compression amount is 2 mm, the actual compression should be 2.1-2.2 mm) to offset the contraction at low temperatures and ensure that there is no gap on the sealing surface. 

4. Strengthen system operation and maintenance: Prevent long-term failure

The long-term reliability of the sealed system needs to be ensured through regular maintenance and monitoring. Key measures include: 

Regular leak detection: After commissioning, use a helium mass spectrometer leak detector (sensitivity ≤ 1×10⁻⁹ Pa·m³/s) to inspect the flanges and valve interfaces every month. Focus on checking the heat affected zone of the weld and the bolt connection area. If a micro-leakage is detected, promptly tighten the bolts or replace the sealing components to prevent the leakage from expanding. 

Avoid sudden temperature changes: During system startup and shutdown, the cooling / heating rate should be controlled (≤ 5℃/min). By slowly adjusting the valve opening, prevent the sealing components from experiencing drastic contraction / expansion due to sudden temperature changes, which could cause the sealing surfaces to separate.

Replacement cycle management: Determine the replacement cycle based on the material of the sealing components (e.g., FFKM sealing components are recommended to be replaced every 3-5 years, and EPDM sealing components are recommended to be replaced every 2-3 years). Even if no leakage is detected, regular replacement is necessary to avoid sealing failure due to material aging.