How to maintain cryogenic valves to extend their service life

Cryogenic valves (working temperature ≤-40℃) are widely used in cryogenic media systems such as LNG, liquid oxygen, and liquid nitrogen. Their maintenance needs to be combined with material properties, sealing principles, and working conditions. The following are systematic maintenance strategies and key measures to extend their service life:

I. Daily inspections and operation monitoring

1. Temperature and pressure monitoring

Real-time data recording: Record the inlet and outlet temperature and pressure fluctuations of the valve through the online monitoring system (such as LNG valves need to monitor temperature fluctuations ≤±5℃) to avoid thermal stress causing material embrittlement or sealing failure.

Abnormal response mechanism: When the temperature or pressure exceeds the design range (such as the design pressure of the ethylene cracking unit valve is 10MPa, the measured pressure exceeds 12MPa and needs to be shut down for inspection immediately), the emergency plan is immediately activated.

2. Leak detection

High-frequency inspections: Use an infrared thermal imager to detect the temperature distribution on the valve body surface every day. Abnormal temperature rise in the low temperature zone may indicate internal leakage; use a laser methane detector (accuracy ≤1ppm) to scan the LNG valve for leakage every week.

Strengthen inspection of key parts: valve stem seal, disassemble and inspect the stuffing box every quarter, and use fluorescent leak detector to confirm the integrity of the sealing surface; valve seat seal, low-pressure helium pressure test every six months (0.1MPa helium, pressure maintenance for 30 minutes, leakage rate ≤1×10⁻⁶ Pa·m³/s).

2. Regular maintenance

Cryogenic sealing system maintenance

Packing replacement cycle: graphite packing, replaced every 2 years (temperature resistance -200℃~600℃, but easy to crack at low temperature); flexible graphite + polytetrafluoroethylene composite packing, replaced every 3 years (both low temperature flexibility and chemical stability).

Valve seat sealing surface repair: metal sealing surface, when scratches appear, use supersonic flame spraying (HVOF) to repair tungsten carbide coating (hardness ≥1200HV); soft sealing surface, PTFE sealing ring is replaced every 5 years, PFA sealing ring is replaced every 8 years (low temperature resistance -196℃).

3. In-depth maintenance of key components

1. Low-temperature embrittlement protection of valve body

Material evaluation: Austenitic stainless steel (such as 304L, 316L) must ensure that the δ-ferrite content is ≤5% (through metallographic testing) to avoid the decrease of low-temperature toughness; aluminum bronze valve body shall be subjected to ultrasonic flaw detection (UT) every 5 years to detect the depth of intergranular corrosion (allowable value ≤0.5mm).

Stress relief: The welded valve body shall be subjected to -196℃ liquid nitrogen impact test after welding to confirm that the Charpy V-notch impact energy is ≥34J (ASTM E23 standard); strain gauges shall be used to monitor residual stress after bolt tightening, and reheat treatment shall be required when the stress value exceeds 50% of the material yield strength.

2. Low-temperature lubrication management

Lubrication process: Liquid nitrogen shall be used to pre-cool the valve stem to -80℃ before lubrication to avoid deformation of the sealing surface caused by thermal stress; gearbox lubrication shall be filled at -40℃ to ensure that the oil viscosity meets the ISO VG 100 standard.

4. Maintenance during shutdown and overhaul period

1. Long-term shutdown protection

Drying treatment: After the valve is closed, use molecular sieve desiccant (dew point ≤-60℃) to fill the valve cavity, or pass 99.999% high-purity nitrogen (flow rate ≥5L/min) to purge for 24 hours. Install a humidity sensor (accuracy ±2%RH) to monitor the humidity of the valve cavity in real time, and immediately alarm when the dew point (-52℃) corresponding to the -40℃ environment is exceeded.

Anti-corrosion treatment: The valve body surface is sprayed with a low-temperature anti-corrosion coating (such as polyurethane + aluminum powder, temperature resistance -196℃~150℃), and the dry film thickness is ≥150μm. Anti-sticking agent (such as Molykote 1000) is applied to valve stems, bolts and other parts to prevent low-temperature bonding.

2. Key inspections during overhaul period

Valve body integrity detection: Phased array ultrasonic testing (PAUT) is used to evaluate the quality of the weld. Defects with an equivalent diameter exceeding φ2mm need to be repaired. The valve body was immersed in liquid nitrogen at -196℃ (24 hours) to detect the low-temperature shrinkage rate (allowable value ≤0.15%).

Sealing performance verification: cold sealing test was carried out under liquid nitrogen medium, and the leakage rate was ≤1×10⁻⁸ Pa·m³/s (helium mass spectrometry leak detection method) when the pressure was maintained for 1 hour. Cycle life test was carried out under simulated actual working conditions (opening and closing times ≥5000 times), and the torque attenuation rate was recorded (required ≤15%).

V. Maintenance records and data analysis

Establish a valve life cycle database to record the following data: installation date, working parameters (temperature, pressure, medium); maintenance time, replacement part model, test results; fault type, repair plan, downtime; analyze historical data through machine learning algorithms to predict the remaining life of the valve (such as predicting the packing failure time based on the LSTM model, with an error of ≤10%).

Conclusion

Through the above maintenance strategy, the average service life of cryogenic valves can be extended from the conventional 5~8 years to 12~15 years, and the failure rate can be reduced by more than 60%.