Threaded connection sealing failure refers to the gradual loosening of male and female thread pairs under thermal cycle vibration, medium pressure fluctuation and creep relaxation of sealing filler. The pre-tightening force of the thread decreases continuously, the gap between fitting surfaces expands, and process medium leaks outward along the thread clearance. In severe cases, the thread completely disengages and the pipeline falls off, triggering material leakage, equipment shutdown and even safety accidents. This kind of failure mostly occurs in detachable threaded joints of branch pipelines and instrument interfaces, which is a common hidden danger in the assembly structure of heating pipelines.
1. Loosening and Leakage Evolution Mechanism
After the thread is screwed in place, the thread tooth bears extrusion preload to compress the sealing raw material or gasket to form a closed sealing surface. During repeated heating and cooling, the difference in thermal expansion coefficient between the pipe body and threaded fittings causes periodic relative micro-displacement between thread teeth. System pressure impact and mechanical vibration will further shear the meshing structure of threads. The initial pre-tightening stress gradually releases and relaxes, the compression amount of the sealing medium reduces, and tiny penetration gaps appear on the sealing surface. Fluid medium continuously seeps along the spiral thread gap. With the loss of sealing filler and continuous wear of thread tooth surfaces, the leakage volume increases day by day, and eventually the thread completely loses meshing restraint and falls off under internal pressure thrust.
2. Typical Positions Prone to Thread Loosening Failure
Branch pipe threaded joints and small-bore instrument take-off interfaces with frequent pressure fluctuation;
Threaded parts near pumps and vibrating equipment subjected to long-term mechanical vibration;
Thread connections in high-temperature alternating temperature zones with obvious thermal expansion and contraction;
Threaded elbows, reducers and conversion adapters bearing bending moment and additional pulling force;
Over-tightened threaded sections with plastic deformation of thread teeth leading to permanent force loss.
3. Key Inducing Factors Accelerating Loosening Damage
No anti-loosening measures after thread screwing, relying only on pure thread meshing for fixation;
Excessive or insufficient initial tightening torque, causing thread tooth damage or insufficient sealing compression;
Long-term temperature alternation and equipment vibration leading to continuous attenuation of pre-tightening force;
Improper selection of sealing filler, which ages, hardens and loses elasticity under high temperature;
External pipeline load and bending force directly acting on the threaded joint to generate shear torque.
4. Full-Link Prevention and Control Technical Measures
① Standardize torque-controlled tightening operation
Use torque wrench to implement rated screwing force to avoid thread sliding and insufficient compression.
② Add thread anti-loosening auxiliary measures
Coat high-temperature resistant thread locker or install lock nuts after assembly to limit relative rotation.
③ Select high temperature resistant and aging resistant sealing materials
Adopt PTFE thread tape or flexible graphite sealing filler suitable for working temperature.
④ Reduce additional external load on threaded joints
Set independent supports near threaded fittings to bear pipeline weight and bending stress.
⑤ Arrange regular inspection and re-tightening during equipment maintenance
Check leakage points and replenish tightening force for relaxed threaded connections periodically.
5. Prevention Effect Comparison Table
表格
| Assembly & Maintenance Mode | Thread Loosening Leakage Risk | Application Suggestion |
|---|---|---|
| Random hand tightening + no anti-loosening treatment + no periodic inspection | Fast leakage and even thread detachment during operation | Retighten with standard torque and add anti-loosening glue immediately |
| Torque fastening + anti-loosening treatment + regular inspection reinforcement | Effectively restrain thread loosening and medium leakage | Standard installation specification for threaded pipeline interfaces |
| Double nut locking + flexible sealing + independent load-bearing support | Minimal sealing failure tendency | Vibration and high temperature working conditions preferred scheme |
Conclusion
Threaded connection failure of 316 stainless steel heating tubes is mainly caused by pre-tightening force attenuation induced by thermal cycle, vibration and stress relaxation, resulting in sealing surface failure and thread meshing loosening. Core prevention strategies include standardized torque assembly, reliable anti-loosening locking configuration, matching high-temperature sealing materials, isolating external bending load and regular on-site inspection and maintenance. Whole-process control of installation process and daily patrol management can effectively eliminate leakage accidents caused by threaded joint failure of heating pipelines.
