Axial tensile separation failure means the PFA lining bears sustained axial pulling force along the pipeline length direction. The bonding force between the lining and steel base pipe is completely overcome, the integral lining is dragged and dislocated axially, detaching from the inner wall of the substrate pipe integrally or sectionally. The flanged flanging part is torn off the flange face, a large gap is formed between lining and pipe body, corrosive medium massively invades the interlayer space, resulting in pipeline blockage and serious substrate corrosion leakage. This failure mostly happens during pipeline thermal displacement, pump startup impact and forced pipeline stretching, belonging to overall interface debonding caused by axial mechanical tension.
1. Tensile Disbonding Evolution Mechanism
The PFA lining is fixed inside the steel pipe mainly relying on sintering adhesion and flange end limit restraint. When axial tension acts on the lining, the shear stress is generated on the bonding interface between fluoroplastic and metal. Once the tensile load exceeds the interfacial bonding strength formed by sintering, the lining starts to slip along the axial direction. Thermal expansion of the lining at high temperature accumulates axial redundant length; when the pipeline cools down and contracts, the lining cannot retract synchronously with the steel pipe, and residual tensile stress accumulates continuously. Under sudden fluid impact during pump startup, the local lining is pulled sharply, the flanging sealing edge is stripped from the flange, and the whole lining loses positioning constraint and is pulled away from the base pipe. Different from local bubbling and partial peeling defects, this failure is overall axial slippage and complete separation of the lining system.
2. Typical Positions Prone to Axial Pull-Off Damage
Long straight single-section pipelines without segmented limit structures, prone to cumulative thermal stretching;
Pipeline sections connected directly to pump outlets with instantaneous fluid impact pulling force;
Flange connection ends with incomplete flanging pressing and weak end limiting effect;
Pipe sections subjected to external pulling force during pipeline displacement and maintenance disassembly;
Lined pipelines with large temperature difference between startup and shutdown, producing obvious alternating axial deformation.
3. Core Inducing Factors Accelerating Pull-Off Failure
No anti-pull-off positioning rings or limit steps set inside the base pipe during lining processing;
Flange flanging is not firmly crimped, lacking effective axial locking for the lining end;
Excessive instantaneous flow impulse when the pump is started without slow boosting operation;
Pipeline overall forced stretching during installation and foundation settlement, transmitting tension to the lining;
Excessive thermal expansion allowance reserved unreasonably, leading to lining accumulation and tension concentration.
4. Full-Link Prevention and Control Technical Measures
① Weld built-in limit retaining rings inside the steel pipe before lining sintering
Mechanically lock the lining to prevent axial sliding fundamentally.
② Adopt double-sided crimped flanging structure at both flange ends
Strengthen end restraint to avoid the lining being pulled out from the flange port.
③ Implement soft start and slow pressure rise procedures for delivery pumps
Eliminate instantaneous water hammer impact and axial pulling load.
④ Optimize pipeline support and foundation design to prevent pipe body stretching deformation caused by settlement
Avoid external tension being transferred to the inner lining.
⑤ Set segmented butt limit structures for ultra-long lined pipelines
Divide the whole pipe into multiple constraint units to prevent overall displacement.
5. Prevention Effect Comparison Table
表格
| Design & Operation Mode | Axial Pull-Off Risk | Application Suggestion |
|---|---|---|
| No internal limit + simple single-sided flanging + direct pump startup | Lining integral pull-off and flange end tearing in short operation period | Replace pipe fitting with built-in limit lining structure and re-install |
| Internal anti-slip limit + double crimped flange + graded startup | Effectively block axial slipping and overall separation of lining | Standard manufacturing design for long-distance PFA lined pressure pipelines |
| Multi-section positioning constraints + full-end locking + pipeline displacement monitoring | Extremely low lining axial detachment hidden danger | Preferred scheme for long-distance process heating transmission pipelines |
Conclusion
PFA lining axial tensile pull-off separation failure arises from axial tension exceeding the interface bonding strength and end limiting force, which makes the lining slip and separate from the metal base pipe along the axial direction. The core prevention methods are adding built-in mechanical limit structures inside the pipe, reinforcing flange end locking restraint, eliminating fluid impact tension through standardized startup, and avoiding external stretching of the pipeline body. Whole-process control of pipe prefabrication structure design, lining forming technology and field operation specifications can prevent large-area lining shedding and interlayer corrosion leakage accidents of PFA lined heating tubes.
