1. What exactly is PTFE expansion joint? — — Structural disassembly and working principle
To put it bluntly, PTFE expansion joints are the products of forcibly "matching" metal bellows and polytetrafluoroethylene (PTFE) lining together. Metal bellows are responsible for carrying pressure and absorbing displacement; The PTFE liner is responsible for blocking corrosive media. Once the two are combined, they become "special forces" in strongly corroded pipelines.
The outermost is a metal bellows-usually 304 or 316L stainless steel, and the elasticity of the crests and valleys allows it to expand and contract axially or swing laterally. The middle is a PTFE lining layer, which is generally 2-3mm thick, and is tightly attached to the inner wall of the bellows by hot rolling or molding process. What about the innermost? Some will add a guide tube (that is, an expansion joint guide tube) to prevent high-speed media from directly washing the root of the bellows.
When the pipe expands, contracts or vibrates, the bellows deforms to absorb displacement-axial compression, stretching, or angular deflection. The PTFE layer follows the deformation, but because of its good flexibility and chemical inertness, it will not crack or leak. And guess what? In some working conditions, the temperature reaches 200℃ and the pressure is 1.6MPa, and this thing can still be dry.
Second, where does it win compared with ordinary metal expansion joints? — — Core advantage of PTFE lining
Ordinary metal expansion joints (such as general-purpose corrugated expansion joints) are a "giving away" when they meet strong acids and alkali. 316L stainless steel can't last three days in concentrated hydrochloric acid, and the bellows is directly perforated. However, the PTFE expansion joint is lined with PTFE, which completely separates the medium from the metal-PTFE is known as the "King of Plastics", and it is scared of almost anything except molten alkali metal and fluorine. Concentrated sulfuric acid, hydrofluoric acid, aqua regia? Come anyway. This is the first advantage: the corrosion resistance grade pulls directly to the full.
The second advantage is anti-adhesion. PTFE has extremely low surface energy, and like Teflon nonstick pans, viscous materials and crystallization media are difficult to hang on the wall. In the polymerization reaction pipeline, the ordinary expansion joint will be blocked in half a year, while the one lined with PTFE can carry it for two or three years.
The third advantage is temperature adaptability. PTFE can be used for a long time at-196℃ to +260℃, and the instantaneous temperature resistance can be higher. With the pressure bearing capacity of metal bellows, it is still reliable under temperature-pressure coupling conditions. Of course, the pressure of pure PTFE lining can't be too high-that's why the selection requires card parameters, which will be detailed later.
In this way, ordinary metal expansion joints are "streaking" before corroding the medium, and lining with PTFE is equivalent to wearing chemical protection clothing. Is that the truth?
Third, what occasions must it be? — — Typical application scenarios such as chemical industry, medicine and desulfurization
Just tell me a few real cases and you will understand.
- Chemical industry: Brine pipeline, hydrochloric acid delivery pipeline and sulfuric acid dilution system in chlor-alkali plant. The medium temperature is 80-120℃, the pressure is 0.6-1.0MPa, and the common expansion joint leaks in three months. With PTFE expansion joints, the design life starts at five years.
- Pharmaceutical industry: The inlet and discharge pipelines of the reactor are often passed through nitric acid, acetic acid and organic solvents. Moreover, the pharmaceutical industry requires the inner wall of the pipeline to be smooth and without dead spots-the PTFE lining just meets FDA and GMP specifications.
- Desulfurization system: Slurry pipeline for wet desulfurization (FGD) in thermal power plant, containing gypsum slurry and limestone slurry, plus trace fluoride ions and chloride ions. Here, desulfurization flue gas baffle door is usually used with PTFE expansion joint, which is wear-resistant and corrosion-resistant.
- Fine Chemicals and Semiconductors: Delivery of high purity chemicals, such as electronic grade hydrofluoric acid and hydrogen peroxide. The pipe requires zero metal ion evolution – the PTFE lining meets perfectly, while the metal bellows is protected on the outside with carbon steel or stainless steel.
As long as the medium running in the pipeline is "toxic, corrosive and has a tendency to crystallize", PTFE expansion joints are just needed.
Fourth, don't step on pits when selecting: how to match pressure, temperature, medium and displacement?
Selection is not a slap on the forehead, it has to be adjusted by four parameters. Let's talk about it one by one.
Temperature: PTFE lining can be used to 260℃, but once it exceeds 200℃, its mechanical strength will drop drastically. When the pressure is high, the temperature must be lower. For example, 1.6MPa and 200℃ are the limits, and 0.6MPa can dry to 250℃. Check the temperature-pressure curve provided by the manufacturer for this.
pressure: The maximum design pressure of PTFE expansion joint generally does not exceed 2.5MPa. Because the PTFE layer is soft, it will be squeezed out if the pressure is too high (the term is called "cold flow"). The solution is to increase the number of layers of metal bellows, or add wire mesh reinforcement to the inner lining-although wire mesh will sacrifice some corrosion resistance and have media requirements.
Medium: In addition to the corrosiveness, it also depends on whether the medium contains particles. If it contains hard particles (such as desulfurization slurry), the deflector must be installed! Moreover, the material of the guide tube should be cemented carbide or ceramic coating, otherwise it will wear out in a few months.
displacement amount: The displacement that a metal bellows can absorb is limited. For example, the axial compression amount is usually only 10%-20% of the bellows length, and the lateral displacement is smaller. If the thermal displacement of the system is large, it is necessary to use the double hinge transverse expansion joint or the straight tube pressure balance expansion joint to distribute the displacement to multiple bellows. The PTFE lining follows the bending, and the radius of curvature cannot be too small, otherwise the PTFE will crack by fatigue.
So, don't just look at the numbers on the sample. When communicating with the manufacturer, explain the working conditions: media composition, temperature fluctuation range, pressure pulsation, pipeline direction and support spacing. If one of these data is wrong, the one selected is scrap metal.
V. Installation and maintenance: Can the tie rod be removed or not? How to use the deflector tube? — — Practical Pit Avoidance Guide
Many customers suffered this loss-after the expansion joint was installed, the tie rod nut was not adjusted, or it was simply not dismantled. As a result, the bellows couldn't move at all, which compensated for loneliness. Gotta be clear here.
Tie rod (also called screw): For the convenience of transportation and installation when leaving the factory, the pull rod is usually tightened to fix the expansion joint into a "zero displacement" state. Once installed in place, the nut on the tie rod must be loosened according to the design displacement-note that it is loosened, not removed! The tie rod itself should be retained to prevent over-stretching of the bellows. How to adjust? Refer to the adjustment guide of expansion joint tie rod nut: first calculate the axial displacement direction, retreat the nut in the corresponding direction to the limit position, and leave a compensation gap on the other side. Symmetric on both sides when adjusting, don't twist off.
guide tube: If there is a high-speed airflow or slurry in the pipe, the guide tube must be installed on the upstream side of the medium flow direction. When the direction is reversed, the deflector becomes a "baffle" and damage is faster. In addition, there should be enough gap between the guide tube and the bellows-generally 3-5mm to prevent it from getting stuck after thermal expansion.
Other Details: Do not let the expansion joint bear torque during installation, the pipe flange must be coaxial. During the pressure test, the water pressure should rise gradually. Don't pressure the lining violently. Daily inspection, take a flashlight to shine on the outer wall of the bellows to see if there are any bulges and cracks-there will be white hair lines on the surface of PTFE when it is cracked, and early detection can avoid major leakage.
Alas, having said so much, there is actually one core: PTFE expansion joint is the "last line of defense" for strongly corroded pipelines. Choosing the right, installing and managing it well can save a lot of maintenance costs. On the contrary, it saves the effort of model selection, and what awaits you behind is the discontinuation and accident.