What exactly are PTFE metal expansion joints? — — Tin wrapped in plastic hybrids
To put it bluntly, PTFE metal expansion joints are relatives of the products in our station-they are essentially the same family as "PTFE-lined hose" and "PTFE compensator". Except the shape has changed from hose to bellows structure. Simply put: lining the inner or outer wall of stainless steel bellows with a layer of polytetrafluoroethylene (PTFE) not only retains the pressure-bearing and displacement compensation ability of the metal expansion joint, but also allows PTFE to carry corrosion. To put it bluntly, he is a hybrid of "tin wrapped in plastic". But this combination just solves the most headache problem of chemical pipelines-media corrosion.
Why do ordinary metal expansion joints always drop the chain in front of acid, alkali and salt?
Two days ago, I met a buddy from a chemical factory. They used to use a general-purpose corrugated expansion joint, but the corrugated root cracked in less than half a year. In fact, stainless steel is afraid of chloride ions and strong acids. Once the passivation film is destroyed, pitting and stress corrosion cracking will all come. PTFE is sandwiched in the middle as an isolation layer, and the medium can't contact the metal body at all, so the corrosion problem is easily solved. And guess what? He replaced it with PTFE, and he hasn't had an accident in three years. Is that the truth?
Which scenarios are the "home" of PTFE metal expansion joints?
Acid pipelines in chemical plants, slurry pipelines in desulfurization systems, wet chlorine gas transportation, bleaching sections in paper mills... It is the first choice for strongly corrosive and the temperature does not exceed 200℃ (PTFE long-term service temperature range). The "desulfurization flue gas baffle door" in our station is equipped with this kind of expansion joint, and sulfide corrosion resistance is the basic skill. However, note that PTFE can't bear the working conditions of high temperature, high pressure and strong corrosion, such as concentrated sulfuric acid above 300℃, so you have to find another way, such as high-temperature axial expansion joint or non-metallic expansion joint.
The three easiest pits to step on when selecting a model
The first pit: only look at nominal pressure and not at temperature reduction.The strength of PTFE at 150℃ is only half that of normal temperature, and the selection should be compared with the temperature-pressure curve. Don't think that the nominal 1.6MPa can be used casually, and it will be wasted as soon as the temperature comes up.
The second pit: ignore the deflector.If there is no guide tube in the expansion joint lined with PTFE, the high-speed medium will scour the PTFE layer, and the life of PTFE will plummet. The specific function of the expansion joint guide we talked about in the question and answer-it is to protect the bellows and lining from direct impact.
The third pit: the installation direction is reversed.The arrows on the bellows must point to the direction of the media flow, otherwise the PTFE lining is easily torn by high pressure fluid. The direction of the arrow of the expansion joint has been specifically mentioned in Q&A, and the reason is the same.
Hardcore knowledge points for installation and maintenance
Large tie rod expansion joint with tie rod nut, pre-compression or pre-stretching amount must be adjusted according to the data given by the manufacturer, don't screw to death when you come up. How to adjust the expansion joint tie rod nut That question and answer said: Find the cold tightness value, twist it symmetrically with a wrench, and don't work hard on the one hand. There are three points in daily inspection: whether there is bulging or cracking on the surface of PTFE, whether the flange sealing surface leaks or not, and whether there is abnormal deformation of the corrugation. Generally, it is enough to remove and inspect the chemical plant once during overhaul. Also, does the screw of the expansion joint need to be disassembled? It depends on the working condition-if it is a transportation fixing screw, it must be removed after installation; If it is a working tie rod, it will not be removed.
Double the price but is it worth it? To settle accounts
There are 2,000 ordinary stainless steel expansion joints, which have been wasted for one year, and it cost 10,000 yuan to replace the labor of the shutdown loss belt; The PTFE metal expansion joint is 4,500 pieces, which is as stable as an old dog after five years. Which is more cost effective? What chemical enterprises are most afraid of is unplanned parking, and the extra 2,500 yuan can buy four years of peace of mind. Besides, now the environmental protection check is tight, and leakage is an accident. I don't need to say more about this account, right?
In the final analysis, PTFE metal expansion joint is not a black technology, but it can solve the actual pain points. When selecting the model, focus on the three details of temperature reduction, guide tube and installation direction, and the money is worth it.