Why must you use special metal expansion joints for toxic media pipes?
Two days ago, I met an engineer of a chemical project, and talked about their pipeline transporting chlorine, which used ordinary stainless steel corrugated expansion joints. As a result, the corrugated pipe was pitted and perforated in less than three months. Fortunately, the inspection was timely, otherwise chlorine leakage was no joke. This makes me think that some money really can't be saved, especially when dealing with toxic media.
Metal expansion joints for toxic media pipelines are simply not replaced by ordinary shelf products. You have to understand that it has to face three big mountains: corrosion, leakage, and temperature and pressure fluctuations.
Three Tests of Toxic Medium on Metal Expansion Joints
Let's start with corrosion. Chlorine gas, hydrogen sulfide, ammonia gas, phosgene gas, any of these toxic media are extremely corrosive. In the environment containing chloride ions, stress corrosion cracking of ordinary 304 stainless steel is almost inevitable-only a matter of time. Once the medium seeps out of the crack, it is not a matter of equipment damage, but a safety accident.
Let's talk about leaks. The expansion joint itself is a flexible structure, and the wall thickness of the bellows is usually only 0.5 to 2 mm, which is much thinner than the pipe body. If the seal structure is not up to standard, even a small amount of leakage may accumulate to a fatal concentration under toxic media conditions. And guess what? Many accidents happen precisely at the sealing surface or weld position.
Finally, there are temperature and pressure fluctuations. In chemical and pharmaceutical processes, pipelines often have to go through heating and pressurization and emergency shutdown, and the expansion joint should absorb axial, lateral and even angular displacement at the same time. If the bellows can't resist repeated fatigue, the consequences... I suggest you directly flip through the Safety Technical Specifications for Special Equipment.
Material selection determines life and death: from PTFE lining to high nickel alloy
What about that? In the selection of materials, the corrosiveness of media must be put in the first place.
For strong acid and strong alkali toxic media (such as concentrated sulfuric acid and hydrofluoric acid),PTFE-lined hoseIs a common choice. Tetrafluorine (PTFE) is resistant to almost all chemical corrosion, and with stainless steel braided mesh and bellows, it ensures both flexibility and media isolation. However, it should be noted that the applicable temperature of PTFE lining is usually-20℃ to 180℃, and the pressure should not be too high. Beyond this range, you'll have to consider higher grade materials.
If the medium contains chloride ions and high temperature (such as above 300℃), it is necessary to use high nickel alloy. Hastelloy C-276 and Inconel 625 are excellent in pitting and crevice corrosion resistance in chloride environments. AndHigh temperature axial expansion jointIt adopts multi-layer corrugated structure and heat-resistant alloy, which can carry flue gas or steam above 600℃. For wet corrosive media such as SO₂ and HCl, which are common in desulfurization and denitrification systems,Desulfurization flue gas baffle doorAnd matching metal expansion joints often use duplex stainless steel (such as 2507) or high nickel alloy, because it is both acid and wear resistant.
The media is not negotiable, the materials must match. If you are not sure, you can check this siteMetal hose pressure standardAndModel and size of expansion joint, there will be a material recommendation table in it.
Sealing structure and safety design: guide tube, double seal, leak prevention measures
Choosing the right material is only the first step, and structural design is also about life and death.
Let's startguide tube。 Many expansion joints will be equipped with guide tubes, which have two functions: one is to divert the medium to reduce the turbulent flow to the bellows; The second is to protect the inner wall of the bellows from being directly impacted by high-speed media. For toxic media, the material of the guide tube should usually be the same as the bellows, and the thickness should be sufficient. Otherwise, once the guide tube is corroded through, the medium will directly flush the bellows.
AgainDouble seal design。 If you flip through this siteDouble-sealed single-axis circular baffle doorOrRound Flap Door (Double Seal)It will be found that this type of product is specifically used in situations where zero leakage is required. Double seal structure means that an isolation cavity is formed between two sealing surfaces. Even if there is a small leakage in the first seal, the leaked medium will be collected or guided to a safe area instead of directly leaking out. For pipelines with toxic media, it is strongly recommended to useDouble seal expansion jointOr configure a leak detection interface.
AndSpecial hose for vacuumAndDirect buried (fully buried) type expansion jointSuch special products also have targeted design in sealing-for example, it is necessary to prevent external pressure instability under vacuum conditions and waterproof and corrosion-proof under buried conditions. Unfortunately, many users only look at the compensation amount and ignore the sealing grade when selecting models.
Hard rules for installation and maintenance: Tie rod adjustment, pre-stretching and regular inspection are indispensable
After choosing the right product, the installation and maintenance messed up, and everything went for nothing. The most common rollover scene isexpansion joint tie rodAdjustment andPre-stretchingDidn't get it right.
When installed, the tie rod is used to fix the initial length of the bellows and avoid damage caused by transportation and hoisting. Before official operation, be sure to adjust the tie rod nut according to the instructions-be careful not to remove it, but to loosen it to the designed position. How exactly to adjust? You can see the question and answer of this siteHow to adjust the tie rod nut of expansion joint, with detailed instructions. If the tie rod is not loosened, the expansion joint is equivalent to a rigid connection, which can't compensate at all. The pipeline stress directly acts on the bellows, and it will soon become fatigued.
What about pre-stretching? For pipelines that need to absorb thermal expansion, the expansion joint should be pre-stretched by half of the thermal elongation of the pipeline (or the design value) when installed in cold state, so that the bellows can be in a near free state and have a long fatigue life in hot state. If you omit pre-stretching, the bellows may be compressed to the limit in the hot state and directly deform plastically.
Finally, regular testing is not a formality. For metal expansion joints of pipelines with toxic media, it is recommended to do an appearance inspection every six months (to see whether there are corrosion spots, deformation and leakage marks), and a pressure test or helium leakage inspection every year. Especially where there is a risk of fatigue, such as bellows peaks and troughs. Don't wait for a leak before repairing it. At that time, the price is shutdown and safety penalties.
Industry practical case: How to accurately select types in chemical, pharmaceutical and desulfurization scenarios?
Case 1: Chlor-alkali chemical hydrogen chloride pipeline
Medium: dry hydrogen chloride gas, temperature 150 DEG C, pressure 0.5 MPa. Hydrogen chloride is extremely corrosive in aqueous conditions, so it must be ensured that the medium is dry. SelectedPTFE-lined hose, lined with PTFE sleeve, outer layer of stainless steel woven mesh, with flanges at both ends. Because the vibration source comes from the compressor, it is also addedUniversal corrugated expansion jointAbsorb horizontal displacement. Operated for 2 years with no leaks.
Case 2: Pharmaceutical intermediate desolvation section
Medium: toluene + trace hydrochloric acid, temperature 180℃, intermittent operation, frequent temperature rise and cooling. In the early days, intergranular corrosion occurred when common stainless steel expansion joints were used. Later replaced withHigh temperature axial expansion jointThe material was upgraded to 316L, and solution treatment was done. At the same time, installedexpansion joint guide tube, reducing turbulence. The effect is obvious and the lifespan is doubled.
Case 3: Desulfurization flue of power plant
Medium: saturated wet flue gas after wet desulfurization, containing SO₂, HCl and HF, with a temperature of 50-80℃ and a large amount of condensed acid. Used here isDesulfurization flue gas baffle doorCooperateRectangular non-metallic expansion jointAndNon-metallic expansion joint (fabric fiber expansion joint)。 Note that non-metallic expansion joints have better corrosion resistance than metals in acid environment, and have higher flexibility. However, if it is a high-pressure or high-temperature area, useCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointThe material is duplex stainless steel.
Tsk, at the end of the day, the toxic medium metal expansion joint is not an ordinary compensator, it is a safety line of defense. Selection, installation and maintenance, every ring can't be made do. If you are unsure of anything, go through the product information and questions and answers of this site again, or talk directly to experienced suppliers.