Metal non-standard expansion joint: how do we design the working condition that standard parts can't handle?
1. Why are standard expansion joints often "insufficient"? — — Real scene of non-standard customization
Two days ago, I met a buddy who made chemical pipelines. When I came up, I spat: "I bought a general-purpose corrugated expansion joint. Within two months of installing it, the corrugated pipe cracked." Parameters: the medium contains chloride ions, the temperature is 350℃, and the pressure is 2.5MPa. Do you say the general purpose type can carry it? Standard expansion joints are designed according to conventional working conditions-temperature range-20℃ ~200℃, pressure within 1.0MPa, no corrosion or weak corrosion in the medium. Once it encounters high temperature (> 400℃), high pressure (> 2.5MPa), strong corrosion (such as wet chlorine and hydrofluoric acid), large displacement (three-way compound displacement), or limited space (straight pipe pressure balance type can't be stuffed into the pipe gallery), the standard parts immediately become "short board".
The desulfurization flue gas pipeline of power plant is not only high temperature, but also a large amount of SO2 and condensate acid. Ordinary stainless steel bellows can't last a maintenance cycle; In the hot air duct of cement industry, the temperature instantly rushes to 800℃, and the metal non-standard expansion joint has to be matched with heat-resistant lining; There is also air-cooled island vacuum pipeline, which needs to bear negative pressure and compensate for lateral displacement, so standard parts can't dry at all. To put it bluntly, standard parts solve 80% of the general problems, and the remaining 20% of "hard bones" have to be gnawed down by non-standard designs.
2. The first step of non-standard design: thoroughly understand the working condition data (pressure, temperature, displacement, medium)
A reliable metal non-standard expansion joint scheme, 60% of the effort is in the early data collection. Many people asked "Give me a non-standard quotation" when they came up, and they didn't even say clearly whether the design pressure was gauge pressure or absolute pressure. Finally, 80% of them would have problems when they were installed.
Not only the working pressure, but also the test pressure and possible water hammer and air hammer peaks. For example, for steam pipelines, the impact pressure of condensate can be doubled when starting up, and the bellows must be checked according to 1.5 times the working pressure when designing.
The permissible stress of metallic materials drops straight with the soaring temperature. The strength of austenitic stainless steel above 550℃ is less than half that of normal temperature. Therefore, in case of high temperature working conditions, it is necessary to use heat-resistant alloys (such as Incoloy 800H), or add insulation layer on the outside of external pressure single axial expansion joint.
Many designers report only the axial elongation and ignore the lateral and angular displacements. The biggest headache is compound displacement-thermal expansion of the pipe and vibration-induced oscillation. This kind of working condition, straight pipe pressure balance type expansion joint is often more suitable than general type, because it comes with its own balance bellows, which can absorb axial displacement without producing blind plate force.
Corrosion media should not only choose corrosion-resistant materials, but also consider the guide tube-the specific function of the guide tube is to protect the inner wall of the bellows from erosion and corrosion, and at the same time reduce the flow resistance. Someone on the Internet asked "Is the guide a must?" I said directly: When the medium contains particles or high-speed fluid, not installing the guide is equivalent to letting the bellows streak naked.
3. Structural selection: From straight pipe pressure balance to compound hinge, which scheme is the most productive?
There is no universal solution to model selection, only the most suitable one. Let's talk about specific products.
For example, the pipeline needs to absorb a large amount of axial displacement and the pressure is high (> 1.6MPa), so it is the first choiceStraight pipe pressure balanced expansion joint。 This structure has two sets of bellows, one working and one balanced, and the internal pressure thrust is digested by itself, without requiring the pipe support to withstand huge thrust. The disadvantage is that it is expensive, long in length, and it can't be stuffed in places with small space.
Space constrained but need to compensate for lateral displacement?Compound hinge transverse expansion jointAndThe curved tube pressure balance expansion joint is ready to go. The former uses two sections of bellows and hinge structure to specially absorb lateral displacement, and the corner is large and does not occupy space; The latter is used on L-shaped pipelines, one in and one out, with its own balance, which is very suitable for the dead angle of the main steam pipeline of power plants.
Another example is the large square air duct (power plant desulfurization, cement kiln tail), which has to be usedMetal rectangular expansion jointOrRectangular non-metallic expansion joint。 The metal rectangle has high pressure resistance, while the non-metal one is more resistant to high temperature and the compensation amount is huge. However, we should pay attention to the replacement cycle of fabric fiber layer. In addition, when you encounter scenes that require isolation maintenance, you can also putElectric plug-in insulation doorOrManual plug-in insulation doorAnd expansion joint arrangement-but that's another topic.
4. Materials and manufacturing: Don't let welds become shortcomings-the key to the life of non-standard parts
Where is the easiest place for non-standard expansion joints to roll over? It's not design calculations, it's welding. You see, the bellows itself is a thin plate (0.5~2mm), which should be welded together with a thick-walled joint (10~20mm). Different materials, different thicknesses, thermal stress concentration, and micro-cracks will appear if you are not careful. There is a case: a high-temperature axial expansion joint used in the cement industry used 316L stainless steel, and as a result, the weld cracked after half a year's operation-because the medium contained sulfur, 316L had poor sulfur corrosion resistance at high temperature, so the problem was solved by changing to Inconel 625 welding wire.
On material selection, general rules: temperature ≤300℃, 304 or 316L for weak corrosion; 300-550 DEG C, 321 or 347 for medium corrosion; Heat-resistant alloy for ≥550℃. Duplex stainless steel or titanium should also be considered as the medium containing chloride ions. But don't be rigid- -likeSpecial hose for vacuumIf you need to resist negative pressure bellows, you have to consider the stiffness, and sometimes you have to increase the wave number or wall thickness.
In the manufacturing process, the forming mode of bellows (hydroforming vs spinning), heat treatment process (eliminating residual stress) and weld inspection (100% PT or RT) are all the keys to life. Many inferior non-standard parts don't even do hydraulic pressure tests before leaving the factory. How long do you think they can last?
5. Installation and maintenance: Is the tie rod removed or not? Does the deflector fit or not? Those pits on site
How to adjust the tie rod nut on the expansion joint? Before answering this question, understand the role of the tie rod-the tie rod is used to protect the bellows from accidental stretching or compression during transportation and installation, and it is also used to limit the amount of displacement during operation. If the tie rod is required to be removed in the design, it must be removed before the pipeline is pressurized or heated, otherwise the tie rod limits the displacement and the bellows will be damaged. Someone asked, "Does the screw of the expansion joint need to be disassembled?" See the drawing mark whether to disassemble it or not: the pull rod with limit function usually loosens the nut but does not disassemble it after operation; If it is a transport tie rod, it must be removed. Bottom line: If you don't understand it, check the instructions for use. Don't screw it blindly.
Does the deflector fit or not? As mentioned earlier, media high speed or containing particles must be loaded. When installing, pay attention to the direction arrow of the guide tube-the direction of the arrow of the expansion joint refers to the direction of medium flow. If the guide tube is installed backwards, it will become a "spoiler tube", which will aggravate the wear.
When the pipe installation temperature is too different from the ambient temperature, the metal non-standard expansion joint needs to be pre-stretched or pre-compressed. For example, if the steam pipe is installed at normal temperature and the operating temperature is 350℃, you have to calculate the thermal elongation, and pre-compress half of the compensation amount of the expansion joint during installation, so that the best stress state can be achieved during operation. The on-site workers didn't understand it, so they installed it directly. As a result, the bellows was pulled out as soon as it heated up-alas, I have seen too many such pits.
In fact, the design of metal non-standard expansion joints is not metaphysics, but hard work. The working conditions are thoroughly understood, the structure is selected correctly, the materials are welded well, and the installation is standardized, so it can basically run for more than ten years. If you also have working conditions that standard parts can't handle, don't hesitate to throw the parameters over, and we will do it one by one.