Disassembly from the name: thick wall, metal, expansion joint, what exactly do these three words refer to?
The expansion joint, also called the compensator, is, to put it bluntly, the "cushion" in the pipeline system-it absorbs the displacement caused by thermal expansion and contraction, and by the way, digests the mechanical vibration. Think about it, a steam pipe hundreds of meters long, whose temperature rises to hundreds of degrees Celsius, if you keep it from moving, the stress on the wall of the pipe can be so great that the flange can be cracked. At this time, you have to rely on the expansion joint to "give way".
The word "metal" is easy to understand. The most common corrugated pipe materials are stainless steel 304 and 316L. If you encounter high temperature and strong corrosion conditions, you can get high-temperature alloys such as Incoloy 825 or Hastelloy. If the material is not selected correctly, no matter how thick the wall is, it will be for nothing.
Here comes the key – what exactly does "thick wall" mean? Many people think that the shell of the whole expansion joint is thick, but it is not. Specifically, the wall thickness of the bellows is thicker than that of the conventional model. The wall thickness of conventional bellows is generally 0.5~1.5mm, while the thick-walled type can be 2~4mm or even thicker. For example, this site'sLarge diameter thick wall expansion joint, is the fist product specially dealing with high-pressure and large displacement scenes.
What is the essential difference between thick-walled metal expansion joints and ordinary corrugated expansion joints?
Ordinary corrugated expansion joints rely on thin-walled corrugated deformation to absorb displacement, which is well used in low-pressure and small-diameter pipelines, and the price is cheap. However, once it encounters the working conditions of high pressure, high temperature and high cycle times, the thin-walled parts are "crispy skin"-the fatigue life is short, and it will crack for you in a few months.
High withstand voltageAndLong fatigue resistance life。 Take the main steam pipeline of the power plant as an example. The pressure is above 10MPa and the temperature exceeds 500℃. Ordinary thin-walled bellows can't hold it, but thick-walled bellows can hold it for several years.
But thick walls also come at a cost — stiffness. With the same displacement, the thick-walled bellows generates more thrust, so it needs to be matched with a tie rod or hinge structure to share the thrust. This site'sCompound hinge transverse expansion jointIt is typical of this combination, with hinges to absorb lateral displacement while withstanding internal pressure thrust.
Where exactly is it used? Industry cases to help you understand
Selection is not a pat on the head, it depends on the actual working conditions. What scenarios are thick-walled metal expansion joints used in? I'll list you a few typical ones.
- Power industry:Power station boiler outlet, turbine bypass pipeline, high temperature and high pressure are common. Recommend this siteCorrugated expansion joint for power station industryAndHigh temperature axial expansion joint, specially optimized for this working condition.
- Cement industry:The exhaust gas pipeline at the end of the kiln is high in temperature and contains much dust, and the bellows is directly exposed to the dust-containing air flow, which causes serious wear. The thick-wall matching guide tube is standard, and the station'sMetal Corrugated Expansion Joints in Cement IndustryWith this design, the guide tube guides away the high-temperature dust-containing air flow to protect the bellows body.
- Desulfurization system:Upstream and downstream of the flue gas baffle door, the medium is extremely corrosive (containing sulfuric acid and sulfurous acid). Thick-walled bellows can be used with PTFE-lined or metal hoses to withstand corrosion. This site'sDesulfurization flue gas baffle doorAnd PTFE-lined hoses are common combinations.
- Vacuum pipe:For example, in the air-cooled island system, there is negative pressure in the pipeline, and the thin-walled bellows is easily deformed by suction. The thick-walled corrugated pipe has strong external pressure resistance.Double hinge expansion joint for air-cooled island vacuum pipelineIs designed for this kind of scenario.
The 3 Most Common Mistakes to Make in Model Selection
I have been in this business for more than ten years, and I have seen too many people step on pits. Say three of the most common ones.
Mistake 1: Only look at the nominal pressure and ignore the reduction of the allowable stress of the material by temperature.Many people choose models with PN, completely regardless of the temperature. In fact, a piece of 304 stainless steel can be directly discounted by 40% with stress at 400℃. The thick-walled parts you select according to normal temperature may exceed the limit when it reaches high temperature conditions. This can't be sloppy.
Mistake 2: Thinking that thick walls can absorb greater displacement.Quite the opposite! When the wall thickness increases, the single-wave compensation amount decreases. You want to absorb the same displacement by either increasing the wavenumber or switching to a duplex structure. Otherwise, it would be embarrassing to install it and find that the displacement is not enough.
Error 3: Ignore the deflector.Thick-walled expansion joints are commonly used in high-speed fluid pipelines. There is no guide tube, and the bellows are directly subjected to fluid scour-especially for particle-containing media, and the life is cliff-like. About the function of the guide tube, this site has a special articleSpecific Function of Expansion Joint Guide TubeMake it very clear. Make sure to provide media flow rate and particle content data when selecting, otherwise you will dig a hole for yourself.
Installation and maintenance: How are they different from ordinary expansion joints?
Thick-walled parts are of great weight, so special lifting lugs must be used for hoisting during installation, and bellows must be hooked directly? Don't even think about it, once the bellows is deformed, it will be crippled.
The amount of pre-tension/pre-compression needs to be accurately calculated. The cold tightness of ordinary expansion joints is about the same, but the thick-walled parts have great rigidity, so the cold tightness is wrong, and the thrust can pull the bracket crooked after installation. It is recommended to directly find the engineering and technical staff of this site to help you calculate.
For maintenance, check the bellows surface regularly for cracks or corrosion pits. Weld repair is difficult once leakage occurs in thick-walled parts – heat-affected zones can change material properties, and direct replacement is usually recommended. If the design is reasonable, the life of thick-walled metal expansion joint can reach 10~15 years under medium working conditions, which is more than twice that of thin-walled metal expansion joint. Whether it is cost-effective or not, settle your own accounts.
To summarize: Should you spend this money?
If the pipe pressure exceeds 2.5MPa, or the temperature exceeds 400°C, or the medium is corrosive and the flow rate is high — any of these three conditions holds, thick walls are a must. Don't be cheap and buy the universal corrugated expansion joint to make do. It's okay if there is no accident, but if there is an accident, the pipeline will crack and production will be stopped for overhaul. The loss will be enough for you to buy dozens of thick-walled expansion joints.
In this matter of type selection, the first is to understand the principle, and the second is to know the working conditions. Figure out "What does a thick-walled metal expansion joint mean?", and you'll be able to avoid 80% of the pits.