What exactly is a round tube type metal expansion joint? — — Start with the structure
To put it bluntly, the round tube metal expansion joint is the "shock absorber" plus "expansion joint" in the pipeline system. The core piece is that corrugated pipe like an accordion fold, usually made of 304 or 316L stainless steel. An end pipe is welded at each end to connect the pipe, and sometimes a guide tube is added inside-the function of this thing, refer to the question and answer on our site, is mainly to reduce the erosion of the bellows caused by the medium flow, and by the way, reduce the resistance. Outside, there may also be tie rods, like what we often call "big tie rod expansion joints", which are used to limit radial displacement or bear blind plate forces. The structure is uncomplicated, but each component has its temper.
And guess what? When many engineers get the products, they are confused at first glance: the guide tube is installed backwards, and the tie rod nut is locked and not loosened. As a result, the bellows cracks in less than half a year. Therefore, understanding the structure is the first step to choose the right and use the right.
How does it work? - -Compensation principle and the role of bellows
Thermal expansion and contraction. As soon as the medium in the pipe heats up, the pipe elongates, creating stress. The bellows in the circular metal expansion joint relies on its own elastic deformation to eat this stretch. For example, a 50-meter-long steam pipeline can expand more than 200 mm when it rises from normal temperature to 300°C. The flange will collapse long ago if the pipeline carries it by itself. At this time, the bellows is like a spring, pressing and pulling to resolve the displacement.
The key to the design of corrugated pipe is the wave number, wall thickness and layer number. The more the wave number, the greater the compensation amount, but the stiffness decreases; The thicker the wall thickness, the higher the pressure resistance, but the elasticity deteriorates. We have general-purpose corrugated expansion joints and high-temperature axial expansion joints on our station, which are subdivided for different working conditions. Don't forget, the direction of the guide tube should be consistent with the direction of the medium flow-the arrow is clearly marked, and if you install it backwards, you will dig a hole for yourself.
The three most critical parameters in model selection: pressure, temperature and displacement
This is the three axes of model selection. Don't place an order if you have one less. Pressure: The nominal pressure ranges from PN0.6 to PN2.5, or even higher. For example, the corrugated expansion joint used in the power station industry is often PN4.0. Temperature: The design temperature determines the material of the bellows. Ordinary stainless steel can reach 400°C, and high-temperature axial type can reach 800°C. Displacement: It is divided into axial, transverse and angular directions. The specific value can be calculated according to the pipe layout and fixed bracket.
"The temperature of my pipe is only 150°C, the pressure is 0.8MPa, and the displacement is 100mm. Which one should I choose?" I directly gave him the parameter table of the straight pipe pressure balance expansion joint-because the pressure is not high but the displacement is large, this model is the easiest to use. If you only give a vague "approximation", it is no different from making a blind box. Therefore, only by accurately providing these three parameters can the manufacturer match you with the most economical scheme.
Installation and maintenance pits, how many have you stepped on?
The transport tie rod is not removed. Many users see a screw on the expansion joint, and think it is OK to tighten it. As a result, the bellows is held tightly, and there is no way to compensate at all. Our Q&A on the site specifically talked about "Does the screw of the expansion joint need to be removed?"-the answer is: the transport tie rod must be removed after installation, otherwise it will be installed for nothing. The second pit: improper adjustment of the tie rod nut. It is common to screw too tightly, limiting normal displacement; Or too loose to guide. Refer to the question and answer "How to adjust the tie rod nut of the expansion joint". Remember that the preloading force should be according to the torque value given by the manufacturer.
Maintenance, in fact, there is no metaphysics. Regularly check whether there are cracks and corrosion pits on the surface of the bellows, especially if the chemical industry contacts acidic media, you can install PTFE-lined hoses or PTFE compensators to isolate them. Also, don't change the pipe bracket randomly-some people think it is easy to change the fixed bracket, but as a result, the stress is all pressed on the expansion joint, and the bellows leaks in less than a year. Tsk, lesson.
Common faults and responses: What should I do if the tie rod is loose and the bellows leaks?
The most immediate cause is vibration. The pipeline near the pump outlet, plus the impact of the valve opening and closing, the tie rod nut will slowly back. The solution is simple: check regularly, tighten to a predetermined torque with a wrench, and add a locking washer or loose nut if necessary. If loosening has caused cracks in the bellows, it has to be replaced-don't think about making do with welding, the corrugated surface of the bellows is more likely to crack after welding.
This is the biggest headache malfunction. The leakage point is usually at a trough or weld. The reasons are none other than dynamic fatigue load, corrosion or deviation of installation. How to treat it? Start by shutting down the machine and confirming the leak location. If the leakage point is outside and the medium is non-toxic, you can temporarily plug it with a clamp and a sealing gasket; But the long-term solution must be to replace the expansion joint with a new one. If the medium is corrosive during the selection, it is recommended to directly use non-metallic expansion joints or PTFE-lined ones-we have rectangular non-metallic expansion joints and rubber compensators on our station, which are specially designed to deal with acid-alkali environments. Remember, don't use the general-purpose model to replace high temperature or corrosion conditions, otherwise you are paying overtime to the maintenance department.
The round tube metal expansion joint is selected, and the pipeline compensation is worry-free for ten years; Wrong choice, fix it three times a year. Understand the structure, keep an eye on the pressure and temperature displacement, avoid the installation and maintenance pit, and learn emergency treatment, and you can also become a veteran of model selection. Is that the truth?