Engineers, who hasn't dealt with pipelines? The pipe is a thing that stretches when it is hot, shrinks when it is cold, and vibrates when the pump is turned on. If you forcefully solder it to death at both ends, the interface will leak at least, and the equipment will be damaged at worst. What about that? Install an expansion joint.
What is an expansion joint?To put it bluntly, it is a section of "elastic joint" installed in the middle of the pipe, which is specially used to absorb the thermal expansion and contraction, vibration and installation deviation of the pipe. You look at those steam lines, flue gas pipes, chemical plants, and you can see a guy with a rippled belly at some distance, and that's probably it.
Are expansion joints and compensators the same thing? Let's make this name clear first
"Is the expansion joint the same as the compensator?" The answer is: the same. These are the two names of the same thing. They are both Expansion Joint in English. There are also compensators and Expansion joints in the industry, so no one has to correct anyone. You just have to know that when you see the words "corrugated compensator", "pipe compensator" and "expansion joint", you know what you know.
Expansion energy saving absorbs displacement, but it does not produce displacement. It's not the same thing as a spring damper, and it's not the same thing as a hose. Metal hoses are mainly used to absorb vibration and end displacement, while expansion joints are components on pipelines that absorb multi-dimensional displacement in a system. These two functions overlap, but the application scenarios are very different.
What does an expansion joint rely on to absorb displacement? What is the use of bellows, guide tube and tie rod
Take apart a typical corrugated expansion joint and you'll find just three things at the core.
**Bellows** is the heart. It is a corrugated shell made of thin-walled stainless steel plate by hydraulic or mechanical forming, which absorbs the expansion and contraction of the pipeline by elastic deformation of the corrugation. The more ripples, the greater the displacement that can be absorbed, but the pressure bearing capacity will decrease accordingly. So when you look at those expansion joints used in high-pressure working conditions, the corrugations are relatively shallow and the number of layers is large, just to take into account both strength and flexibility.
The **guide tube** is an inner liner. It is mounted inside the bellows and is in direct contact with the medium. What is the role? The first is to reduce the erosion wear of the bellows when the medium flows, and the second is to reduce the fluid resistance. Think about it, there are potholes inside the bellows, and the medium flow will definitely run around in the past. With the guide tube to straighten the channel, the flow will be much smoother. There is another detail-the installation direction of the guide tube is particular. Generally, it should be installed along the direction of medium flow. If it is installed backwards, it will not only fail to divert the flow, but also may blow the bellows out.
**Puller** is the bodyguard. It is attached to the end plates at both ends of the expansion joint, restricting the bellows from expanding and contracting only in the axial direction, preventing it from twisting and twisting. Like the transverse expansion joint of compound hinge and the pressure balance expansion joint of straight pipe, the tie rod system is more complicated, and the purpose is to make the bellows deform according to the designed direction instead of turbulently.
Metallic, non-metallic, sleeve type... What are the common types of expansion joints and what working conditions are they suitable for
There are many ways to classify expansion joints. According to the material, the three most common categories are metal, non-metal and sleeve type.
Metal corrugated expansion jointIs the absolute workhorse. It is resistant to high temperature and high pressure, and is suitable for steam pipeline, power station smoke air duct and chemical pipeline. There are general corrugated expansion joints, external pressure single axial expansion joints, double hinge transverse expansion joints and straight pipe pressure balance expansion joints. The name sounds around, and the core idea is one: choose different structural forms according to the layout of the fixed bracket of the pipeline and the displacement direction that needs to be absorbed. For example, the metal corrugated expansion joint used in the cement industry has to intentionally strengthen the wear-resistant design, because the cement dust wear is too severe.
Non-metallic expansion jointAlso called fabric fiber expansion joint or rectangular non-metallic expansion joint, the flexible non-metallic material is used as a loop belt. It is not afraid of large displacements, and is especially suitable for rectangular pipes, such as the inlet and outlet of desulfurization tower, the flue near the flue gas baffle door. Non-metallic expansion joints also have an advantage that metal can't compare-good thermal insulation performance and stronger corrosion resistance, which is suitable for flue gas environment containing acid and alkali. Of course, it has limited pressure resistance and cannot be used on high-pressure pipelines.
Sleeve type pipe expansion jointTook another path. It relies on the relative sliding of the inner and outer sleeves to absorb displacement, has simple structure and low cost, and is mainly used for buried pipelines or heating pipelines with low pressure and large diameter. The disadvantage is also obvious-the seal is prone to failure and the packing needs to be maintained regularly.
There is also a category calledRotary compensatorIt absorbs the thermal displacement of the pipeline through the rotation of the rotating cylinder, and can top several ordinary expansion joints on a long-distance steam pipeline, but the installation accuracy requires high requirements, and not everyone can play it.
Don't just look at the name when choosing a model. Also called "expansion joint", the design logic of corrugated expansion joint and double hinge expansion joint of air-cooled island vacuum pipeline used in power station industry is completely different from the same channel. The former has to withstand the smoke erosion of hundreds of degrees Celsius, while the latter has to be kept sealed in a vacuum environment. The material, wavenumber and wall thickness are all calculated separately.
When choosing expansion joint, only look at the caliber: pressure, temperature and displacement are the key parameters
Two days ago, I met a customer who was doing HVAC engineering. When I came up, I asked, "How much is the expansion joint of DN500?" I said, don't ask for a price in a hurry. What pressure, temperature and displacement does your pipeline want to absorb? These numbers can't be determined, and the quotation is meaningless.
When choosing the expansion joint, the diameter is only the basic parameter, and the real core is the following three:
- Stress.The pressure capacity of bellows is determined by wall thickness, number of layers and wave height. The expansion joint of PN16 is used on the pipeline of PN25, and the bellows bulges for you in minutes.
- Temperature.When the temperature is high, the allowable stress of stainless steel plummets. 800℃ flue and 200℃ steam pipe, the materials of corrugated pipes are completely different. Heat-resistant alloy steel should be used in high temperature conditions, and even heat insulation layer should be added.
- The amount of displacement.Axial displacement, transverse displacement and angular displacement should be calculated separately. Axial expansion joint can absorb axial expansion and contraction, and you have to let it eat lateral deviation, and it will be fatigued and cracked within two months.
Fatigue life。 Expansion joints are flexible pieces that consume life every time they expand or contract. The design life is usually calculated according to 1000 to 5000 cycles. If you start and stop the boiler frequently, the fatigue life of the bellows has to be longer, so don't save this money.
Poor installation of expansion joints is equal to buying for nothing: the easiest pit to step on in installation and maintenance
Expansion joint, three points depends on type selection, seven points depends on installation. If you don't pay attention to it, no matter how expensive the expansion joint is, it will be useless.
The first pit isThe tie rod nut is not adjusted properly。 The nut on the tie rod when the expansion joint leaves the factory is locked, that is in shipping condition. When installing, loosen the positioning nut so that the bellows can expand and contract freely. Some people try to save trouble, and they weld the pipe directly without even loosening the nut. The expansion joint becomes a decoration, and the pipe should crack. On the other hand, if it is a pressure-balanced expansion joint, the adjustment of the tie rod nut is particularly specific, so it can't be completely loosened at once, so the bellows on both sides must be stressed evenly.
The second pit isInstall the guide tube in reverse direction。 As mentioned earlier, the guide tube is directional. Install it backwards, and the medium flushes the bellows directly, which can wear out in a few months.
The third pit isForget the pre-deformation。 When the ambient temperature at the time of installation is far from the operating temperature of the pipe, the expansion joint should be pre-stretched or pre-compressed. For example, if the steam pipeline installed in winter runs at more than 100 degrees, the expansion joint has to stretch half of the compensation amount in advance, otherwise the displacement will be exceeded after operation.
The fourth pit isSave the bracket。 The expansion joint is not a universal glue, it can only absorb the thermal displacement of the pipeline itself, and the fixed bracket and guide bracket of the pipeline should be set. If the bracket is not set correctly, the expansion joint will be forced to bear the force it should not bear, and its life will be greatly reduced.
One more word on maintenance. If the protective sleeve on the outside of the metal bellows is broken and rainwater seeps in, the bellows will easily rust and pitting. The ring belt of non-metallic expansion joint is even more delicate. If the cotton head is broken, it should be repaired in time, otherwise it will tear a big hole when the wind blows. It is much more cost-effective to take a second look during regular inspections than to stop production for emergency repair afterwards.
With all that said, back to the question at the beginning:What is an expansion joint?It is the elastic joint in the pipeline system that silently carries all the thermal expansion and contraction. Understand its principle, type and selection logic, and when you look at the pipeline drawings, you will have a bottom in your heart. Next time someone comes to ask for a price with a caliber, you can also ask him the other way around, "What is your operating pressure?"