Structural disassembly: the bellows is responsible for compensation, and the sleeve is responsible for carrying pressure and diversion. How can the two cooperate?
The name of the sleeve corrugated metal expansion joint is a mouthful to pronounce, but the structure is actually not complicated-a layer of corrugated pipe inside and a set of sleeves outside. The bellows is working, absorbing the displacement caused by thermal expansion and contraction of the pipe, and relying on the flexible deformation of the corrugation. What about the sleeve? One function is to bear internal pressure and reduce the burden of bellows directly carrying pressure; Another function is to divert the flow, so that the medium goes along the inner wall of the sleeve, and don't directly wash the ripples.
Let's just say that bellows are like springs and sleeves are like armor. The medium passes through the sleeve, and the bellows is hidden outside the sleeve, so that it will not be worn by the medium and can be deformed with peace of mind. When the two cooperate, the service life is much longer than that of bare bellows.
Note, however, that there is a gap between the sleeve and the bellows. How big this gap is and how the end of the sleeve is closed directly determines that the expansion energy saving can't adapt to the medium and pressure of your pipe. Some manufacturers have thick sleeves, good carrying pressure but small compensation; Some are made thin, with large displacement but easy to deflate. So don't just look at the shape, you have to ask about the structural parameters.
Before selecting the model, answer three questions: what is the medium, how big is the displacement, and how high is the pressure and temperature
Two days ago, I met a customer, and when he came up, he asked, "Is there a sleeve corrugated metal expansion joint of DN500?" I asked him what medium went in the pipeline, and he said steam. Asked again about the pressure and temperature, he said "about the same". There is no choice. The expansion joint is not a hardware sold according to the caliber. Different working conditions have different material thickness, corrugation wave number and sleeve length.
The medium determines the material. Go steam, 304 stainless steel is enough; To take sulfur-containing flue gas, you have to use 316L or even more corrosion-resistant alloys. Are there any particles in the medium? In some cases, the inner wall of the sleeve has to be wear-resistant, or simply choose the structure with a guide tube.
The amount of displacement determines the number of ripples. The axial compensation amount is calculated by how many millimeters each wave can absorb. The more corrugations, the greater the compensation amount, but the axial stiffness will also become lower, and the risk of pipeline instability will increase. You have to tell the manufacturer about the distance between the fixed brackets at both ends of your pipeline and the temperature change range.
The pressure temperature determines the wall thickness and the number of layers. High pressure, bellows may be double or multi-layered; When the temperature is high, the high-temperature creep characteristics of materials have to be accounted for. Don't put normal temperature parameters on high temperature conditions. It's the same as climbing snow-capped mountains with sandals.
Installation is not just screwed on: pre-tension, guide bracket, cold tightening are indispensable
Many people think that the expansion joint will be completed once it is welded at both ends, but in fact, installation accounts for more than half of the responsibility. Sleeve corrugated metal expansion joints usually have pre-stretched quantities when they leave the factory, or require cold tightening during installation. What's the meaning? It is to let the bellows be pre-elongated or compressed by a certain distance in the installed state, so that when the pipeline runs to a hot state, the bellows can work near the middle position, and will not push it to the end.
The guide bracket should not be saved. The expansion joint can only absorb axial displacement. If the pipeline has lateral deflection, the bellows will be forced laterally, and the life will be greatly reduced. Therefore, a guide bracket must be set near the expansion joint. How close the first bracket is to the expansion joint and how large the distance between the second bracket is, are regulated on the drawings, so dress according to the clothing.
What should I do if I miscalculate the amount of cold tightness? In light case, the compensation amount is insufficient, and in worst case, the bellows is cracked. To calculate this, you need to know the linear expansion coefficient, installation temperature, working temperature and pipe length of pipe material. The formula is not complicated but the parameters should be accurate. If it is really unclear, let the manufacturer directly give the installation instructions according to your working conditions.
Don't wait for a leak before replacing: Where are sleeve wear, bellows fatigue and corrosion failures most likely to occur
The most common failure mode of sleeve corrugated metal expansion joints is wear on the inner wall of the sleeve. When the medium flow rate is high and contains particles, the sleeve is worn out, and the medium directly passes to the root of the bellows. The bellows is washed by particles for a long time and leaks quickly.
Then there is bellows fatigue. The pipeline starts and stops frequently, the temperature changes repeatedly, and the bellows bends back and forth every wave. In design, if the number of cycles is estimated less and the actual working conditions far exceed expectations, fatigue cracks will come out from the trough first. The trough is the stress concentration point, and the crack propagates along the ripple once it appears.
Corrosion failure is mostly on the outer surface of the bellows. In the gap between the sleeve and the bellows, water vapor or corrosive gas accumulates, forming gap corrosion. Therefore, although this structure protects the inner wall of the bellows, the outer wall environment should be paid attention to. When selecting, ask: Does the sleeve have a hydrophobic hole? If not, the condensate will not drain out, and problems will occur sooner or later.
How to match the corresponding products of this site: Application boundary of universal corrugated expansion joint and sleeve pipe expansion joint
In the product line of this station, general corrugated expansion joint and sleeve pipe expansion joint are two kinds of ideas. Universal type without sleeve, suitable for clean media, low pressure and high temperature occasions, low price, large amount of compensation, but the bellows directly contacts the media, and the life is affected by media corrosion and wear.
The sleeve-type pipe expansion joint is the finished form of the protagonist of this paper, which is designed for high-pressure, particle-containing or high-flow pipelines. For example, ash removal pipelines in power plants and kiln tail flue gas pipelines in cement industry have strong media abrasiveness, so sleeve type is much more reliable than general-purpose type. Many of the corrugated expansion joints used in the power station industry and the metal corrugated expansion joints in the cement industry have been targeted and strengthened in the sleeve structure.
How to judge which one to use? If the medium is clean water or steam and the pressure is not high, the general-purpose type is enough. If you have abrasive debris, catalyst powder in your pipeline, or the pressure is over 1.6MPa and the flow rate is high, then honestly choose the sleeve type. Don't save thousands of dollars in price difference. As a result, the expansion joint is changed once every two years, and the downtime loss is much more expensive than the equipment money.
The matter of type selection, in the final analysis, is to change the working conditions for the structure. Make the medium, displacement, pressure and temperature clear, so that the manufacturer can match you with a suitable sleeve corrugated metal expansion joint. If these parameters are vague, then buying them back and installing them is like opening a blind box-whether it can be used or not depends on luck.