First, let's figure out a question: When do you need shaped metal expansion joints?
Two days ago, I met a customer with a sad face holding the drawing-the diameter of the pipe is standard, but the tee position has a 30-degree bevel angle, and the standard corrugated expansion joint can't fit in. This kind of scene is not rare at all at the engineering site. Standard parts solve 80% of the conventional working conditions, and the remaining 20% are either space deformity, special medium, or wonderful displacement combination. At this time, you have to take the customized route, that is, special-shaped metal expansion joint.
Then when exactly do you have to go on Alien? Several typical signals:
The pipe section is not round? The standard circular expansion joint won't work, so it has to be replacedMetal rectangular expansion jointOrRectangular non-metallic expansion joint。
The pipeline itself is curved, but also absorbs both axial and lateral displacements?Compound hinge transverse expansion jointOrCurved tube pressure balance expansion jointOnly this kind of combination product can be done.
The medium temperature exceeds 600℃, and there is also corrosive smoke?High temperature axial expansion jointAdd the lining, or simply put it onNon-metallic expansion joint (fabric fiber expansion joint)。 These are things you can't find in the standards catalog.
To put it bluntly, the judgment standard is very rude: the standard parts can't be installed, the life span can't run for a maintenance cycle, and the displacement can't be calculated-choose one of the three, and go to the manufacturer to talk about customization.
Power plants, cement, desulfurization-the three most frequent places of special-shaped expansion joints
To say which industry is the hardest hit area of special-shaped metal expansion joints, power plants rank first. Especially in the boiler smoke duct system of thermal power plants, the pipe size is as large as three or four meters wide at every turn, and the shape is irregular. For example, for air-cooled island vacuum pipesDouble hinge expansion joint for air-cooled island vacuum pipeline, not only to bear the vacuum negative pressure, but also to absorb multi-directional displacement, and when designing, you have to consider the anti-scour of the guide tube-you read correctly, it is mentioned in the FAQSpecific Function of Expansion Joint Guide TubeSo as to prevent high-speed flue gas from directly grinding corrugations. Under this working condition, the standard parts simply can't bear it.
The cement industry has more headaches. The high-temperature gas pipe at the outlet of the rotary kiln has large temperature fluctuation and high dust concentration, and the pipe is often of non-standard elliptical cross section. Often matched at this timeMetal Corrugated Expansion Joints in Cement IndustryIf the temperature is higher, simply useNon-metallic expansion joint
(Fabric Fiber Expansion Joint)
Instead. However, the life of non-metallic parts is usually only about two years, and the maintenance cost is not low.
The desulfurization system is another wonderful scene. The limestone slurry in the slurry pipeline has high solid content, low pH value, corrosion and thermal expansion, and the conventional stainless steel bellows leaks in one month. We supply the desulfurization tower for import and exportDesulfurization flue gas baffle doorCooperateRubber PTFE compensatorOnly to pull the life span to more than three years. Do you think this thing is an alien? The size and angle of each interface are different, and all of them have to be mapped on the spot-customized, without negotiation.
From design to factory: Three hard bones that can't be avoided by special-shaped expansion joints
The first hard bone isMechanical calculation。 Standard parts have ready-made stiffness formulas and fatigue life curves (such as those mentioned in the FAQStiffness and Calculation Formula of Bellows), but the remodeling of the profile. Let's sayCompound straight pipe bypass pressure balanced expansion jointThe internal pressure thrust has to be balanced by the bellows itself. If the calculation is slightly poor, the pipeline will collapse. We once helped a chemical project to design a double seal structure. Because of the high internal pressure, we finally choseSingle-axis double-flapper doorThe sealing scheme, only to stabilize.
The second hard bone isManufacturing process。 The bellows of special-shaped parts can often not be formed with standard molds, so the mold must be opened separately, or hydroforming plus manual dressing. LikeLarge diameter thick wall expansion jointThe wall thickness exceeds 10mm, the forming pressure is high, the rebound control is difficult, and the weld must be 100% radiographic inspection. If the material is super duplex stainless steel, it is easy to embrittle if the welding parameters are wrong.
The third hard bone isFactory inspection。 The special-shaped parts cannot comply with the general national standard, and the test scheme must be signed by three parties. Such asSpecial hose for vacuumPump to an absolute pressure below 0.1 mbar and hold the pressure for 24 hours;Rotary compensatorTo simulate rotational displacement 100,000 times. You also usually have to check the direction of the arrow during acceptance —Arrow direction of expansion jointPointing to the direction of media flow, it is useless if the guide tube is installed backwards.
It is not enough to choose the right one. If these details are not handled properly during installation, they will still be wasted
Do you think that everything will be fine when the product is in place? Wrong. I have seen too many cases: the special-shaped expansion joint was transported to the site, and the installer looked at the drawings in confusion and hard-fitted it. As a result, it was scrapped before a maintenance cycle arrived. You better keep an eye on the construction team for the following four details.
First, the position of the guide bracket and the fixed bracket.The special-shaped expansion joint usually absorbs the displacement in multiple directions, and the bracket is not set correctly, which will twist the bellows. Such asStraight pipe pressure balanced expansion jointFixed brackets must be set on both sides, otherwise the pressure thrust will push the pipe away. Asked in the FAQCorrect installation method of expansion joint of large tie rodThe core is that the nut of the tie rod can't be tightened, so the displacement margin should be left.
Second, pre-stretching/pre-compression.Many special-shaped parts have been cold tight some displacement when they leave the factory, and they should be adjusted according to the direction marked by the nameplate during on-site installation. Such asUniversal corrugated expansion jointIt is common to pre-stretch 40%. If you don't pull to the specified length, the bellows is prone to over-compression cracking in hot state.
Third, tie rod nut adjustment.This question is asked a lot in the FAQ- -How to adjust the tie rod nut of expansion joint？ Standard practice: After installation, loosen the nut twice to ensure that the bellows can expand and contract freely. However, if the special-shaped part has angular displacement or lateral displacement, the tie rod may also have to bear the limiting role. At this time, the nut cannot be completely loosened, so it must be locked according to the calculation book. In additionFunction of expansion joint tie rodIt is to limit the excessive stretching of the bellows. Don't dismantle it for transportation protection.
Fourth, pay attention to the direction of the guide tube.Especially for the special-shaped expansion joint in the air duct of the power plant, the guide tube is installed backwards, and the airflow directly washes the bellows, which will be perforated in half a year. There is a special article in the FAQSpecific Function of Expansion Joint Guide TubeThat figure is very intuitive-the inner diameter of the guide tube is slightly smaller than the inner diameter of the pipe, forming an annular gap to guide the medium to the inside and avoid the corrugation from contacting the high-temperature and high-speed airflow.
From design to landing, the essence of special-shaped metal expansion joint is custom engineering, not standard product procurement. It takes two more days to communicate with the manufacturer about the installation details when the drawings are confirmed, which saves three months of construction period than replacing the parts if there is a problem. What do you think?