What is Shaped Metal Expansion Joint? Why can't conventional products handle it?
Special-shaped metal expansion joints, to put it bluntly, are those corrugated compensators that can't be applied to the standard model. You flip through the manual,Universal corrugated expansion joint、Axial expansion jointThese conventional products, the size, wave number and flange are fixed, and are only suitable for straight pipe sections and regular pipeline systems. However, when you encounter corners with limited space, misaligned equipment interfaces, and need to absorb lateral and angular displacements at the same time, the standard parts are blinded.
In the power plantAir-cooled island vacuum pipeUsefulDouble hinge expansion jointIt needs to absorb the three-dimensional displacement of the pipeline in an extremely narrow gap, and the direction of the bellows and the angle of the tie rod have to be redesigned. Another exampleCement industryThe dust removal pipe, the section down from the cyclone separator, the interface is square, and the back is connected with a round pipe. At this timeMetal rectangular expansion jointOr the non-standard part of the square-circle transition is the positive solution. Regular products? It doesn't fit at all.
Therefore, the essence of special-shaped metal expansion joints is "customized for specific working conditions". There are no ready-made drawings, and each one has to be calculated from scratch.
Difficulties in the design of special-shaped structures: How to balance space constraints and displacement compensation
What are the biggest challenges in designing shaped metal expansion joints? Space and displacement are always fighting.
The waste heat recovery pipeline of an iron and steel plant is installed close to the wall, and the length of the expansion joint can only reach 200mm, but it needs to absorb the axial displacement of 30mm and the transverse displacement of ±15mm. According to the conventional bellows stiffness calculation (refer to theStiffness and Calculation Formula of Bellows), in order to achieve sufficient compensation for such a short bellows, the wave number must be increased, but as the wave number exceeds, the overall stiffness decreases, and the internal pressure thrust will deform the top of the pipe. What to do? Only adjust the wall thickness and the number of layers of the bellows, and install them at the same timetie rodTo withstand internal pressure thrust. The length of the tie rod, the preload force of the nut (corresponding toHow to adjust the tie rod nut of expansion jointThat problem) all have to be matched precisely, otherwise either the tie rod is unevenly stressed or the displacement is limited.
guide tube。 The internal medium flow rate of the special-shaped expansion joint is high and contains particles (such as dust in cement industry). The guide tube must be designed at an angle according to the actual flow direction and maintain sufficient clearance with the inner wall of the bellows. If the gap is too small, the guide tube will hit the bellows after thermal expansion; If the gap is too large, the diversion effect will be discounted. Many problems on the spot lie in the design of the deflector.
From power stations to cement industries: What operating conditions must be fitted with special-shaped metal expansion joints?
To be honest, as long as the pipeline layout doesn't follow the standard drawings, you have to consider non-standard schemes. The following industries are frequent visitors of special-shaped metal expansion joints:
- Power station industry: Main steam pipeline, reheating hot section pipeline, high temperature (above 500℃), high pressure, often need to be arranged in a narrow spaceHigh temperature axial expansion jointOrCurved tube pressure balance expansion joint。 Especially from the boiler outlet to the steam turbine, the thermal displacement is very complicated, which often requires the cooperation of hinges and tie rods in multiple directions.
- Cement industry: Preheater, decomposition furnace, kiln head and kiln tail pipeline, medium temperature is high, ash content is large. in common useMetal Corrugated Expansion Joints in Cement IndustryAndRectangular non-metallic expansion jointFor example, at the square-circle interface, the metal rectangular expansion joint absorbs axial displacement and the non-metal segment absorbs lateral displacement.
- Desulfurization and denitrification system: Flue gas baffle door front and rear pipes, because the baffle has to be switched frequently (Electric plug-in insulation door、Double-sealed single-axis circular baffle door), the thermal expansion and cold contraction of the pipeline are violent, and the special-shaped expansion joint must leave sufficient compensation margin, otherwise the baffle door will jam.
- Chemical Pharmaceuticals: Applications where corrosion resistance is required, such asPTFE-lined hoseOrPTFE compensator, pipe sizes are often non-standard and can only be customized.
The pipeline direction itself is non-standard, such asCompound hinge transverse expansion jointUsed in U-bend sections, orStraight pipe pressure balanced expansion joint
Used in situations where fixing brackets cannot be set. None of these can be solved by just buying a general-purpose part.
The easiest pits to step on in model selection: stiffness, matching of guide tube and tie rod
When choosing a special-shaped metal expansion joint, many people ask "how much pressure to bear" and "how much compensation". These two parameters are of course important, but what is really prone to problems is the matching relationship between stiffness, guide tube and tie rod.
Let's start with stiffness.Some purchasers are greedy for cheap, so they choose a bellows with small stiffness. As a result, after installation, the internal pressure thrust cracked the weld of the pipe support. Or conversely, if the stiffness is too large, the expansion joint cannot effectively absorb the displacement, and the bellows is prematurely fatigued. How to calculate stiffness? We have a special stationStiffness and Calculation Formula of BellowsYou have to calculate according to the actual working pressure, temperature and allowable stress of materials, and you can't pat your head.
guide tubeThe pit is more secluded. Many designers do not know the specific function of the guide tube (you can see our FAQSpecific Function of Expansion Joint Guide Tube), thinking that it is just blocking the medium. In fact, the guide tube reduces the scouring of the bellows by the medium, reduces the flow resistance, and also prevents particle deposition. However, if it is a special-shaped expansion joint, the shape of the guide tube has to follow the shape of the pipe. For example, for a rectangular to circular special-shaped part, the guide tube should be made into the shape of a round sky, and the thickness should be considered for wear. I have seen a cement plant project where the deflector steel plate was too thin and worn out for half a year, and the bellows was directly exposed to the dusty airflow and leaked.
tie rodThe problem is more common. The function of the tie rod is to bear the internal pressure thrust and protect the bellows from being over-stretched (corresponding toFunction of expansion joint tie rod）。 However, due to the structural asymmetry of the special-shaped expansion joint, the arrangement of the tie rod cannot be symmetrical according to the center, otherwise a torsional moment will be added to the pipeline. The number and angle of tie rods should be reasonably arranged according to the pipeline support conditions and displacement direction. After installation, the adjustment of the tie rod nut is also critical, refer toCorrect installation method of expansion joint of large tie rod-The expansion joint will not work properly without loosening the locking nut during transportation.
How to accept non-standard products? Critical dimensions and fatigue testing cannot be omitted
There is no national standard for special-shaped metal expansion joints that can be directly applied (JB/T 12235-2015 for non-metal expansion joints, but mainly refer to GB/T 12777 and design drawings for metal non-standard parts). Therefore, the acceptance must be according to the drawings and the agreement between the two parties. Several key points:
- Critical dimensions: Flange bolt hole pitch, bellows length, wave pitch and tie rod length, all of which must be checked on site with vernier caliper or tape measure. Especially the installation length-special-shaped parts often have pre-stretching or pre-compression requirements, and the actual length deviation exceeds ±2mm and may not be installed.
- Fatigue testing: If it is high pressure or high frequency vibration condition (such as air-cooled island pipeline), the manufacturer should provide fatigue life calculation report or sampling test report. On our stationDouble hinge expansion joint for air-cooled island vacuum pipelineThis kind of product will do a simulated fatigue test before leaving the factory to verify the number of cycles.
- Hydraulic or pneumatic test: Check the welds and bellows for leaks. Pay attention to the irregular shape of the special-shaped part, and consider the force of the end blind plate during the pressure test to avoid damage during the test.
- Appearance and Material: Stainless steel materials need to provide material report, corrugated pipe surface should not have scratches, pits, cracks. The threads of the tie rod and nut should be intact and can be adjusted smoothly.
Acceptance records must be kept well, and in case of problems at the site, there is a traceability basis.
Installation and Maintenance: Field Adjustment and Frequently Asked Questions for Shaped Metal Expansion Joints
Install shaped metal expansion joints, first thing: straighten the pipe. Many on-site pipelines have deviations after welding, and the expansion joint bolts are hard screwed on. As a result, the expansion joint is unevenly stressed and the bellows is twisted. The correct way is to first adjust the pipe support so that the two flange surfaces are parallel and the spacing meets the requirements of the drawings, and then install the expansion joint.
Loosen the transport lock nut first, then tighten the working nut evenly with a torque wrench according to the amount of pre-tension or pre-compression required by the drawing. How exactly to adjust? refer toHow to adjust the tie rod nut of expansion jointAndDo you need to remove the screw of the expansion jointThese two articles-the basic logic is: the nut should not be locked, and the expansion joint should be allowed to move freely, but it should not be too loose, otherwise the internal pressure thrust will be borne by the bellows.
What about FAQs? One is that the guide tube falls off. The guide tube of the special-shaped part is not welded firmly or deformed by heat, and is blown off by the medium, and then the bellows directly washes the perforation. Second, the tie rod is loose. Vibration during operation causes the nut to retreat and the expansion joint to fail. Therefore, during maintenance, check the tightening state of the tie rod nut every six months to see if there is any rust or looseness. Third, dust accumulation in bellows. There is a lot of dust in cement and electric power industry. If the guide tube is not installed or the design is unreasonable, dust will accumulate in the trough of the bellows. During thermal expansion and contraction, the dust will grind the bellows and accelerate fatigue. Periodically purge with compressed air, or consider installing it outside the bellowsMetal hoseProtective sleeve.
Because the special-shaped metal expansion joint is non-standard, it is particularly troublesome to change it if there is a problem. Therefore, every step of selection, design, installation and maintenance can't save effort. If you are unsure, send the working drawings to the manufacturer, and let the manufacturer help you with the calculation book and scheme.