Angular Expansion Joints Structural Principle & Multi-angle Pipeline Rotation Compensation Application

In the layout design of industrial pipelines,a large number of elbow and bent pipe structures need to be arranged to avoid equipment and building barriers.When the pipeline is heated and expanded,the bent pipe section will produce obvious angular deflection rotation displacement.If ordinary axial expansion joints are used for compensation,the bellows will bear excessive lateral shear force,resulting in early fatigue cracking damage.Angular Expansion Joints are specially designed for pipeline angular rotation displacement compensation products.The hinge assembly composed of ear plate and alloy pin shaft is added on the basis of stainless steel bellows.The hinge can rotate freely around the pin shaft within the design angle range,absorb the angular deflection of the bent pipe,and the hinge structure bears the pipeline blind plate force,so that the bellows only bear the flexible rotation load without axial tensile damage.At present,angular expansion joints are divided into single hinge type and double hinge type two mainstream structural forms,which are widely matched with heating pipe network elbows,chemical process bent pipes,power plant steam turbine auxiliary pipelines and other pipeline sections with angle change displacement.

The working principle difference between single hinge angular expansion joint and double hinge angular expansion joint determines their applicable pipeline layout scenarios.The single hinge product is provided with a set of symmetrical hinge ear plates and pin shafts on one side of the bellows,and can only realize unidirectional plane angular rotation compensation,with a single maximum rotation angle of±5°,which is suitable for single-plane elbow pipelines with simple displacement direction,such as horizontal horizontal 90°elbows of heating main pipelines.The double hinge angular expansion joint is equipped with two groups of mutually perpendicular hinge assemblies,which can realize two-way plane angular rotation at the same time,and the single direction rotation angle can reach±8°,which can adapt to the three-dimensional bent pipe sections with simultaneous horizontal and vertical angle changes,such as multi-layer three-dimensional layout process pipelines of chemical plants.The core load-bearing component of both structures is the alloy steel hinge pin shaft,which bears all the axial blind plate thrust of the pipeline,and the stainless steel bellows only plays the role of flexible sealing and rotation angle adaptation,which fundamentally avoids the problem that the bellows of ordinary expansion joints bears both pressure load and displacement load at the same time,and greatly extends the fatigue service life of the bellows.

The material collocation of each component of angular expansion joints has strict working condition matching standards.The bellows flexible main body still adopts multi-layer stainless steel hydraulic forming process,304 stainless steel for low-temperature heating pipelines,316L for chemical weak corrosive medium pipelines,and Hastelloy alloy or PTFE lined structure for strong acid and alkali corrosive process pipelines.The hinge ear plate is made of thickened carbon steel or stainless steel plate,and the surface is hot-dip galvanized or sprayed with anti-rust paint to prevent outdoor rain corrosion and rust deformation.The hinge pin shaft is the most critical load-bearing part,which must adopt high-strength chromium-molybdenum alloy steel for overall quenching and tempering treatment,with high tensile strength and shear resistance.If ordinary carbon steel pin shafts are used to reduce costs,shear fracture accidents will occur under the long-term blind plate force of high-pressure pipelines,resulting in pipeline separation and medium leakage.The two ends of the expansion joint adopt flange or butt welding connecting ends,and the sealing gasket at the flange surface is selected according to the medium temperature:rubber gasket for low-temperature water medium,metal wound graphite gasket for high-temperature steam medium.

In the actual pipeline engineering layout,a single angular expansion joint can only compensate the angular displacement of one elbow,and the complex bent pipe system needs to be equipped with multiple groups of angular expansion joints to form a combined compensation system to release all pipeline thermal stress.Taking the three-dimensional multi-bend chemical process pipeline as an example,two groups of double hinge angular expansion joints need to be arranged at the horizontal and vertical elbows respectively,and matched with a small number of sliding supports to limit the pipeline axial displacement range.In the heating municipal pipe network project,single hinge angular expansion joints are installed symmetrically on both sides of the 90°elbow,and the two groups of expansion joints rotate cooperatively to absorb the thermal expansion angle change of the horizontal pipe section and the vertical riser section at the same time,without setting a large number of reinforced fixed supports near the elbow,reducing the construction difficulty of the pipe network support foundation.When designing the layout,engineers need to calculate the thermal expansion elongation of each pipe section,convert it into the angular deflection value of the elbow,and select the single or double hinge expansion joint with matching maximum rotation angle according to the angle data,to avoid the rotation angle exceeding the design limit leading to hinge jamming and bellows damage.

The standardized installation operation of angular expansion joints focuses on the protection of hinge rotating parts and the alignment of pin shaft rotation plane.Before hoisting,the factory will install temporary fixing steel bars on both sides of the hinge ear plate to lock the rotation angle,which can only be removed after the pipeline welding is completed and the fixed support is installed in place.When aligning the expansion joint,the rotation plane of the hinge pin shaft must be consistent with the deflection plane of the pipeline elbow,otherwise the pin shaft will bear lateral shear force and be stuck and worn quickly.The welding operation needs to avoid splashing welding slag on the hinge pin shaft and ear plate,and the high welding temperature cannot bake the bellows part,to prevent the bellows from producing thermal fatigue cracks.After welding,the pipeline pressure test is carried out,and during the pressure maintaining process,manually push the pipeline elbow to simulate thermal expansion rotation,observe whether the hinge rotates flexibly without jamming,and check whether there is permanent deformation of the ear plate and pin shaft under pressure load.If the hinge rotation is blocked during the test,the fixed support position needs to be adjusted to reduce the pipeline unilateral pulling force on the expansion joint.

The daily operation fault types of angular expansion joints are relatively concentrated,mainly including hinge pin shaft rust jamming,ear plate deformation and bellows fatigue crack leakage.The hinge jamming fault is mostly caused by the lack of anti-rust protection of the pin shaft,and rainwater and dust invade the rotation gap to produce rust scale;the maintenance solution is to disassemble the pin shaft,polish and remove rust,and fill high-temperature anti-rust lubricating grease into the rotation gap,and add a dust-proof rubber protective cover outside the hinge for long-term protection.The deformation of the hinge ear plate occurs when the pipeline fixed support fails and the blind plate force exceeds the bearing capacity of the hinge,and it is necessary to reinforce the fixed support and replace the deformed ear plate assembly.The fatigue crack of the bellows is caused by the long-term rotation angle exceeding the design limit,and the pipeline layout needs to be optimized to increase the number of expansion joints to share the rotation displacement,and replace the cracked bellows components.In the regular maintenance work every six months,the maintenance personnel need to check the lubrication state of the hinge pin shaft,remove the accumulated dust and rust around the ear plate,and ensure the flexible rotation of the hinge structure.

Compared with axial tie rod expansion joints,the core advantage of angular expansion joints is targeted compensation for bent pipe angular displacement,which can avoid the lateral shear damage of bellows caused by elbow thermal expansion.In the long-distance pipe network project with many elbows,the use of angular expansion joints can reduce the number of pipeline fixed supports and simplify the overall stress design of the pipeline system.Its application limitation is that it can only absorb angular rotation displacement,and cannot compensate large axial stretching displacement,so it needs to be used with a small number of axial expansion joints in ultra-long straight pipe sections.In terms of procurement cost,single hinge angular expansion joints have a moderate price,and double hinge products are slightly higher,but the comprehensive maintenance cost in the later stage is much lower than that of ordinary axial expansion joints used in elbow positions,which has outstanding economic benefits for enterprises with long-term continuous operation of pipelines.

In conclusion,Angular Expansion Joints are professional compensation components oriented to pipeline elbow angular deflection displacement.The hinge ear plate and pin shaft rotating structure is its core technical feature,which separates the pressure load bearing function and the flexible displacement absorption function,and greatly improves the anti-fatigue performance of the bellows.Pipeline design engineers and pipe network operation and maintenance units need to distinguish the applicable scenarios of single hinge and double hinge products,master the combined layout scheme of multiple groups of expansion joints and hinge anti-rust maintenance skills,so as to reasonably solve the thermal stress damage problem of all kinds of bent pipe sections,and ensure the stable and safe operation of heating,chemical industry and power plant pipeline systems.