Classification and application of bellows expansion joint in high temperature pipeline
Why use bellows expansion joints in high temperature pipelines?
When there is a temperature change in the pipeline transportation medium or the environment where the pipeline is located, the thermal expansion and contraction caused by the temperature of the pipeline are inevitable. If the size change is not compensated in a certain way, a high stress will be generated in the pipe wall, which will be transmitted to the fixed pipe rack or equipment through the pipeline. When the temperature difference is over a certain range, the temperature difference stress is greater than the stress range that the pipe can bear. Then Compensation must be considered.
In the compensation design of piping system, the most economical one is natural compensation. Natural compensation uses the flexibility of natural bending shape of pipeline to compensate the thermal displacement. Obviously, the capacity of natural compensation is limited. When natural compensation cannot be required, expansion joints should be considered generally.
The loads on the pipe system are mainly external force loads (self weight of pipeline and flowing medium, internal pressure, wind load, earthquake load, etc.) and displacement loads. The purpose of setting up the pipe rack is to eliminate the forces exerted on the equipment or pipeline by the external load, and the complex pipe system can be divided into relatively simple and independent expansion pipe sections to ensure the best use effect of the expansion joint.
The purpose of setting expansion joint is to absorb the thermal deformation which can not be absorbed by the pipeline itself and minimize the displacement load. According to the use characteristics of different types of expansion joints, this paper presents several compensation design schemes for typical pipe systems.
This paper analyzes and calculates the force and moment on the pipe rack or equipment of the system composed of three hinge type expansion joint (including universal hinge type), double pull type expansion joint and elbow pressure balance type expansion joint, and discusses the corrosion of bellows in expansion joint.
The types of expansion joints and compensation design of typical pipe sections are mainly divided into the following types:
Single axial expansion joint
A flexible device consisting of a corrugated pipe and two end pipes (or flanges) that can be connected with adjacent pipes and equipment is mainly used to compensate the axial displacement of the straight pipe section, and also can absorb a small amount of lateral displacement. Figure 1 is an example of using axial expansion joint.
Figure.1 Example of axial expansion joint setting
In the case of lateral displacement or axial and lateral combined displacement, the use of single expansion joint is limited by its ability to absorb lateral displacement. In addition, when the working pressure, temperature and diameter are high or the main fixed support or multiple guide supports cannot be installed on the structure, the axial expansion joint may not work.
Double pull rod type expansion joint
The flexible device consisting of two bellows connected by the middle connecting pipe, pull rod, end plate, etc. compensates the thermal displacement of the plane or three-dimensional bending pipe section by means of transverse displacement. The pull rod device shall be able to bear the pressure thrust and its additional external force.
The double tie rod type expansion joint is particularly suitable for absorbing transverse displacement. In addition, this design form can also be used to absorb axial displacement, angular displacement and any displacement synthesized from these three forms. The general usage is to set the expansion joint with connecting rod in the middle arm of the 90 ° Z-shaped pipe system, and adjust the connecting rod to prevent the external axial displacement. Fig. 2 and Fig. 3 There are two application examples.
1-end plate 2-pull rod 3-intermediate pipe 4-bellows 5-spherical washer 6-end pipe
Fig.2 Double tie rod type expansion joint
Fig. 2 shows that the double connecting rod type expansion joint absorbs the transverse displacement of the single plane “Z” bend, the thermal displacement within the connecting rod of the intermediate pipe arm is absorbed by the axial displacement of the expansion joint, and the thermal displacement of the horizontal pipeline is absorbed by the transverse displacement of the expansion joint.
Since the pressure thrust is borne by the connecting rod, the intermediate fixed support is used at both ends. Because the axial force acting on the pipeline is the deformation reaction force generated by the expansion joint, only the guide support is needed. The part of the middle pipe arm outside the connecting rod, such as the thermal expansion of the elbows at both ends, must be absorbed by the bending of the horizontal pipe.
Fig.3 Universal expansion joint with connecting rod used in space “Z” bend
Fig. 3 is a typical example of using universal expansion joint with connecting rod in space “Z” bend. Since the universal expansion joint can absorb the lateral displacement in any direction, the two horizontal pipe arms can be in any direction on the horizontal plane.
Elbow pressure balanced expansion joint
A flexible device composed of two or one working bellows, one balance bellows and end pipes, end plates, elbows, heads, pull rods, etc. is used to compensate for the axial displacement, lateral displacement or combined displacement of the pipe sections, and does not make the fixed pipe rack or connected equipment bear the pressure push, and the pull rod device bears the pressure thrust and other additional external forces.
The main advantage of the elbow pressure balanced expansion joint is that when it absorbs the axial displacement from the outside, it will not make the system subject to the internal pressure thrust. The force caused by the overall rigidity of the bellows is not eliminated. In fact, the force is generally greater than the elastic force caused by the displacement of the single expansion joint.
Because both the working bellows and the balance bellows are subject to compression or tension, the force acting on the pipeline or equipment is the axial force of both.
Figure 4 is a typical example of using elbow pressure balanced expansion joint when there is combined axial and lateral displacement. The supports at the end of the pipeline and on the turbine are intermediate fixed supports, and only guide supports are needed,
Reasonable design can make the guide bracket above the turbine bear the force that makes the expansion joint produce axial displacement and avoid the force acting on the turbine. Only the force that makes the expansion joint produce lateral displacement can act on the turbine.
Figure.4 Use elbow pressure balance expansion joint when there is combined axial and transverse displacement
Figure 5 shows a common case that is very suitable for the use of elbow pressure balanced expansion joint. In the process operation, the expansion capacity of the vessel and the vertical pipeline may be different. Install a pressure balanced expansion joint as shown in the figure. The vertical displacement difference can be absorbed by the axial displacement of the expansion joint, and the thermal expansion between the vessel centerline and the pipeline can be absorbed by the lateral displacement of the expansion joint.
Fig.5 A common case which is very suitable for the use of elbow pressure balanced expansion joint
Hinged expansion joint
The flexible device consists of a bellows, two sets of hinge plates connected with the end pipe and a pair of pin shafts. Generally, two or three hinge expansion joints are used as a group to absorb the lateral displacement in one or more directions in a single plane pipe system.
In this system, each expansion joint is restricted by its hinge plate, resulting in pure angular displacement. However, each pair of hinge expansion joints separated by the pipe segment can absorb the lateral displacement by cooperating with each other. Given the angular displacement of a single expansion joint, the lateral displacement absorbed by each pair of hinged expansion joints is directly proportional to the distance between the hinge pin axis,
Therefore, in order to make full use of the expansion joint, this distance should be increased as much as possible. The hinge plate of the expansion joint is usually used to bear all the pressure and thrust acting on the expansion joint. In addition, it can also be used to bear the weight of pipes and equipment, wind load, etc.
Figure 6 is a double hinge system to absorb the main thermal expansion of single plane “Z” bend. As the pressure thrust is borne by the hinge plate on the expansion joint, only the middle fixed bracket is set at both ends of the pipe system. Because the expansion joint is restricted by the hinge, only pure angular displacement can be generated,
The thermal expansion of the middle pipe arm containing the expansion joint must be absorbed by the bending of the pipe arm which is perpendicular to it. The bending deflection of the two long pipe arms is controlled by the correctly designed guide bracket and bracket.
Fig.6 Absorption of main thermal expansion of single plane “Z” bend with double hinge system
If the middle arm of the single plane “Z” pipe system is too long, the system of three hinged expansion joints can be used.
Figure 7 shows the three hinge expansion joint system in the single plane elbow. The thermal expansion of the vertical pipe section will be absorbed by the actions of B and C expansion joints, and the thermal expansion of the horizontal pipe section will be absorbed by a and B expansion joints. Obviously, the angular displacement of expansion joint B is the sum of a and C.
Figure.7 Three hinge expansion joint system in single plane elbow
As in the previous example, only the middle fixed bracket is used to fix the two ends of the pipe system. In this example, all the thermal deformation is absorbed by the expansion joint, because the load of the thermal deformation of the pipe on the fixed bracket is very small,
If the distance between the left fixed bracket and the first hinged expansion joint is large, a guide bracket shall be set near the expansion joint to bear the rotating force of the expansion joint, so as to reduce the bending of some pipes between the expansion joint C and the left fixed bracket,
In order to keep the pipe system in the plane and eliminate the bending force that may be exerted on the hinge by the external load, one or more guide supports can be added. The support of the pipe system can adopt various ways. When the pipe between expansion joints is supported without hindering its free movement, spring supports and hangers can be adopted.
figure 8 is an example of using a hinged expansion joint when the elbow angle is not equal to 90 °. The main advantage of the hinged expansion joint is that it is compact in size, easy to install, and can make its hinge plate have great rigidity and strength,
They can usually be used to compensate the thermal expansion of complex pipelines with irregular configuration. It is often not feasible to use other expansion joints on such pipelines. Because the hinge structure can transmit loads, the force exerted by the pipe system of the hinge expansion joint on the fixed support is very small,
The support point of this system can be located at any position without impeding the system activity, which brings great freedom to the system design.
Figure.8 Example of using hinged expansion joint when bend angle is not equal to 90 °
Universal hinge expansion joint
The flexible device consists of a bellows, a universal hinge ring and two pairs of hinge plates connected with the universal hinge ring and the end pipe.
Generally, two universal hinge type or two universal hinge type and a single hinge type expansion joint are used together. As shown in Figure 9, two universal hinge type expansion joints cooperate to absorb the combined displacement of the upper and lower horizontal pipe arms, while the hinge type expansion joint and the upper universal hinge type expansion joint cooperate to absorb the displacement of the vertical pipe arms.
The system made up of universal hinge expansion joint has similar advantages to the system made up of hinge expansion joint mentioned in the previous section, but the application of universal hinge expansion joint has greater flexibility, which is not limited to single plane system.
Figure.9 Two universal hinge type expansion joints cooperate to absorb the combined displacement of the upper and lower horizontal pipe arms, while the hinge type expansion joint and the upper universal hinge type expansion joint cooperate to absorb the displacement of the vertical pipe arms
Simulation analysis of expansion joint by CAESARII
Taking the modeling and analysis of the common double tie rod expansion joint as an example, the pipeline is a high-temperature extraction pipeline, the design temperature is 269 ℃, the design pressure is 0.5MPa, and the material is carbon steel. The pipe model is shown in Figure 10:
Fig.10 Stress calculation model of extraction pipeline
In node 10, the equipment has an additional displacement of 15mm vertically downward on the pipeline. Now, the primary stress and secondary stress of the pipeline are checked. From the stress analysis results, it can be concluded that the secondary stress at node 80 is 1.36 times of the allowable stress, seriously exceeding the allowable stress of the pipeline material, and the pipeline system is at risk.
Figure.11 Calculation results of primary stress and secondary stress
Figure.12 Calculation results
Stress condition of relevant joints of pipeline
In the pipeline, it can be seen from the stress conditions of each node that the supporting forces at nodes 10, 80 and 140 are all too large; the primary stress is within the required stress range, and it can be concluded from the design conditions of the pipeline system that the secondary stress is mainly caused by the thermal expansion caused by temperature, which causes the deformation of the pipeline, and the deformation is constrained by the support to generate the thermal stress;
Therefore, in order to reduce the secondary stress of the pipeline, a feasible design must be adopted to absorb the displacement caused by the thermal expansion of the pipeline, so as to reduce the thermal stress. Now, double tie rod expansion joint is added to the vertical pipe at the equipment outlet, and the added model is shown in Figure 13,
The specific modeling process will not be described here. Considering the additional vertical downward displacement of the equipment to the pipeline, only node 10-50 of the whole pipeline system has a vertical pipeline direction, so the expansion joint is added to the vertical pipeline.
Re check the stress calculation of the pipeline. Since the primary stress before expansion joint is not added does not exceed the allowable stress, only the secondary stress can be checked here. The calculation results are shown in Figure 13.
Figure.13 Stress analysis model with expansion joint
Figure.14 Secondary stress calculation results of pipeline after expansion joint is added
Figure.15 Joint force after expansion joint is added
It can be seen from the above secondary stress settlement results and the force of each node that after the expansion joint is added to the pipeline, the secondary stress is greatly reduced, the maximum stress is only 18.2 of the allowable stress, and the force of each node is also greatly reduced, which shows that the expansion joint can effectively absorb the thermal expansion in the pipeline, thus greatly reducing the thermal stress of the pipeline.
Source: China Expansion Joints Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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