In addition, it is further proposed to use the stress classification method for the strength design of the pipeline, and at the same time, the stability requirements of the pipeline under pressure are also proposed.
1 Stress classification method 1.1 Plastic deformation and failure of steel The steel is a good plastic material. It can be seen from the tensile test that the plastic deformation of the steel from yield to fracture is 10 to 15 times larger than the maximum elastic deformation corresponding to the yield limit. Therefore, plastic deformation does not mean steel damage.
The plastic deformation of steel can be divided into limited and infinite. Only infinite plastic deformation will cause the plastic flow of the steel to cause damage; and the limited plastic deformation of the size equivalent to the maximum elastic deformation will not cause damage.
In fact, people often use this limited plastic deformation in production and life. From the car body in machining, to the fluorescent lampshade in the living room, and even the steel pipe discussed in this article, all are made by plastic deformation.
1.2 Action load) All the factors that cause the internal force and deformation of the pipeline are called the action and load. When the steel yields, different types of action cause the pipe to produce different types of plastic deformation, which may further lead to different ways of damage.
Depending on the type of plastic deformation produced, the effects can be divided into two different categories: force is a given force, independent of pipe deformation, such as pipe weight and medium pressure.
The displacement action can be a given displacement or deformation, such as thermal expansion or pipe settling, or a force related to displacement or deformation, such as axial friction of the soil and lateral compression reaction.
1.3 Stress classification The stress generated by the force action is called the primary stress, which depends on the static equilibrium condition. If the primary stress exceeds the limit state, the pipe will undergo an infinite plastic flow, which will immediately cause a burst or break. Therefore, the primary stress should be controlled so that it does not produce plastic flow, and a limit analysis of the primary stress should be performed for this purpose.
The stress generated by the displacement is called the temperature stress generated by the temperature in the secondary stress directly buried pipeline, which depends on the deformation coordination condition. If the secondary stress exceeds the limit state, the pipe will only produce limited plastic deformation. However, due to the plastic deformation on the internal structure of the steel pipe, the plastic deformation of the cycle will cause the pipeline to break with time. Therefore, the secondary stress should be controlled so as not to produce cyclic reciprocating plastic deformation, and for this purpose, the secondary stress should be analyzed for stability.
In addition, stress concentration occurs at the discontinuity of the local structure of the pipe, and the corresponding stress is called the peak stress. Due to the limited area of ​​stress concentration and the constraint of the surrounding elastic region, the peak stress exceeds the limit state and does not cause plastic flow. However, the peak stress of the cyclic change will also cause damage to the internal structure of the steel pipe, resulting in fatigue failure of the pipeline. Stresses at elbows, tees, and corners are peak stresses. The magnitude of the peak stress, ie the fatigue analysis of the peak stress, should be controlled according to the number of fatigues allowed. Design method 2.1 Characteristics and failure mode of direct buried pipelines Due to the uniform support of the soil, the self-weight of the pipeline can be neglected. Since the nominal wall thickness of the heat network pipe is much larger than the design wall thickness required for the medium pressure, the internal pressure stress is much smaller than the yield stress of the pipe. Therefore, in direct buried pipelines, the effects of self-weight and internal pressure are relatively small.
On the other hand, due to the constraints of the soil, the thermal expansion deformation of the straight pipe section cannot be released freely, and the temperature stress formed during the temperature change may cause cyclic plastic deformation, and the temperature rise axial pressure formed at the operating temperature may cause instability, and straight The thermal expansion and deformation of the pipe section, the transfer to weak parts such as tees, elbows and chamfers can also cause fatigue damage, and the transfer to the valve can also cause the valve to break. therefore. In direct buried pipelines, temperature has a greater impact on pipeline strength.
For direct buried heat networks with a diameter less than DN500, there are generally several failure modes: cyclic plastic failure and overall instability failure of straight pipe sections; damage caused by weak parts such as tees, elbows, chamfers and heads, and valves. .
The strength design is to adopt an appropriate design method to prevent the above-mentioned damage from occurring and to ensure the safe and economic operation of the pipeline.
2.2 Design method of straight pipe 2.2.1 Design method to prevent cyclic plastic failure The stress change caused by the change of pipe temperature between the highest temperature and the lowest temperature of the working cycle is the cause of cyclic plastic failure. Since the stress change is independent of the installation temperature, the prestressed installation does not solve the problem of plastic damage in the cold installation cycle.
When the straight pipe section in the anchored state satisfies the stability condition, the uncompensated pipe section is allowed to exist, which of course includes the cold-installed uncompensated pipe section.
Otherwise, the compensation device can only be set at a certain distance in the straight pipe section, so that the pipe becomes a compensation pipe section.
2.2.2 Design method to prevent overall instability damage The temperature rise axial pressure generated when the pipeline temperature rises from the installation temperature to the highest temperature of the pipeline working cycle is the cause of the overall instability failure.
Under cold installation conditions, cold-loaded uncompensated pipe sections are allowed to exist when the straight section of the anchored condition meets the stability conditions. Otherwise, the following measures should be taken: increase the depth of soil cover, increase the reaction force to prevent instability; preheat or install a one-time compensator by prestressing installation method to reduce the size of thermal expansion deformation; set compensation device to release absorption thermal expansion Deformation.
2.2.3 Straight pipe design general law Generally speaking, the operating temperature is not higher than 130', the buried depth is below 1m, and the pipe diameter is not more than DN500. From the perspective of straight pipe strength, the cold-installed uncompensated pipe section is in strength. There is no problem. However, from the point of protecting the weak parts and reducing the fixed thrust, it is often necessary to provide a compensation device in the partial pipe section.
As for the prestressed installation method, since the stability problem can only be solved, it is usually more economical to solve the stability problem by increasing the depth of the soil or setting the compensation device, so the use of the prestressed installation method becomes less and less.
2.3 Design method to prevent damage of weak components The stress of weak components depends on two aspects. Whether the structure of the component itself is prone to stress concentration, and the thermal expansion deformation of the two straight pipes is transferred. Therefore, damage can be avoided from the following three aspects.
2.3.1 Structural Dimensions of Weak Parts From the perspective of the pipe parts themselves, structures with a low degree of stress concentration are used to reduce the peak stress: use a three-way that increases the wall thickness or uses a stiffener; increases the thickness of the bend or the wall thickness Elbow; replace a large angle with a curved tube of large bending radius or a small angle of small stress concentration; replace the angle or elbow of 15.~70. with a U-bend; use multiple steps to change the size The head replaces a large head with a large change in pipe diameter; a steel welded valve with good strength characteristics is used.
2.3.2 Method of setting the fixed raft From the arrangement of the pipeline, the fixed enthalpy can be set to prevent the thermal expansion deformation from being transferred to the stress-concentrated component: a fixed raft is arranged on the curved arm of the curved pipe; and fixed on the branch pipe of the three-way pipe镦; Set a fixed 镦 on the straight pipe near the corner, the head, the valve and the tee main pipe to limit the transfer of the thermal expansion deformation of the straight pipe to these parts.
2.3.3 Local compensation method From the pipeline arrangement, it is also possible to absorb the thermal expansion deformation to the components with stress concentration by setting the compensation device. The compensation device may be a bellows compensator, or may be formed by using a pipeline to form a bend compensator: a compensating device is arranged on the bent arm of the elbow; a compensating device is arranged on the branch of the tee; the chamfer, the size head, the valve A compensation device is provided on the straight pipe near the main pipe of the tee.
3 Conclusion The design of the direct buried pipe network belongs to system engineering. There are many failure modes in the pipe network, and the causes of various failure modes are different, and the generated parts are also different. In the design, only the temperature of the straight pipe is different. Stress stability analysis is not enough. It is one-sided to use the uncompensated installation method suitable for straight pipe to negate the compensation installation method required for pipe fitting design. On the basis of discussing the stress classification method, this paper proposes a design method for different stresses on different components, and organically combines the compensation installation and the uncompensated installation method to ensure that all components are in a safe state from the perspective of the system. .
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