Starting from a pipeline accident: If the force analysis is not done thoroughly, the expansion joint will also fail
Last year, I visited a thermal power enterprise and found that on a DN600 steam pipe, circumferential cracks appeared on the surface of the bellows of the axial expansion joint. When removed, there is an obvious fatigue fracture at the trough of the bellows. The workshop director said that the expansion joint was used for less than eight months. After asking around, I realized that when I selected the model, I didn't calculate the stress at all, only because the price was cheap. This case is not unusual, but it is the best footnote for the necessity of stress analysis of metal expansion joint.
The expansion joint does not undertake the function of pipe support in the system, its job is to absorb thermal displacement. However, while absorbing displacement, the bellows itself has to bear various forces. Regardless of the force, it is equivalent to letting someone catch the flying knife with their bare hands. Sooner or later, something will happen.
Break apart and crush: What are the types of forces that metal expansion joints bear?
The forces on the metal expansion joint are nothing more than these types: axial thrust caused by internal pressure, which is often called blind plate force; Stiffness reaction force caused by bellows deformation; The displacement load imposed by the thermal expansion of the pipeline; Alternating loads caused by system vibration; There is also the consumption of wall thickness by media corrosion and erosion. The stress analysis in engineering mainly focuses on the first three.
How do you understand blind board force? There is pressure in the medium in the pipe, and the pressure acts on the effective area of the bellows, which will create a force that attempts to straighten or bend the expansion joint. The magnitude of this force is equal to the medium pressure multiplied by the effective area of the bellows. The higher the pressure and the bigger the caliber, the more frightening this force is. DN1000 pipeline, 1MPa pressure, blind plate force can reach 70 to 80 tons. Without dealing with this force, the fixed bracket and adjacent equipment will suffer.
Don't just look at bellows: Different structural forms of expansion joints have completely different force logics
Many people think that the expansion joint is a bellows, and this perception needs to be changed.
The universal corrugated expansion joint absorbs axial displacement by the bellows itself, and the blind plate force must be borne by the external pipe support. When selecting the type, the blind plate force must be transmitted to the structure professional. The balanced bellows is added in the straight pipe pressure balanced expansion joint, which can offset the blind plate force by itself, and the pipe support only bears gravity. Compound hinge transverse type expansion joint restricts axial displacement with hinge structure, only lateral deflection is allowed, the force path is clear, and the blind plate force is transmitted to the pipe through the hinge pin shaft. Curved tube pressure balance type expansion joint should be more comfortable with external pressure and lateral displacement.
The structural form determines the transmission path of the force. Different paths have different objects and conclusions of force analysis. Not all the forces on the expansion joints are calculated in one account.
Calculate the account clearly: How to calculate the internal pressure thrust, stiffness reaction force and displacement compensation?
F = P × S. P is the design pressure and S is the effective area calculated from the average diameter of the bellows. This data is available on the product sample, so you don't have to take a ruler to measure it yourself.
F = Kx × x. Kx is the axial stiffness in N/mm, indicated on the sample; x is the actual amount of compensation. The higher the temperature, the more the wave number, the smaller the stiffness, but when the displacement is large, the single wave displacement is too large and it is easy to become unstable. Therefore, the number of layers of multi-layer bellows should be selected to balance stiffness and pressure resistance.
Δ L = α × L × Δ T. α is the linear expansion coefficient, about 12×10⁻⁶/°C for carbon steel and slightly higher for stainless steel; L is the length of the pipe between the two fixed brackets; Δ T is the difference between the installation temperature and the operating temperature. What is calculated is the total compensation amount, which is then distributed to each expansion joint.
These three accounts must be settled before the selection. Otherwise, when the expansion joint is installed and the pipeline is tested, it will be found that the thrust of the bracket is over the limit or the compensation amount is insufficient, and the rework cost is enough to buy several expansion joints.
Practical case: How to implement the stress analysis under the conditions of power station, cement and desulfurization?
The corrugated expansion joint used in the power station industry for the main steam pipeline of the power station has an operating temperature above 550°C and a pressure of more than ten MPa. Under this condition, heat-resistant alloy should be selected as the material of corrugated pipe, and the core of stress analysis is creep fatigue interaction. It is not enough to only look at the normal temperature strength, but it has to be calculated as the high temperature durable strength. When we made a plan for a power plant, in addition to the conventional internal pressure thrust, we also specially checked the influence of blind plate force of pipeline on the pushing of steam turbine interface, and finally selected the external pressure single axial expansion joint with external pressure guide structure, so that the external pressure of bellows is more stable.
Cement industry kiln head cover and grate cooler air duct, working condition is high temperature dust gas. Cement Industry Metal Corrugated Expansion Joint In this scenario, the bellows should not only absorb heat displacement, but also prevent dust accumulation. Additional loads increased by dust accumulation should be considered during stress analysis. A cement factory customer used non-metallic expansion joints at first, which wore out too quickly. Later, he replaced them with metal rectangular expansion joints, but the thermal stress was not checked. The stress in the four corners of the rectangular structure was concentrated, and the weld cracked. After re-doing the stress analysis, reinforcing ribs were added to the four corners, and the problem was solved.
The desulfurization system is a low-temperature corrosion environment, and the flue gas baffle door and expansion joint are next to each other. After using the metal rectangular expansion joint, the slurry crystals fall on the bellows. Once the expansion joint deforms, the hard crystals directly wear through the thin wall. Nickel-based alloy resistant to chloride ion corrosion should be considered in the material selection of this working condition, and the additional weight of fouling should be taken into account in the stress analysis. Some projects directly add heat tracing or purging outside the expansion joint to prevent crystal accumulation, which can also be regarded as solving the stress problem.
How to use the force analysis results? Type selection, arrangement and tie rod adjustment in one step
After calculating the strength, the next thing to do will see the true skill.
Firstly, the model is selected by the results of force analysis. If the blind plate force is so large that the pipe support can't bear it, change the straight pipe pressure balance expansion joint or the curved pipe pressure balance expansion joint. The displacement is mainly transverse, and the double hinge transverse expansion joint is used. Three-dimensional displacement is required but space is limited, and a general-purpose corrugated expansion joint is used to match the guide bracket. The boundary of the problem that each structure can solve, the stress analysis has already told you the answer.
Secondly, the arrangement of the fixed bracket and the guide bracket should be designed according to the thrust value of the force analysis. The fixed bracket should bear the blind plate force, and the guide bracket should ensure that the pipe can only slide axially to prevent it from becoming unstable laterally. The installation of the expansion joint next to the fixed bracket is the layout principle derived from the conclusion of force analysis.
Finally, talk about the pull rod. The tie rod is not a pretense, it can transfer the internal pressure thrust to the tie rod itself. How to adjust the expansion joint tie rod nut? Loosen it first when installing, so that the pipe can expand and contract freely; After the pipeline is cold tight in place, lock it according to the specified torque. The locking sequence is particular, and it should be carried out symmetrically and alternately, otherwise excessive force on one side will pull the bellows.
Stress analysis is not completed by delivering a calculation book. It should run through four links: selection, layout, installation and operation. Each link is not matched, and it will be reflected in the bellows in the end. Instead of filling the hole afterwards, it is better to calculate this account in the early stage.