Where does the thrust come from? — — The Nature of Pressure Thrust and Blind Plate Force
Nine times out of ten, those who engage in pipeline design have been tossed by "thrust". Metal expansion joint thrust, to put it bluntly, comes from two sources: pressure thrust and blind plate force. Many people think that these two are the same thing, but in fact, they are far from each other.
Pressure thrust is the axial force generated by the medium pressure acting on the effective area of the bellows of the expansion joint. As long as there is pressure in the pipe, this force is there, facing outward, trying to stretch the expansion joint. What about blind board power? It occurs at the end of the pipe or where the diameter is reduced-such as where the valve is closed and the blind plate is blocked. The medium pressure acts on the closed end face and the force is transmitted directly to the pipe and support.
A DN500 steam pipeline, the pressure is 1.6MPa, the effective area of the bellows is about 0.2m², and the pressure thrust alone can reach 320kN. This is equivalent to 32 tons of weight hanging from the pipe. Think about it, can the brace hold it?
Therefore, the thrust of the metal expansion joint is not what the bellows itself can carry, but the pipe, bracket and equipment can carry it together. I can't figure this out, and the later selection and installation are all pits.
How to calculate? — — Thrust calculation formula and key parameters
Don't be afraid to calculate this matter. There is only one core formula:F = P × A。 F is the thrust force (N), P is the design pressure (MPa), and A is the effective area of the bellows (mm²). Note the effective area, not the diameter area.
How to take the effective area? RightUniversal corrugated expansion jointAndExternal pressure single axial expansion jointFor this kind of product, the manufacturer will give it directly in the technical parameters. Really can't find it. The rough algorithm is:Effective area ≈ (mean diameter of bellows ² × π) /4。 But this can only be estimated, and the formal design must use manufacturer data.
There is also one parameter that is easily overlooked – temperature correction. Under high temperature, the material strength decreases, and the safety factor should be increased. For example, the power station industryHigh temperature axial expansion jointAt the design temperature of 550℃, the allowable stress may be only 1/3 of the normal temperature. At this time, the thrust is unchanged, but the rigidity of the bellows changes, so it should be considered synchronously when calculating the displacement compensation amount.
In addition, the elbows and valves in the pipe layout will also change the direction of thrust. Lateral thrust is generated at the elbow, which isCompound hinge transverse expansion jointAndDouble hinge expansion joint for air-cooled island vacuum pipelineIt is particularly critical in the selection of models.
How do pressure balanced expansion joints "defuse" thrust?
The thrust is thrown directly to the bracket, which is costly and occupies a large area. So there's the pressure balanced expansion joint-which eats the pressure thrust itself through the internal structure. TypicallyStraight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion joint。
Design one more group of bellows so that the effective area of the two groups of bellows is equal, and the pressure and thrust direction are opposite, which cancels each other. And guess what? The main bellows is tensioned and the balanced bellows is pressurized, with a net thrust of zero. Then there is no need to set the main fixing bracket on the pipeline, just keep the guide bracket.
But at the cost of increasing length and costs. Moreover, the balance bellows works under pressure, which requires high stability, and it is easy to lose stability if it is not designed properly. SoCompound straight pipe bypass pressure balanced expansion jointSuch products have stricter requirements for manufacturing processes and materials.
A few days ago, a customer in the cement industry, the pipeline pressure is 0.8MPa, DN600, originally intended to use fourMetal Corrugated Expansion Joints in Cement IndustryAdd a bunch of brackets. Later, the pressure balance type was changed, the number of brackets was cut in half, and the overall cost was still low. Is that the truth? Don't just look at individual items when calculating the general ledger.
How to consider thrust when selecting a model? — — From pipe layout to support design
Many engineers choose expansion joints, only focusing on the compensation amount and pressure temperature. The impact of thrust is often underestimated. The consequences are bracket deformation, bellows cracking and flange leakage.
- The first step, calculate the thrust.Calculate the axial thrust generated by each expansion joint based on the design pressure and effective area.
- Step 2, look at the layout.The thrust of straight pipeline is simple, and the component force should be considered if there is an elbow or a reduced diameter. arrangementManual plug-in insulation doorOrElectric plug-in insulation doorThe position and thrust direction will change suddenly, so it must be checked.
- The third step is to set the bracket.The main fixed bracket must be able to withstand the sum of all thrust forces, and the guide bracket only withstands lateral forces. When the thrust is large, give priority to pressure balanced expansion joints.
By the way, don't be superstitious that "bellows stiffness can resist thrust". The stiffness can only resist a small part, mainly by the bracket. Many people thinkSleeve type pipe expansion jointNo thrust, that's a misconception-the friction at the sleeve seal and the thrust from the medium pressure still exist, but the calculation is different.
Installation and maintenance: Common problems caused by thrust and countermeasures
The tie rod nut of the expansion joint is not adjusted properly. The function of the tie rod is to prevent the bellows from being accidentally stretched or compressed during transportation and installation, but it must be loosened after installation, otherwise the expansion joint will not work and the thrust will be fully pressed on the tie rod, resulting in the destabilization of the bellows.
How to tune it? Refer to what is said in the Q&A of this site: After installation, adjust the tie rod nut according to the design requirements, so that the bellows is in the initial position under the design working condition. If it isExternal pressure single axial expansion jointPay attention to the direction and positioning of the external pressure sleeve, otherwise it will get stuck.
In the maintenance stage, the failures caused by thrust are mostly manifested as cracking of bellows welds or falling off of guide tubes. In addition to protecting bellows and reducing medium erosion, the guide tube can also balance some pressure pulsation. However, under high thrust conditions, if the connection between the guide tube and the bellows is not firm, it is easy to desolder.
aboutDesulfurization flue gas baffle doorAndDouble-sealed single-axis circular baffle doorThe thrust problem often occurs at the moment when the baffle door is closed-the pressure rises suddenly, and the thrust increases instantaneously, resulting in the deformation of the baffle door. The countermeasure is to set a safety valve or pressure relief device on the pipeline, or add an expansion joint in the front and rear of the baffle door to cushion it.
I ran a lot of scene, and the most outrageous thing I have ever seen is: a chemical plantDirect buried (fully buried) type expansion jointDirectly buried in the soil, without thrust calculation, as a result, the expansion joint is compressed and deformed, and the compensation amount returns to zero. When I dug it out, I saw that the bellows had been squeezed into discus.
Therefore, the thrust of metal expansion joints, from design, selection, installation to maintenance, can't be careless. You don't value it, and it makes you blow up the pipe. Of course, pipe explosion is an extreme situation, and the common ones are loose brackets, flange leakage and early fatigue of bellows. Don't wait until something goes wrong to check the thrust-it will be costly then.