Negative pressure and positive pressure are not the same thing
Find out the working conditions first. The medium pressure in the negative pressure pipeline is lower than the atmospheric pressure, and the outside of the bellows is subjected to atmospheric pressure. What about positive pressure pipes? The pressure is pushed outward from the inside, and the bellows bear tensile stress. One drums outward, the other presses inward, and the direction of force is completely reversed.
The difference is terrible. Under positive pressure, as long as the material strength and wall thickness of the bellows are sufficient, there will be no major problems. However, negative pressure is different. What negative pressure metal expansion joints fear most is not insufficient strength, but instability. Once it is unstable, the bellows is like a flattened can, and the whole deflates. To put it bluntly, it's not that the material is crushed, it's that the geometry can't hold up.
How does external pressure instability happen
External pressure instability, in vernacular, means that the bellows buckles under external pressure. The bellows has a thin wall and a wavy cross section, so its stiffness is not great. When the external pressure exceeds its critical value, the crest and trough will undergo irreversible plastic deformation, and the whole expansion joint will be wasted.
The relationship between instability and external pressure is not linear. The pressure may be fine just as it reaches the critical value, but a little more, and the deformation suddenly occurs. Therefore, sufficient safety margin must be left in design, and the pressure calculation of working condition cannot be stuck. In actual engineering, the pressure fluctuation of the vacuum system is easily ignored. When the operating temperature rises, the yield strength of the material decreases, and the critical instability pressure also decreases-the two factors are stacked together, and this is often the working condition that happens.
There are clear formulas and methods for the stability calculation of bellows in GB/T 12777. The core is to calculate the permissible external pressure and critical instability pressure. But formulas are dead, engineering is alive. The more layers and the larger the wave distance, the worse the stability; The larger the diameter, the easier it is to become unstable. Negative pressure metal expansion joints with a diameter of more than 500mm should be designed with extra care.
Selection focuses on three parameters
Look at the stiffness first when selecting the model. The stiffness determines the ability of the expansion joint to absorb displacement, but if the stiffness is too large, the thrust of the pipeline will be large, and the bearing will not be able to stand it; The stiffness is too small, the bellows is too soft, and the ability to resist instability is poor. There needs to be a trade-off between this, not the softer the better.
Look at the wave number again. The number of waves is large, and the amount of compensation is large, but each wave is a potential risk point. Under negative pressure conditions, one more wave has more possibility of instability. Therefore, as long as the compensation amount is enough, don't blindly pile up wavenumbers.
Finally, talk about reinforcing rings. This thing is a key structure to resist external pressure instability. The reinforcing ring is welded at the trough, which is equivalent to adding reinforced skeleton to the bellows, which can significantly increase the critical instability pressure. A reinforcing ring can also be added in the middle of the two layers of bellows, and the effect is similar. However, it should be noted that after adding the reinforcing ring, the axial stiffness of the expansion joint will become larger, and the displacement compensation ability will decrease. Users have to make a trade-off between stability and compensation. The external pressure single axial expansion joint of this station is often equipped with a reinforcing ring for vacuum working conditions. The design idea is to let the external pressure cylinder bear part of the pressure when the bellows is externally pressurized, and cooperate with the reinforcing ring to improve the stability as a whole.
Pits common to installation and site
Install this pit quite a few. Let's talk about the difference between vacuum hose and negative pressure metal expansion joint first. Some users use metal hoses as expansion joints, which is a misunderstanding. The metal hose mainly absorbs vibration and a small amount of displacement, and the bellows has thin walls and low stiffness, so it is easy to suck flat when vacuum is pumped under negative pressure. Special vacuum hoses are usually built-in spiral steel wire support, just to prevent external pressure collapse. However, the negative pressure metal expansion joint is designed according to the standard of pipeline displacement compensation with end connection. The two structures are different and cannot be replaced.
The direction of the deflector is also often mistaken. The medium flow direction in the negative pressure pipeline is from A to B, and the opening direction of the guide tube should face the medium flow direction. When installed backwards, the medium directly washes the bellows and wears out quickly. In addition, the guide tube itself should also open a pressure relief hole, otherwise the pressure difference between the inside and the outside will deflate the guide tube.
The tie rod nut is also a typical pit. When leaving the factory, the tie rod and the nut are to fix the transportation state. After installation, the nut must be loosened to allow the expansion energy saving to deform freely. Some sites forget to loosen, the expansion joint becomes a rigid body, and the thermal displacement of the pipeline all acts on the fixed bracket, ranging from the deformation of the bracket to the cracking of the pipeline. The correct method is: after installation, return the tie rod nut to the specified clearance value to allow the expansion joint to work normally.
Practical industry application cases
In the power station industry, the exhaust pipeline between the condenser and the low-pressure cylinder is a typical negative pressure condition. Here, the temperature is not high but the diameter is large, often exceeding DN2000. The negative pressure metal expansion joint needs to bear the vacuum external pressure, and at the same time absorb the thermal expansion when the steam turbine is started. In this case, the external pressure single axial expansion joint with reinforcing ring is usually selected to constrain the transverse displacement with the hinge structure. Among the corrugated expansion joint products used in power station industry, there are designs specifically aimed at this working condition.
Negative pressure operation is very common in the flue gas system of kiln head and kiln tail in cement industry. The flue gas is dusty and the temperature fluctuates greatly. The expansion joint should not only resist external pressure, but also be wear-resistant and high temperature-resistant. Non-metallic expansion joints have weak resistance to external pressure under negative pressure conditions, so large-diameter negative pressure pipelines are more reliable than metal.
Desulfurization systems are more complex. Both slurry pipeline and flue gas pipeline have negative pressure sections, and the medium is highly corrosive, so corrosion-resistant materials have to be considered. At the same time, the desulfurization system starts and stops frequently, and the pressure fluctuates violently. The fatigue life of negative pressure metal expansion joint must be designed according to high cycle fatigue. Compound hinge transverse and straight pipe pressure balanced expansion joints are often used in flue systems in combination. The former absorbs transverse displacement, and the latter eliminates blind plate forces to ensure that the pipe is not overloaded.
Choosing negative pressure metal expansion joints is, in the final analysis, to fight against instability. By thoroughly understanding the working conditions, calculating the parameters correctly, and smashing the installation details, the system can run safely for several years. If you only copy the positive pressure parameters, you will have to pay your tuition fees on the spot sooner or later.