Why Pipelines "Can't Hold" Thermal Expansion and Cold Contraction
Everyone knows that the pipe elongates when heated, but why can a carbon steel pipe, whose temperature rises from normal temperature to 200℃, eject several tons or even more than ten tons of force when it is several tens of meters long?
Because the two ends of the pipe are tightly restricted by the fixed bracket and the equipment interface, the thermal elongation can't be released, and the force is fully pressed on the pipe wall, ranging from flange leakage to torn the equipment interface. The work of the compensator is not simply "absorbing displacement"-displacement is just the appearance. What it really wants to solve is the force balance of the pipe system, so that the deformation caused by thermal expansion and contraction has a place to go, instead of carrying it hard.
Therefore, when selecting a compensator, look at the system constraints first, and then the displacement amount. You just count the elongation and not the reaction force, just like looking at the scene of a car accident without looking at the speed, it doesn't make sense.
Bellows are the soul
The core of the corrugated expansion joint is the bellows. Take the general-purpose corrugated expansion joint, for example, it relies on the elastic deformation of the corrugation to "eat" the axial displacement-the corrugation is compressed, the displacement is absorbed, and then the pipe is pulled back to its original position by the elastic restoring force. This principle sounds simple, but the shape, number of layers, and material of the ripples directly determine the compensation ability.
External pressure single-type axial expansion joint is clever in putting the bellows outside, and internal pressure acts on the end, not the corrugated inner wall. In this way, columnar instability can be avoided, and it is suitable for large displacement and high pressure scenarios. What is the amount of compensation? It is not determined by patting the head. The ripple stiffness, single wave displacement and fatigue life restrain each other. For example, if the design life requires 1000 full displacement cycles, the single wave displacement must be controlled within a certain value; If you have to overdisplacement, your life will drop exponentially. Many scene ripples cracked after two or three years of use. Nine times out of ten, it was only greedy for large compensation when selecting the model, not fatigue.
Ripples alone are not enough
While the bellows absorbs displacement, it will generate a huge blind plate force, which will be transmitted to the fixed bracket and adjacent equipment. The pressure balance type digests the blind plate force by itself through the internal balance corrugated structure or tie rod system, and the pipeline and equipment don't have to carry extra force.
What about the curved tube pressure balance type? It is specially used in elbow sections to absorb lateral displacement while also balancing pressure and thrust. Compound hinge transverse expansion joints rely on hinges to constrain axial displacement, and only allow transverse deformation to occur-it works where there is not enough space and axial displacement is large.
However, if you have simple working conditions and low pressure, it is easier to use a rotary compensator. It uses the rotation angle to change the direction of the pipeline, has a large compensation amount and low flow resistance, and is used a lot in steam pipelines. People don't rely on the corrugated hard top, but "borrow their strength".
Nonmetallic and metallic, compensation logic is completely different
Non-metallic expansion joints (fabric fiber expansion joints) and rubber compensators are made of flexible bodies, which absorb multi-directional displacement by the flexibility of fabric or rubber itself, and can also absorb shock and noise. For example, in the low-temperature and strong corrosive environment of desulfurization flue, the metal bellows can't bear acid, but the non-metallic material is solid, so it is convenient to replace. The vibration damping effect of rubber compensator at the inlet and outlet of pump is simply incomparable to that of metal parts.
But you don't expect non-metals to withstand high pressure and heat. Don't stick metal on those occasions, and vice versa. If the material is chosen wrong, the compensator is a pile of scrap iron.
With the compensator all will be fine? Wrong
The direction of the guide tube is installed backwards, and the medium directly washes the inner wall of the corrugation, which will wear out in less than half a year. The tie rod nut is used for positioning when it leaves the factory. After installation, it must be adjusted according to the instructions, not to be welded to death. Installing pre-stretching is even more specific-for the expansion joint that requires pre-stretching, you don't do it, it looks fine in the cold state, but it overdisplaces when it runs in the hot state.
These details are not done in place, and the compensator not only fails to compensate, but becomes a weak point in the pipeline. We have seen too many cases, and customers said, "The quality of your products is not good". As a result, when we looked at the scene, the draw rod was not disassembled, the guide tube was installed backwards, and the bracket was in the wrong position. It was purely the installation and backing the pot.
Don't just look at the compensation amount when selecting a model
Medium, temperature, pressure and fatigue life have to be calculated together. For corrugated expansion joints used in power station industry, the typical working condition is high-temperature steam, so the thermal displacement and the number of fatigue cycles must be calculated. The expansion joint matching the desulfurization flue gas baffle door contains sulfur and dust in the medium, so it is necessary to consider corrosion resistance and wear resistance. At this time, it is more reliable to use non-metal expansion joint or metal rectangular expansion joint with guide tube.
Two days ago, I met a customer and said, "Our compensation amount is enough, why is it still bad?" When asked, there were particles in the medium, and the bellows were directly worn out. You can't just look at the amount of compensation, media abrasion is more deadly than displacement. Therefore, when selecting the model, give all the working condition parameters, and don't just lose a "compensation amount of 50mm".
The compensator looks like a small part, but it does the work of "moving a thousand pounds". You treat it as an ordinary joint, and it will give you a look; If you treat it as a precision bullet, it will keep your pipeline safe for twenty years. The truth is this truth, the key is not to think of its "compensation" simply.