Find out first: How did pre-compressed 50% come from?
When many on-site masters heard about the installation of expansion joints, their first reaction was to "press half first and then talk about it". Where did this habit come from? Some people say that it is to save trouble, some people say that they are afraid of insufficient heat expansion, and others think that the more they press, the more durable the bellows will be. Do you look through the design manual and find the "pre-compression 50%" rule? Can't find it. The expansion joint is not a spring, the displacement capacity of the bellows is a fixed stroke, and the pre-compression of 50% just hard presses the bellows to the middle position, which sounds like a "centering safety" and actually blocks both sides.
Two days ago, I met a customer and told me that the high-temperature axial expansion joint in their factory began to leak after less than half a year of installation. When I removed it, there was a crack at the bottom of the bellows trough. Ask how to pre-press it during installation? The answer is "pressed by 50% as usual". You calculate the actual thermal elongation of the pipe again, and it only takes 20% to press at full load. With this 50% pressed in, the bellows is in the wrong initial position from day one, and it would be weird if nothing happened.
Disadvantage 1: The axial compensation ability is sacrificed, and the bellows will be broken if the thermal displacement of the pipeline is slightly larger
What does it mean to pre-compress 50%? Assume that the rated axial displacement of the bellows is ±40mm and the total stroke is 80mm. If you press 40mm in advance, the remaining compressible stroke is only 0, and the stretching stroke is only 40mm. What if the actual thermal expansion of the pipe is 50mm? The bellows was either pressed to the limit or pulled overhead, and the crest of the wave cracked directly. Products such as general-purpose corrugated expansion joints and high-temperature axial expansion joints have a good displacement range when they leave the factory. The amount of pressure should be calculated according to the actual thermal displacement, which is not one size fits all.
Amount of pre-compression = (actual working displacement/2) - (displacement that has occurred at installation). It's better for you to press 50% when you come up, which is equivalent to throwing half of the compensation ability calculated by the design institute. For example, if you buy shoes in size 43, but don't wear a size 42, you still say "it will loosen after you hold it up"-who do you blame for worn feet?
Disadvantage 2: The corrugated pipe is in a high stress state for a long time, and the fatigue life falls by a cliff
Bellows absorb displacement by elastic deformation of wave peaks and valleys. Pre-compressed by 50%, the trough is pressed deeper, the crest is stretched more open, and the stress level does not rise linearly, but doubles up. For stainless steel bellows, the direct problem caused by high stress is stress corrosion cracking-even if there is a little chloride ion in the medium, the crack can penetrate the wall thickness in a few months.
Some people take "pre-compression can extend life" as an example, so don't be led off. What are the conditions for pre-compression to extend life? It is the amount of pre-compression that just offsets the cold tight displacement during installation, allowing the bellows to return to the middle position during operation. For example, the installation temperature of the pipeline is 20℃, the operating temperature is 200℃, and the calculated elongation is 60mm. When the pipeline is installed, it is pre-pulled or preloaded by 30mm, and it just completes half a cycle during operation. This is called "cold tightness", not "blind pressure". If you press 50%, if it does not match the calculated value, the bellows will always work in an over-limit state, and the fatigue life will be directly reduced by an order of magnitude.
Disadvantage 3: Instability or buckling is easy to occur after installation deviation and medium pressure are superimposed
In addition to absorbing displacement, bellows have to withstand the thrust generated by internal pressure. After 50% pre-compression, the initial deflection of the bellows is already present and a large chunk of the stability margin is eaten. At this time, once there is pressure fluctuation, water hammer, or even just normal pressure fluctuation in the pipeline, the bellows may become laterally unstable-the pipe itself does not move, and the bellows itself "bulges". Instability is not slowly leaking, but instantaneous twisting and deformation, and the whole pipeline system has to stop.
Particular attention should be paid to directly buried expansion joints and externally pressurized single axial expansion joints, which have strict limitations on pre-deformation. There is soil constraint around the direct buried type, and if it is pressed in the wrong direction, the lateral force of soil will directly crush the bellows; External pressure single axial type external pressure bellows itself works in the reverse state. If you precompress it by 50%, it will add insult to injury. The pre-deformation data in the installation manual are calculated by the manufacturer one by one, not copied.
So how much exactly should we pre-compress?
Amount of pre-compression = (actual working displacement/2) - (displacement that has occurred at installation). It is recommended that you copy this formula next to the device. For example: the maximum thermal elongation of the pipeline is calculated to be 80mm, and it has been elongated by 10mm at ambient temperature during installation, so the pre-compression amount is 80/2-10=30mm. This is called on-demand preloading.
If the pipeline has no thermal displacement at all, such as some lined pipelines, the pre-compression amount is 0, and compression instead creates stress. If it is cold after installation and heats up after operation, the pre-compression amount should be greater than 0. On the contrary, if it is installed in a hot state and cools down after operation, it has to be "pre-stretched", not compressed.
Finally, let's be honest
Expansion joint pre-compression is a technical job, not a matter of patting your head to set proportions. If you have installed a pre-compressed 50% expansion joint, quickly go through the running record to see if there are any abnormalities in temperature and pressure, and then squat down to see if there are any cracks, bulges or scratches on the bellows surface. If you find anything wrong, contact the manufacturer immediately for re-accounting. If you haven't installed it yet, you are unsure about the selection or installation. Don't be superstitious about the "experience value" of the old master. Find an expert who makes expansion joints to calculate it with a calculation book, which will save much money than changing the pipe afterwards.