What exactly is a full high wave metal expansion joint? Find out the relationship between corrugation height and compensation amount first
We in the plumbing business are familiar with expansion joints. Thermal expansion and contraction, equipment vibration, foundation settlement, all of which have to be "discounted" by it. However, there is a word on the market called "full-height wave metal expansion joint", which sounds quite bluffing, but in fact, the meaning is very simple: the wave height of the corrugation is full, maximized, and the deformation ability of the wall thickness of the corrugated pipe is fully utilized to absorb the displacement.
Then the higher the ripple, the stronger the compensation? Don't be in a hurry to nod. When the wave height is high, the single wave compensation amount can indeed go up, but there is a physical limitation here-once the wave height exceeds a certain range, the stress difference between the root of the wave and the wave peak will increase sharply, which will reduce the fatigue life. This is not an arithmetic problem, or the compensation amount doubles when the wave height doubles, and there is an "optimal range" hidden in the middle. The design of full-height wave pays attention to "squeezing dry the force that each wave can produce on the premise of ensuring fatigue life".
When actually calculating the compensation amount, engineers will use the formula in EJMA standard to throw in the wave height, wave pitch, wall thickness, number of layers and elastic modulus of materials, and iterate them step by step. For example, the pipeline of DN400 has a working temperature of 300℃ and an axial compensation amount of 80mm. With the standard wave height, it may get about ten waves. If it is replaced with a full-height wave structure, seven waves can be completed. At what cost? The transverse stiffness becomes lower, and if the pipeline has lateral displacement, it must be calculated separately.
Compared with ordinary corrugated expansion joints, under which working conditions can the full-height wave structure bear more?
Ordinary corrugated expansion joints, the wave height is generally according to the standard series, and the design is conservative. They can be used in any working condition, but everything is "just right". Full-height wave is different. It is specially prepared for those occasions where "there is not enough space and displacement is not small". For example, the main steam pipe of a power plant has a large pipe diameter, high temperature and large heat displacement to be absorbed, but the pipe gallery in the plant is so wide. If you use ordinary bellows and string a string of more than a dozen waves, the length will directly exceed the limit. The full-height wave metal expansion joint achieves the same compensation amount with less wave number, and the axial size is shorter, so the pipe frame layout is much faster at once.
Another example is the flue gas pipeline of kiln head and kiln tail in cement industry. The working condition is not only high temperature, but also the vibration of the fan and the negative pressure pulsation in the pipeline. The full-height wave structure has low stiffness, small reaction force on pipelines, and will not tear the interface of equipment. With a guide tube, the high-speed dusty flue gas directly washes the guide tube, and the bellows body basically does not touch the medium, so the service life is naturally long.
But then again, full-height waves are not all-purpose. If you take it to the high-pressure oil system, the working pressure is above 5MPa, the wall thickness of the bellows must be thickened, and the advantage of wave height will be immediately weakened. At this time, it is more stable to use ordinary bellows or compound tie rod structure. What to choose, after all, depends on the working conditions.
Don't just focus on wave height when selecting: how to balance pressure, temperature and fatigue life
Two days ago, I met a customer and asked, "Is there any expansion joint with the largest wave height?" I said you should report the pressure and temperature to me first. Why? Selection is essentially a multi-objective balance.
Pressure determines the pressure resistance of bellows, depending on the wall thickness and the number of layers. When the wall thickness goes up, the corrugation is not easy to be deflated, but it is difficult to form, and the fatigue life will also be reduced. Temperature determines the material. Carbon steel can't do work above 450℃, and stainless steel basically reaches the top at 304 to 700℃. If you go up, you have to change Incoloy 800H or GH3030 superalloy, and the price doubles several times. What about fatigue life? It is directly linked to the number of cycles the bellows is subjected to. Your pipeline starts and stops twice a year, and it starts and stops once per shift. The design life is orders of magnitude different.
When selecting the full-height wave metal expansion joint, the correct posture is: first determine the working condition parameters (pressure, temperature, medium, displacement, number of cycles), and then let the manufacturer design and calculate the bellows to determine the wave height, wave pitch, wall thickness and number of layers. Wait until the drawings come out, and finally look at the structural form-whether it is a general-purpose corrugated expansion joint, an external pressure single axial type, or a straight pipe pressure balance type. The order is reversed, and the back is full of pits.
Several details that are easy to overlook during installation and routine maintenance
Install this piece, but any metal expansion joint, the first thing to do is check the tie rod and guide bolts. When manufacturers deliver goods, they generally lock the bellows with transportation rods to prevent the ripples from being damaged by transportation bumps. Some people forgot to dismantle and install it directly. As soon as the pipeline is heated, the displacement is all suppressed on the equipment, and the expansion joint becomes a decoration.
Because of the low stiffness of the full-height wave structure, it is more afraid of "not working hard" when installing. If the pipe is inaccurate and forcibly deadpulled in place, an initial stress is generated inside this thing, which is enough for you to "enjoy" slowly after running. The correct way is: first fix both ends of the pipe, naturally align the expansion joint, and then tighten the bolts evenly. If you need to pre-stretch or pre-compress on site, be sure to follow the value given in the drawing, and it may be wasted if you twist half a turn.
Don't just look at the corrugated surface for routine maintenance. The guide tube is worn out, the liner is off, and the surface is not visible at all. It is recommended that each time the machine is shut down for maintenance, light the inside of the bellows with a flashlight, or use an endoscope to see the wear of the guide tube. Especially in the cement industry, the flue gas has a large dust content, and it is the norm for the guide tube to wear out. Once you find that the deflector is broken, don't hesitate to change it quickly, otherwise the bellows will be the next to wear out.
From power stations to cement industries, practical application cases of full high wave metal expansion joints
Tell me a practical case. The air-cooled island vacuum pipeline of a power station, with a diameter of DN1400, has a working pressure of only a few tens of kPa, but the vacuum requirement is high, and the thermal displacement of the pipeline is particularly large. The original design used ordinary corrugated expansion joint, but the length was too long, and a lot of pipe frame reinforcement was done. Moreover, the elastic reaction force of the corrugated pipe to the pipeline was large, resulting in micro-cracks at the interface of the vacuum box. Later, it was replaced with a full-high wave metal expansion joint, and the wave number was reduced from 14 to 9, the axial size was shortened by nearly 40%, and the reaction force was reduced by half, so the problem was directly solved.
The cement industry is even more typical. The temperature of the pipeline between the kiln tail smoke chamber and the humidifying tower fluctuates from 200℃ to 900℃, with large thermal displacement, and it has to bear a large air volume of dusty flue gas. We have done a project, using a full-high wave structure thickened wall guide tube, lined with wear-resistant material, and ran for three consecutive overhaul cycles-that is, six years-without leakage problem. However, the non-metallic expansion joint on the same production line has been replaced once in less than two years.
After all, the full high wave metal expansion joint is a tool. Whether the tool is suitable or not depends on how you use it. Wave height is only the appearance, and design calculation and working condition matching are the core. Next time someone tells you, "The higher the ripple, the stronger the compensation," you can ask him, "Then how do you calculate your fatigue life?"