First, absorbing heat displacement: the most fundamental mission of metal expansion joint
Pipes, to put it bluntly, are victims of thermal expansion and contraction. Once the steam pipe heats up, the pipe of tens of meters can be extended by several centimeters. If it is carried hard without giving it space, it is common for the pipe interface to collapse and the equipment to deform. The core mission of metal expansion joints is to do this-eat the thermal displacement in.
The bellows deform axially or transversely, like the bellows of an accordion. But when it comes to type selection, the learning is big. Like what we always sayUniversal corrugated expansion jointMainly cope with axial expansion and contraction; WhileHigh temperature axial expansion jointTo work in the flue gas pipeline at 800℃ or even higher, the corrugated pipe material must be made of heat-resistant alloy steel, otherwise it will crack after one thermal cycle. Two days ago, I met a customer who insisted on using an ordinary rubber compensator on a 200℃ steam pipeline. As a result, it was scrapped in three months. Metal expansion joint function this piece, the temperature level is dead line, don't make do.
Deflector tube. Many people ask "the specific function of the expansion joint guide tube", which is simply to protect the bellows from being washed by high-speed media. If the flow rate of the medium in the pipeline is fast and there are many particles, if there is no guide tube, the elbow of the bellows can be worn out in one month. When installing, pay attention to the direction of the arrow pointing to the direction of the media flow, and don't install it backwards-I have seen this error more than once in the field, and it is labor-consuming to remove and reinstall it.
2. Vibration and noise reduction: the underestimated protector of pipelines
Thermal displacement is only a superficial problem, vibration is the hidden killer. Pump outlet, compressor pipeline, fan inlet and outlet... The pipelines in these places are wrapped in pulsating flow all the year round, and the amplitude may be only a few millimeters, but the frequency is high and the duration is long. One of the most important functions of metal expansion joints is vibration isolation: the flexible structure of the bellows itself can cut off the propagation path of vibration and prevent it from transmitting along the pipeline to valves, brackets and even factory structures.
Which do you say is better for vibration isolation, it or rubber compensator? Depends on the working conditions. Rubber for ordinary water circulation pipelines is cost-effective, but when high temperature, corrosive medium or pressure exceeds 1.6MPa, rubber can't bear it-at this time, it must be made of metal. In a cement plant project we did, the kiln tail flue gas pipeline was usedMetal Corrugated Expansion Joints in Cement IndustryAn external pull rod is installed, and with the double hinge structure, the vibration noise is directly dried from 95 decibels to below 75 decibels.
Pipe rack setting, damper and soft joint must be matched. The expansion joint only uses the bellows as a natural spring medium. The lower the stiffness, the better the vibration isolation effect, but the low stiffness means that the pressure bearing capacity decreases-this is a trade-off. Therefore, when selecting the type, we need to look at the matching of stiffness and the number of bellows layers. We have a detailed description of the stiffness and calculation formula of bellows on our website, which you can refer to.
3. Compensation for installation error and foundation settlement: coping with unforeseen displacement
You thought everything would be fine when the plumbing was installed? It's not that simple. On-site welding deviation, unaligned flange alignment, support settlement... These "man-made disasters" plus the "natural disasters" of uneven foundation settlement make the extra stress bearing by the pipeline exceed the limit in minutes. The function of metal expansion joint is "covering the bottom" here: it can absorb multi-directional displacement, not only axial expansion and contraction, but also lateral and angular deflection.
Such asCompound hinge transverse expansion jointSpecially used to absorb lateral displacement, the two bellows cooperate with the hinge structure, which can make the pipe move tens of millimeters away in the horizontal direction without transmitting force to the equipment at both ends. AndStraight pipe pressure balanced expansion jointIt has a balanced bellows inside, which can eliminate the blind plate force, and is especially suitable for being placed between the fixed brackets of large-diameter pipes.
The vacuum pipeline of air-cooled island of power plant has complicated pipeline direction and large spacing between supports. The displacement caused by foundation settlement is 3-5mm every year. We made it forDouble hinge expansion joint for air-cooled island vacuum pipelineThe two hinges allow the pipe to deflect arbitrarily in the plane, and it has been in operation for eight years without a single leak. You see, the function of metal expansion joint is not only to solve "thermal expansion and contraction", but also to leave fault-tolerant space for the whole pipeline system.
4. Selection logic of different working conditions: general type, high temperature type, pressure balance type... Don't choose the wrong one
There are a basket of expansion joint models on the market, how to pick? Don't just look at the name of the product, but first grasp three factors: medium temperature, pressure grade and displacement direction.
A steam or water line with a temperature less than 500 DEG C and a conventional pressure,Universal corrugated expansion jointIt's basically enough. If it exceeds 600℃, it has to be changedHigh temperature axial expansion jointThe corrugated pipe material is upgraded to Incoloy 800 or a similar heat-resistant alloy, and usually requires an additional insulation layer. High-pressure scenarios, such as the main steam pipeline of a power station, need to useCorrugated expansion joint for power station industryThe wall thickness is larger, the number of layers is large, and the welding process is special.
If the displacement direction is complicated, it can't be solved by an axial type alone. For example, if you need to absorb both axial and lateral displacements, you have toCompound straight pipe bypass pressure balanced expansion jointWhich is handled by separating axial and lateral displacements by means of bypass tubes and pressure balancing elements. There's another pit: many people can't tell the difference between an "expansion joint" and a "compensator". As explained in our FAQ, expansion joint and compensator are different names of the same type of equipment in industrial pipeline system. Don't worry about the names, just look at the parameters.
Make sure to provide the medium flow direction, operating temperature range, installation space, support and hanger position of the pipeline when selecting the type. Only manufacturers can do stress analysis and life check. Slap the head yourself from experience? That metal expansion joint is useless no matter how good it is.
V. Key points of installation and maintenance: Is the tie rod removed or not? Which way does the deflector face?
If the product is selected correctly, it will still be scrapped if there is a problem with installation. Take the tie rod for example-should the tie rod nut on the expansion joint be removed or not? This question ranks in the top few in the Q&A.Transportation and installation tie rods (also called transport tie rods) must be removedOtherwise, the expansion joint cannot be freely expanded and contracted, which is equivalent to white installation. But if it isLarge tie rod expansion jointOr the length-adjusting pull rod on the pressure-balanced expansion joint, which is used to limit the displacement range, cannot be disassembled after installation, and the nut position must be adjusted according to the requirements of cold tightening or hot tightening.
The direction of installation of the deflector is mentioned earlier-the arrow points to the direction of media flow. But there is another detail: the gap between the guide tube and the bellows should meet the design requirements. If the gap is too small, it will be stuck and deformed, and if the gap is too large, it will not play a protective role. In addition,rubber compensatorAndNon-metallic expansion joints (fabric fiber expansion joints)The installation requirements are stricter-it cannot be welded directly on the metal end face, and it must be fixed by spot welding first to avoid hot deformation.
What about maintenance cycles? There is no fixed "must be replaced in a few years" for metal expansion joints. We have seen low-pressure cold water pipelines that are still intact after fifteen years of use, and high-temperature flue gas pipelines that are perforated after two years. There are three key points: whether there are corrosion pits or cracks on the surface of the bellows, whether the tie rod and ear plate are deformed, and whether the guide tube is seriously worn. During regular inspection, measure the wave pitch change of the bellows with a gauge. If it exceeds the original value by 10%, it is time to warn. If you are really unsure, flip through the Q&A on our website "The Service Life of Expansion Joints", which has more detailed judgment criteria.