Metal expansion joint reinforcement: when do you need to add it and how do you add it reliably?
The expansion joint is born to be "soft"-it absorbs heat displacement and holds vibration, relying on the flexibility of the bellows. But flexibility has a fatal shortcoming: it can't withstand the blind plate force generated by medium pressure. The internal pressure of the pipeline acts on both ends of the expansion joint, which will form a large axial thrust. If this force is not held by the pipe rack or equipment, it will pull the expansion joint into deformation at the slightest, and directly pull the bellows out of the corrugation at the worst. Scrapping is considered light. Therefore, "reinforcement" is not superfluous, but installing a skeleton for the expansion joint-making it soft where it should be soft (it can expand and contract when it should absorb displacement) and hard where it should be hard (steadily transmitting pressure and thrust to the fixed pipe frame). To put it bluntly, reinforcement means that the expansion joint can safely transmit the blind plate force while bearing the displacement.
Which scenarios are most likely to make the expansion joint "unable to bear"?
The first category is large-diameter pipes. For metal expansion joints with nominal diameter exceeding DN1000, the bellows stiffness is not high to begin with, and the pressurable thrust increases with the square of the cross-sectional area-doubling the diameter and quadrupling the thrust. Without reinforcement, the pipe rack can't hold it at all. Once the scene is suppressed, the pipe rack will be directly crooked for you to see. The second category is high-temperature and high-pressure steam pipelines, such as the steam supervisor in the power station industry. The temperature exceeds 500℃ and the pressure exceeds 4.0MPa. Ordinary general-purpose expansion joints can't bear it at all, so high-temperature axial expansion joints must be fitted with tie rods or pressure balance structures. The third category is the occasion where there is frequent vibration or water hammer impact-air-cooled island vacuum pipeline and desulfurization flue gas pipeline all belong to this category. Vibration will accelerate bellows fatigue. At this time, the abnormal displacement can be effectively constrained by using compound hinge transverse expansion joint or structure with limit tie rod. In addition, when the pipeline is complicated and needs to absorb axial and lateral displacements at the same time, the straight pipe pressure balance or curved pipe pressure balance expansion joint is a "one-step" reinforcement scheme-the structure is self-balanced, and there is no need to set up an additional main fixed pipe frame, which saves space, but the cost is indeed high.
What are the categories of reinforced structures? Each has its own applicable scenario
The simplest is the tie rod type-weld ear plates at both ends of the expansion joint and connect the ends with a tie rod through the bellows. The tie rod constrains only axial tension and does not restrict compression and lateral displacement. For typical products, such as general-purpose corrugated expansion joint with tie rod, the tie rod nut should be adjusted to the appropriate preload after installation, and then the transport screw should be removed after installation (for specific operation, refer to FAQ Article 13 of this site: Does the screw of expansion joint need to be removed?). Further up is the hinged type, with hinge plates instead of tie rods, allowing angular displacement but limiting axial displacement, suitable for L or Z pipe systems. Compound hinge transverse expansion joint is a combination of two hinges, which can absorb a large amount of transverse displacement, and is often used in air-cooled island vacuum pipeline. A bit more advanced is the pressure balance type-which uses bellows or sleeve structure to counteract internal pressure thrust. Straight pipe pressure balance type, curved pipe pressure balance type and double straight pipe bypass pressure balance type all belong to this category. This kind of product is self-balancing, and does not need to set an additional main fixed pipe rack, which is suitable for occasions with limited space. As for the large-diameter thick-walled expansion joint, the thick wall itself is a kind of "self-reinforcement"-by increasing the wall thickness and number of layers of the bellows, the pressure resistance and instability resistance are improved, but the cost is that the stiffness becomes larger and the displacement compensation ability decreases, so it is necessary to weigh it when selecting the type.
How many of the two most common mistakes you make in selection?
Just look at the amount of displacement, regardless of pressure and thrust. Many people use the expansion joint as a spring-only concerned about how much heat it can absorb and elongate, but not how much blind plate force caused by the internal pressure of the pipe. As a result, once the scene was suppressed, the pipe rack was pushed askew, and the whole system had to be reworked. Second: the installation direction of the reinforcement is reversed. There are often arrows on the expansion joints indicating the flow direction of the media, which must be installed in line with the flow direction. Why? Because the direction of the guide tube determines whether the medium flushes the bellows or the guide tube, installing it backwards will accelerate the corrosion of the bellows or cause abnormal force on the tie rod (refer to FAQ Article 10: The direction of the arrow of the expansion joint refers to). There is another detail after installation: the tie rod nut cannot be screwed to death, and the compression allowance allowed by the design should be allowed, otherwise the expansion joint will have nowhere to stretch during thermal expansion, and the bellows may be squashed. For the adjustment method, refer to FAQ Article 12: How to adjust the tie rod nut of the expansion joint-to put it bluntly, it is to pre-tighten to the design value first, and then loosen it half a turn to one turn to leave some room for thermal expansion.
Only when the reinforcement scheme is selected correctly can the life of the expansion joint be guaranteed
According to industry experience, under the premise of reasonable selection and correct installation, the service life of metal expansion joints can usually reach 10-15 years-but the premise is to find out the working conditions clearly. For example, the metal rectangular expansion joint matched with the desulfurization flue gas baffle door of power station has a large fluctuation in flue gas temperature and contains acidic corrosive medium. Not only the mechanical strength, but also the corrosion resistance and temperature resistance diversion lining should be considered to reinforce the structure. Looking at the cement industry again, there is much dust and high temperature. It is recommended to use external pressure single axial expansion joint-the shell can protect the bellows from dust accumulation and mechanical damage, which is much more reliable than the open structure. Two days ago, I met a customer who installed an ordinary general-purpose type on the cement production line. As a result, the bellows was worn out by dust in two months. After changing the external pressure type, it took four years without any problems. Tsk, this gap is not a matter of money, but a matter of choosing ideas. Finally, don't put the whole pipeline system in order to save thousands of reinforcements-it is too expensive, and rework costs and shutdown losses are much more expensive than reinforcements.