1. Why should the expansion joint be reinforced? — Pressure thrust is the invisible killer
Two days ago, I received a phone call from a customer, saying that the metal expansion joint on a steam pipe in their factory had been used for less than two years, and the bellows had bulged. At the scene, good guy-the expansion joint was directly pushed into a "pregnant woman's belly" by pressure thrust. Many people think that the expansion joint is a soft connection, as long as the pipe doesn't leak, but the real killer is the blind plate force (pressure thrust).
The pressure of the medium in the pipe acts on the end of the expansion joint, which will generate a huge axial force, which will push the bellows outward like a bulldozer. If the expansion joint itself is not structured enough to withstand this thrust, the bellows can overstretch, become unstable or even crack. Especially in large-diameter and high-pressure working conditions, such as the corrugated expansion joint used in the power station industry, the pressure thrust is often tens of tons, and the bellows itself can't bear it at all. Therefore, adding a tie rod and a hoop is essentially to "insure" the expansion joint and transfer the pressure thrust to the rigid structure, so that the bellows is only responsible for absorbing displacement and does not bear the principal stress.
Second, three mainstream reinforcement methods: tie rod, hoop, pressure balance type, how to choose?
- Tie rod type: Most common. Weld ear plates at both ends of the expansion joint, and connect the two ends with several screws. The screw has a nut on it to adjust the preload force. Like oursUniversal corrugated expansion jointOrLarge tie rod expansion joints rely on tie rods to restrain axial displacement and bear pressure thrust at the same time. Advantages: simple structure and low cost; Disadvantages: It can only constrain the axial direction and cannot absorb the lateral displacement.
- Hoop type: To put it bluntly, it is to ring a reinforcing ring on the outer side of the bellows, or weld a ring of ribs. It is mainly used to prevent radial bulging of bellows, and is suitable for vacuum conditions or external pressure scenarios. Such asVacuum special hoses andLarge diameter thick wall expansion joint, often equipped with hoops.
- Pressure balance type: This is "advanced gameplay". Two corrugated cavities, one left and one right, are designed internally, and the medium pressure is self-balanced, and the pipeline does not generate thrust. Like oursStraight pipe pressure balanced expansion jointAndCurved tube pressure balance type expansion joint is used in high temperature and high pressure steam pipeline. At the expense of large volume and high cost.
How to choose? Look at two parameters: pressure and displacement. High pressure, large axial thrust-> tie rod or pressure balanced type; Vacuum or external pressure-> hoop; Both need to absorb lateral displacement without wanting to transmit thrust to the pipe frame-> pressure balanced type.
3. Working condition determination scheme: reinforcement logic in high temperature, large diameter and vacuum environment
The same reinforcement method is very different under different working conditions. For example:High temperature axial expansion jointUsed on the boiler flue, the temperature is above 800℃. The corrugated pipe material is made of superalloy, but the material strength decreases at high temperature. At this time, the spacing of the tie rods has to be densified, otherwise the extra stress caused by thermal expansion and contraction will pull the screw and bend.
What about the large caliber? The caliber exceeds DN1000,Large diameter thick wall expansion jointThe self-stiffness is already good, but the pressure thrust increases in the square (pressure × area). We did it for the cement factoryMetal Corrugated Expansion Joints in Cement Industry, caliber DN2000, pressure 0.25MPa, calculated thrust close to 80 tons. Under this working condition, the number of tie rods should be at least 6, and the diameter of each tie rod should not be less than 30mm. Otherwise guess what? During the pressure test, the tie rod broke directly, but fortunately, no one was hurt.
In a vacuum environment, the direction of pressure thrust is reversed-the outside atmospheric pressure presses the bellows inward. At this time, you can't use a tie rod (the tie rod is tensile and can't be pressed), and you must put on a hoop or external pressure structure. LikeThe double hinge expansion joint of air-cooled island vacuum pipeline relies on the external hinge and hoop to resist negative pressure together.
IV. Reinforcement practice in installation and commissioning: Is the tie rod nut adjusted? Can the screw be removed or not?
Do you want to adjust the tie rod nut?To adjust, but in the right direction. When the expansion joint leaves the factory, the tie rod nut is usually in a "pre-tensioned" state, with the aim of preventing bellows from sloshing during transportation and installation. After installation in place, the nut should be loosened to the clearance required by the design (generally 1~2mm) to ensure that the bellows can expand and contract freely when the pipe is thermally expanded. Don't screw it to death. Screwing it to death is equivalent to turning the expansion joint into a rigid connection, and there is no compensation effect at all. WeThe function of the expansion joint tie rod is to "limit the position but not limit the death".
Can the screw be removed or not?According to the situation. If it is a screw for temporary transport fixing (welded or bolted at factory), it must be removed after installation. But if it is a tie rod as a permanent reinforcement, then don't dismantle it, only adjust the nut. Many engineering teams tried to save trouble and removed all the tie rods. As a result, as soon as the pipeline heated up, the bellows directly cracked. This lesson is too much.
Regarding "How to adjust the expansion joint tie rod nut", there is a special article on our official website, which will not be repeated here. Remember one principle: after adjustment, ensure that no less than 2 buckle threads are exposed at both ends of the screw to prevent slipping off.
5. The more reinforcement is not the better-excessive constraint may pull the pipeline away
Do you think that the more reinforcement the safer it is? Wrong! Over-reinforcement is equivalent to adding a "tightening spell" to the pipeline.
Of a chemical plantThe compound hinge transverse expansion joint itself is designed to absorb the transverse displacement. As a result, four additional tie rods were added to completely lock the transverse displacement in order to be "safe". When the pipeline is thermally expanded, the deformation stress has nowhere to be released, which directly leads to the fracture of the pipeline support. The expansion joint itself is not broken, but the pipeline is scrapped. The so-called "reinforcement" must match the design displacement. The core function of the expansion joint is to absorb displacement, and reinforcement only limits those "displacements that should not occur", such as excessive tension caused by pressure thrust, and should not limit "displacements that should occur".
How to judge "degree"? One is to look at the displacement marked on the design drawings, and the other is to look at the stiffness curve provided by the manufacturer. Such asFor straight pipe pressure balance expansion joint, the manufacturer has calculated the internal balance force, and the external tie rod is only auxiliary, so don't add additional lock; WhileGeneral-purpose corrugated expansion joint, the pre-tightening force of the tie rod should strictly follow the instructions, and can't pat the head.
Reinforcement is a subtraction to the expansion joint – removing unnecessary thrust rather than adding weight to it.
In the selection stage, the reinforcement method is set, which is 10,000 times stronger than on-site rework.