What exactly is Lajin? It's not the same thing as a tie rod and a screw
Two days ago, I met a customer and called in a hurry: "The bellows on our site is cracked again. Hurry up and send me two expansion joints with tension bars!" Guess what? He already had a tie rod on his tube, but he used it as a tie bar.
A pull bar (also called a limit pull bar) and a pull bar are two different things. The tie rod generally appears on the expansion joint of large tie rod or the transverse expansion joint of compound hinge, and its main function is to bear the internal pressure thrust and control the bellows from swinging laterally. What about Lajin? It is more like a safety rope-the flanges at both ends of the expansion joint are connected with wire rope or flat steel to prevent the bellows from excessively extending or compressing under pressure. As for those screws that adjust the pre-deformation during installation, they are temporary and have to be removed after installing the pipeline (refer to the question and answer in our station "Do the screws of expansion joints need to be removed").
To put it bluntly, tie bars are permanent passive protection devices, while tie rods are structural parts under design conditions. Don't get mixed up, the consequences are serious.
Under what circumstances must Lajin be installed? Three core scenarios
Not all metal expansion joints have to be fitted with a tie bar. But when you encounter the following three scenarios, you can wait for rework without pretending.
Scenario 1: High pressure steam pipeline.With high steam temperature and high pressure, once the bellows accidentally fails (such as fatigue crack), the internal pressure thrust can instantly push the two ends of the pipe away by several centimeters or even tens of centimeters. The expansion joint with the guide tube is better, but if the straight tube pressure balance expansion joint or the external pressure single axial expansion joint has no tension bar restriction, the consequence is a flying tube. We did a post-event analysis for a power plant. One of their DN600 steam pipes used a universal corrugated expansion joint without a tie bar. As a result, the pipe was misaligned by 15cm after the bellows was torn, and the bracket was directly pulled down.
Scenario 2: Pipelines in earthquake-resistant or settlement areas.In chemical parks, earthquake fortified areas, or where the foundation has soft soil settlement, the pipeline will generate additional unexpected displacement. At this time, the function of the tie bar is not to absorb the displacement, but in case the bellows body is damaged, the tie bar can hold the bottom and prevent the pipe from disengaging. To put it bluntly, it is the last line of defense, similar to a safety rope. Like the metal corrugated expansion joint we supply to the cement industry, if the site passes through the factory building, we generally suggest installing tension bars.
Scenario 3: Large diameter expansion joint mounted vertically.The expansion joint is installed on the vertical pipe section, and the gravity plus the self-weight of the bellows will cause the bellows to stretch unexpectedly if the internal pressure fluctuates. This kind of thing is especially common in the double hinge expansion joint of air-cooled island vacuum pipeline-the bellows itself has to bear negative pressure under vacuum conditions. Once the selection margin is insufficient and there is no tension bar to limit the elongation, the bellows will easily be deflated or cracked.
Consequences of not installing tension bars: Pressure thrust can push the pipe flying
You think the bellows tear is just a minor problem? Wrong. How big is the internal pressure thrust? A simple calculation: DN500 pipeline, working pressure 1.0MPa, thrust = pressure × effective area ≈1.0× (π/4×0.5²) ≈196kN, that is, nearly 20 tons of force. This force is usually balanced by the rigidity of the bellows itself and the pipe support. However, once the penetrating crack appears in the bellows, the 20-ton force instantly loses its constraint and directly acts on both ends of the pipeline. Think about it, can ordinary supports and hangers block the sudden release of 20 tons of thrust?
On high-temperature, high-pressure, toxic and flammable media pipelines, it is equivalent to violating the regulations if the metal expansion joint is not equipped with tension bars. To put it harder, this is risking your life to save money.
By the way, some designers will confuse a tie bar with a tie bar, thinking that you don't need a tie bar if you have a tie bar anyway. Typical logical loophole: the tie rod is an active force-bearing part, and it will also fracture by fatigue. Lajin is a standalone secure backup, and the two do not conflict.
Everything will be fine with Lajin? Pits for installation and adjustment
Alas, loading Lajin looks simple, but in fact, there are many faults.
Pit 1: The preload force is adjusted too much.The tie bar is usually wire rope or flat steel with threaded adjustment at both ends. For the sake of "insurance", some installers tighten the tension bar tightly, and as a result, the compensation ability of the expansion joint is directly locked. The bellows should be expanded and contracted, but it can't move by the tension bar, so it can only absorb stress in frustration-the fatigue life plummets. This is the same as when you jam the retractors of your seatbelt.
Pit 2: Tie bar and bellows interference.Especially on large-diameter thick-walled expansion joints or rectangular expansion joints, if the position of the tension bar is not properly arranged, it will rub against the bellows surface. After the protective layer is worn off, the bellows wait for stress corrosion to crack. The FAQ of "How to adjust the tie rod nut of the expansion joint" in our station is very detailed, but the adjustment logic of the tie bar is similar: the gap should be reserved, and it can't be stuck to death.
Pit 3: The thermal adjustment is ignored.Some expansion joints are loose when installed in cold state, but the displacement changes after the pipeline heats up, and the tension becomes tight instead. The correct method is to adjust the expansion joint itself according to the designed cold tightness or thermal pre-bias, and then install the tension bar. After the tension bar is locked, the actual displacement margin of the bellows is left. We are inrubber compensatorAndUniversal corrugated expansion jointThis is emphasized in the installation instructions of.
How to judge whether the expansion joint on your hand needs to be installed with a stretch? Self-test checklist
Don't listen to the outside blind fooling, directly judge according to the following comparison:
- Is the pipeline medium pressure ≥0.6MPa? Yes → Suggested to install.
- Does the pipeline transport high-temperature steam (≥200℃) or flammable and explosive media? Yes → Must be installed.
- Are expansion joints used in settlement areas or earthquake fortified areas? Yes → Strongly recommended to install.
- Is the expansion joint mounted vertically and has a diameter ≥ DN300? Yes → Suggested to install.
- Is the existing expansion joint already equipped with a tie rod (e.g. large tie rod, double hinge type)? -There are tie bars but it is still recommended to evaluate additional tie bars, especially single hinges may not be able to constrain accidental failure elongation.
- The expansion joint on your hand isCorrugated expansion joint for power station industryOrDouble hinge expansion joint for air-cooled island vacuum pipelineSuch demanding products? When leaving the factory, it will usually be equipped with a stretch bar.
Then again, some customers like to buy their own wire ropes to tie on, which I don't recommend. The tension bar must be designed and checked together with the expansion joint, including the breaking tension, fatigue life and corrosion resistance grade of the tension bar-for example, the high-temperature axial expansion joint next to the desulfurization flue gas baffle door has to be made of 316L or even C-276, otherwise it will rust and break in half a year, and installing it is equivalent to white clothing.
So, should metal expansion joints be fitted with tension bars? My answer is: install it when you need to install it, and don't wait until the bellows are torn, the pipeline is misaligned, and the production is shut down for emergency repair. Leave professional matters to professional people, but at least you have to understand this logic, otherwise you won't know if you are fooled.