Two days ago, a customer who made steam pipes called and asked, "Your duplex metal expansion joint, with two bellows strung together, can really withstand the lateral displacement?" I asked him back, "Try with a bellows alone. Regardless of whether it is axial or horizontal, it can translate you by a few millimeters at most. If it is bigger, it will suffocate to death." He chuckled and placed the order.
To put it bluntly, the core of the duplex metal expansion joint is three words:Division of labor and cooperation。 If a bellows can't bear it, then two of them, with a tie rod and hinge, eat the displacement dry and wipe it off.
1. Where is the duplex structure? — — Break clear with single expansion joint
Single type corrugated expansion joint, simple structure, just one bellows plus two end connections. It can absorb axial expansion and contraction and can also withstand some lateral displacement, but of limited magnitude. For example, a single expansion joint with a nominal diameter of DN200 allows a lateral displacement of more than ten millimeters at most. If you let the pipe turn a corner in a small space, the temperature rises, the thermal expansion runs tens of millimeters sideways, and the single type strikes directly-the bellows becomes laterally unstable and leaks.
What about the duplex? The two bellows are connected with an intermediate pipe in the middle, and the two ends are equipped with pull rods or hinges. The function of the tie rod is to bear the thrust generated by internal pressure and prevent the bellows from being pulled into "noodles". The hinge restricts the bellows from bending in only one direction. In this combination, each bellows shares a part of the lateral displacement, and the total displacement can be doubled. Calculate, the two bellows each bear 30mm transverse, which adds up to 60mm. Is this the reason?
Moreover, the duplex structure can also absorb angular displacement, like twisting twists. Single-style? Don't even think about it.
2. What are the common compound metal expansion joints on the market? From compound hinge to bypass pressure balanced type
Among the varieties we have listed, there are several models related to duplex, so don't get confused.
- Compound hinge transverse expansion joint: This is the most typical. Two bellows, with a set of hinges in between (usually two hinge plates appearing in pairs), allow lateral movement only in a plane perpendicular to the pipe. Suitable for scenarios where pipe routing needs to be shifted sideways but axially does not allow to be compressed. For example, if you connect it from the outlet of the equipment, and the thermal expansion of the equipment pushes it to the side, you can use this.
- Compound straight pipe bypass pressure balanced expansion joint: This structure is a bit more complicated, in addition to two working bellows, there is also a balanced bellows (or bypass pipe). The purpose is to offset the blind plate force generated by internal pressure, so that the expansion joint does not bear axial thrust. It is particularly fragrant when used on high-pressure large-diameter pipes. Do you think that the DN500 steam pipe with a pressure of 2.5MPa has tens of tons of thrust alone. Do you let the pipe support carry it hard? That's not seeking death. With this, balance yourself, and the bracket only bears the weight of the tube.
- Straight pipe pressure balanced expansion joint: Similar to the above one, but without a bypass tube, the pressure is balanced by the internal structure of two bellows. It also absorbs axial and lateral displacement, making it suitable for places with tight spaces.
- Double hinge expansion joint of air-cooled island vacuum pipeSpecially used for air-cooled island exhaust pipe, the double hinge structure ensures that it can flexibly absorb displacement under vacuum, and does not leak.
We have detailed parameters and drawings on these products on our website, so you can turn them over by yourself. Remember when choosing:Displacement direction, displacement amount, pressure level, pipe materialNone of the four elements can be missed.
Third, under what circumstances must we use duplex? Space, displacement, pressure, all are indispensable
Why should I spend more money on a duplex if I can solve it with a single style? Tsk, reality often doesn't allow it.
Space constrained。 For example, in the pipe gallery, there is only 300mm between the pipes. If you want to absorb the thermal expansion by natural compensation (L-shaped or Z-shaped elbow), there is no place to turn at all. At this time, plug a duplex hinge expansion joint, move across and eat it, perfect.
Large lateral displacement。 Some pipes, such as the outlet flue of power plant boilers, can have a thermal expansion displacement of more than 100mm. Single bellows can't hold it at all, so you have to use duplex, or add a set of universal hinge duplex.
High pressure needs to be balanced。 Just like the steam pipe mentioned earlier, the pressure is over 1.6MPa, and the axial thrust is scary. If you use an ordinary duplex, the pull rod has to be as thick as your arm. It's better to go directly to the pressure balance type to digest the thrust. It saves the cost of the bracket and is safe.
When you find that the allowed displacement value in the parameter table of single bellows is not enough, or the thrust is too large for the bracket to hold, you should decisively go to double bellows.
Fourth, selection and installation, don't step on pits: how to match the tie rod, hinge and guide bracket
Installing duplex metal expansion joints, I've seen too many rollover cases. Most common errors:Forgot to remove the transport fixture。 In order to prevent transportation vibration damage when leaving the factory, manufacturers usually screw the nut or weld the stud on the tie rod. Some people install it directly without dismantling it. As a result, as soon as the pipe heated up, the bellows didn't move at all, and the pipe buzzed. When you take it apart, the fixtures are still welded! Alas, do you think it is irritating.
Improper adjustment of tie rod nut。 The tie rods on the expansion joints are usually pre-stretched or pre-compressed when they leave the factory, and are adjusted according to the actual installation temperature of the pipeline. Many construction teams get them, whether they screw them to the tightest or loosen them completely. The correct way is to adjust the nut with a special wrench according to the cold tightening value given by the design. How to adjust it specifically, refer to the question and answer "How to adjust the tie rod nut of the expansion joint" on our website. There are pictures and truth.
Guide bracket is not fitted correctly。 Generally, the double expansion joint can only absorb the lateral displacement in one direction. If you don't add a guide bracket to restrict the freedom in other directions, the bellows will twist randomly, and the life will be greatly reduced. For example, the double hinge transverse type, you must add guide brackets on both sides of the expansion joint to ensure that the pipe only moves in the design direction. The spacing of brackets is also particular, generally no more than 4 times the nominal diameter of the pipeline.
By the way, be sure to check the coaxiality of the pipeline before installation. If the deviation exceeds 1%, don't hard install it. Adjust the pipeline and do it again.
5. How many years did it go wrong? Common faults of compound expansion joints and on-site emergency measures
It is impossible for equipment to never fail, the key is that it will be dealt with if it does.
Corrosion perforation of bellows。 Especially used in pipelines of flue gas, desulfurization and chemical plants, the medium contains acid gas. When you choose materials, if you don't choose the right stainless steel grade, or don't add a deflector, it will leak after a few years. Emergency measures: The small hole can be plugged with metal repair agent, but don't last too long. It must be replaced within two months at most. Big hole? Just stop the line and change, don't hesitate.
Fatigue crack。 If the pipeline vibrates frequently, or the displacement margin is left too little when the expansion joint is selected, the fatigue life of the bellows will be greatly shortened. You can use ultrasonic thickness measurement on the spot. If you find that the wall thickness is thinned by more than 20%, prepare spare parts. Emergency? Drill crack stop holes at both ends of the crack to temporarily reduce vibration, or add an outer protective sleeve.
Loose or broken tie rod。 The tie rod is subjected to a huge internal pressure thrust, and if the nut is loose, the bellows will be elongated and deformed. When you hear the "click" sound of the pipeline, quickly stop the machine for inspection. Reinforcement method: Replace high-strength bolts or install anti-loosening gaskets. Sometimes hinge pins also wear out, and regular oil filling can extend life.
Guide bracket stuck。 The bracket is rusty or deformed, the pipe can't move, and the expansion joint is hard to carry. This situation is very covert. The temperature of the tube is normal when you touch it, but the appearance of the expansion joint has bulged. Solution: Clean the bracket, add grease, and replace the sliding pad if necessary.
Of course, the best emergency is prevention. Regular inspection, conduct bellows appearance inspection every six months, and measure whether the displacement is within the design range. Don't wait until something happens to regret it. It costs more than one expansion joint.
Okay, that's all for today about the duplex metal expansion joint. If you have specific working conditions at hand, you may wish to flip through the product information on our website, or contact the technology directly to help you calculate which one to use. Remember, if you choose the right one, the pipeline will be steady; Wrong choice, air leaks every three days.