Why is there a "diameter reduction" demand in the pipeline system?
A main steam pipe of DN800, after entering the heat exchanger, the outlet becomes DN500-how to connect it? Just use the reducer to attack? It works, but what about thermal expansion and cold contraction? The thermal displacement of the pipe will not disappear because of the change of diameter, but the stress will be more concentrated at the reduction diameter. That's when it's timeMetal expansion joint with variable diameterEnter the stage. It combines the two functions of "reducing diameter" and "compensating displacement" into one, and one component solves two pain points.
The reduction scene goes far beyond steam pipes. For example, in the desulfurization flue gas system, the flue is cut from rectangular to circular, and the diameter is reduced from 2m to 1m. The temperature and corrosive airflow act at the same time-the ordinary expansion joint can't be plugged in, so the variable diameter metal expansion joint becomes the only choice. Another example is the pipeline at the pump outlet. The interface size of the pump is often different from that of the main pipeline, and it needs to absorb vibration and thermal displacement. You can't weld a reducer in the middle of two flanges in order to align the ports, right? Space often doesn't allow it either.
To put it bluntly, the variable diameter metal expansion joint was not thought up by the designer, but forced out by the scene-thermal displacement, space constraint and mismatch of interface sizes of different equipment. These three conditions are put together, and it is the optimal solution.
What is the special structure of variable diameter metal expansion joint?
Asymmetry. The diameter of the two ends is different. How to design the bellows? If you directly weld a section of standard corrugated pipe with a large flange at one end and a small flange at the other end, the corrugated pipe will generate an axial force component under pressure-because the force area at both ends is different. This force will push the expansion joint to displace in the direction of small diameter, which may break the pipe support.
Therefore, the corrugated pipe of the reduced diameter metal expansion joint mustTapered designOrSegmented taperLet the wave crest line gradually change along the axis to ensure that the meridional stress of each wave is uniform. Another approach is to add a section of tapered joint in the middle of two universal corrugated expansion joints of different diameters, but this structure takes up a lot of space and has an extra weld, which increases the risk of leakage. For the truly mature expansion joint with variable diameter, the bellows body is tapered, and the wave height and pitch change gradually along the axial direction.
Another detail that is easy to miss isguide tube。 The diameter of the two ends is different, and the guide tube has to change the diameter according to it, otherwise vortex will occur when the medium flows through the cone section, which will accelerate wear. The reducing angle of the guide tube is generally controlled within 15°, and the pressure loss increases sharply beyond this value. It is mentioned in the FAQ that the specific function of the guide tube is to protect the bellows from erosion and reduce the flow resistance. This iron law on the reducing model is equally applicable, and it is more critical.
The three easiest pits to step on when selecting a model
First Pit: Pressure Rating by Big End or Small End?Some people think that it is safe to press the big end, but in fact, it is just the opposite-the pressure bearing capacity of the bellows depends on the smallest section (small end), because the small end has the smallest trough radius and the most serious stress concentration. The correct approach is that the pressure level of the small end meets the system requirements, and the large end naturally meets the system requirements. If you choose the big end in reverse, the small end is likely to be overloaded.
The second pit: Should the deflector be equipped?In many working conditions, in order to save money and trouble, Party A said that "the medium is clean, and there is no need for a guide tube". However, the internal fluid state of the variable diameter expansion joint is complicated. Even if the medium is clean, the high-speed airflow will generate strong turbulence when passing through the conical section, which directly washes the root of the bellows. The correct logic is: as long as the medium flow rate exceeds 8m/s or contains solid particles, it must be equipped with a flow guide tube. Moreover, the length of the reducing diameter section of the guide tube should not be too short, otherwise the streamline transition will not be formed.
The third pit: How to adjust the tie rod?The function of the expansion joint tie rod is to limit the excessive stretching or compression of the bellows. Due to the unbalance of axial force in the reducing diameter model, the adjustment of the tie rod is more particular. When installing, screw the tie rod nut to the standard length (the position calibrated by the manufacturer when leaving the factory), and then fine-tune it after the system presses. In the FAQ, there is a special article "How to adjust the tie rod nut of the expansion joint", which is very clear-first loosen the locking nut, adjust it to the design compensation amount, and then lock it. However, it must be noted that the force at both ends of the tie rod is different. When adjusting, it is best to twist it symmetrically with a torque wrench to prevent the bellows from deflecting caused by too much twisting on one side.
Compared with the general corrugated expansion joint and the straight pipe pressure balance expansion joint, should the reduced diameter model be used?
To be honest, the first reaction of many engineers when they encounter a reducing connection is to put two firstUniversal corrugated expansion jointAdd a reducer in the middle, or useStraight pipe pressure balanced expansion jointAdd transition flange. The advantages of these two combination schemes are that they use standard parts, are cheap and quick to purchase. But at what cost?
- Universal scheme: Two expansion joints + reducer + four pairs of flanges + eight bolts, the length is increased by at least 300mm, and the installation space is often insufficient. Moreover, each flange connection surface is a potential leak point.
- Straight pipe pressure balance type schemeThis expansion joint can absorb axial forces and pressure thrust by itself, but it is designed for equal diameter pipes. For reducing diameter, you have to add an extra large and small head flange. The result is a spike in axial length and a doubling in cost.
Is space constrained?If restricted, go to the reduced diameter model.Leak risk tolerance?The fewer connection points, the better the reducer model.Cost budgeting?The purchase price of a single piece of reducing diameter is higher than that of the standard combination, but from the perspective of comprehensive installation, maintenance and reliability, the cost of one overhaul is enough for you to buy how many reducing diameter expansion joints. Do the math for yourself.
Those "hidden rules" at the installation site
Old masters who have installed expansion joints know that there are several details that are wrong and directly reworked. First,Can the screw be removed or not?The transport screw is used to fix the bellows and is installed in placeMust be removedOtherwise the expansion joint does not work. However, note: due to its own unbalanced force, sometimes the manufacturer will keep a pair of tie rods as protection. Don't disassemble the protective tie rods when disassembling the transportation screw. How to distinguish? Look at the logo-the transport screw is generally painted with red paint, and the protective tie rod is painted with yellow paint. The frequently asked question and answer "Does the screw of the expansion joint need to be disassembled" is more detailed, so you can refer to it.
Second,How do you see the direction of the arrow?The arrow on the shell of the reducing-diameter expansion joint is not the medium flow direction, but isInstallation direction-The arrow is pointing at the small end. Why? Because the tapered structure of the guide tube is directional, when the guide tube is installed backwards, it becomes a flared opening, and a suction effect occurs when the medium flows through. The back of the bellows is sucked by negative pressure, and it will crack by fatigue in less than half a year. Just point the arrow in the direction of the small end pipe before installation.
Third,Which way to put the reducer end?This problem is strongly related to the flow direction of the medium. The general principle is: the small end is close to the pump outlet or the heat exchanger outlet, and the large end is close to the main pipe. Because the small end has a high flow rate, proximity to the device can reduce eddy current disturbances. However, if the medium contains particles, it is recommended that the large end rely on equipment and the small end rely on downstream, so that the particles can slow down and settle in the expansion section. There is no absolute standard, it depends on the specific working conditions. Remember one sentence: Don't blindly change the installation guidance given by the manufacturer. Calling to ask for clarity is a hundred times worry-free than exchanging goods afterwards.