Transverse Metal Expansion Joint Selection and Installation: 5 Truths Engineers Won't Tell You
I met a customer two days ago, and the bellows cracked after only three days of testing the steam pipe. Check the reason-the lateral displacement was incorrectly calculated by 50mm, and the ordinary axial type was installed. You say it was wrong or not? Talk about transverse metal expansion joints today. This thing is seen every day in power plants, chemical and metallurgical projects, but if you really choose the right one and install the right one, six out of ten have to be reworked. I've been in this business for more than ten years, and I'll talk about five truths that engineers don't say and the manuals are vague.
I. What is a transverse metal expansion joint? What is the relationship between it and axial type and duplex hinge type?
Figure out the concept first. The transverse metal expansion joint is designed to absorb the displacement of the pipe perpendicular to the axis-the force of the pipe "running" to the side. It is different from the axial type (only push-pull), and it is not a separate category. There are two main kinds of common varieties on the market that realize lateral displacement:Compound hinge transverse expansion jointAndStraight pipe pressure balanced expansion joint。 In addition, there are variants such as curved tube pressure balance type and air-cooled island vacuum pipe double hinge.
Compound hinge transverse expansion joint, to put it bluntly, is to add a group of hinge structures in the middle of two bellows, and the two ends are transversely offset by the swing of the hinges, while the bellows itself is almost free of axial force. The straight pipe pressure balance type uses a set of tie rods and balance bellows to offset the pressure thrust, so that the main pipe bellows is only responsible for absorbing lateral displacement. Which one to choose? Depends on the working conditions. More on that later.
By the way, don't confuse "horizontal" with "sideways". The lateral displacement is the vertical movement of the tube in the horizontal plane, and the lateral direction may be vertical. Many design institutes are not clearly marked on the drawings. When you get it, you should check it yourself with the direction of the pipe system.
Second, how big is the lateral displacement? Three parameters that must be clearly calculated before selection
The first step in selection is not to turn over samples, but to calculate data. Three parameters one less, the back is all blurred.
First: Total lateral displacement.It is not the 20mm set by you patting your head, but it should be calculated according to the thermal elongation of the pipe, the position of the fixed bracket and the distribution of the middle hinge. For example, a 30-meter DN400 steam pipeline has a working temperature of 350℃ and an installation temperature of 20℃. The linear expansion coefficient is calculated as 0.012mm/m·℃, and the total elongation is 30×0.012×330≈119mm. If the pipeline is L-shaped and the short arm is 5 meters long, the lateral displacement is 119×5/30≈20mm? Wrong! This is only the lateral component caused by theoretical thermal elongation, plus installation deviation and equipment foundation settlement. To be conservative, multiply by a safety factor of 1.2 to 1.5 and start at 30mm.
The second: the angular displacement capacity of the bellows.The rotation angle that each bellows can bear is limited. Take the double hinge transverse expansion joint as an example. The rotation angle of a single bellows generally does not exceed ±4°. The transverse displacement Δ is divided by the distance L between the two hinges to obtain the angular displacement θ = arctan (Δ/L). Suppose you want to compensate for 30mm lateral displacement, hinge spacing is 500mm, θ ≈ 3.4°, in the range. But what about the spacing of only 300mm? θ ≈ 5.7°, exceeding! Either lengthen the middle pipe section or choose a larger bellows.
Third: Pressure thrust.This is easily overlooked by many engineers. The tie rods and hinges used in transverse expansion joints are designed to resist pressure thrust. Calculate: the internal pressure is 0.6MPa, the effective area of the bellows is about 0.13m² (the reference value of DN400), and the pressure thrust is 78000N, which is close to 8 tons. If there is no pull rod, the thrust directly acts on the pipe bracket, how strong must the fixed bracket be? Therefore, it is necessary to confirm whether the tie rod specification and hinge strength of the expansion joint match the working pressure when selecting the model.
3. For the same kind of displacement, why do some use compound hinges and others use straight pipe pressure balance?
Since both can compensate the lateral displacement, why should one use a double hinge transverse expansion joint and the other use a straight tube pressure balance expansion joint? The answer lies in spatial constraints.
The structure of the double hinge transverse expansion joint is relatively compact, and the hinge occupies little space, which is suitable for the crowded place of pipeline arrangement. But it has an Achilles heel: the hinge cannot withstand axial tension, so it can only be used where the pipe has sufficient axial constraint-the fixed bracket must eat the axial force to death. I have seen a project where the installer loosened the limit nut twice because the hinge was in the way, and as a result, the bellows was screwed directly like noodles during pressure test.
Straight pipe pressure balance type expansion joint is different. It comes with its own balance bellows and tie rod, the pressure thrust is internally cancelled, and there is no axial force on the external bracket. This means that it can be installed in the middle of the pipe without the need for main fixing brackets at both ends. What about the cost? The length is long, the weight is heavy, and the price is 20% to 30% expensive. So which one to choose? Simple:There is enough space at the corner of the pipeline, and the axial force is carried by a fixed bracket, and a compound hinge is used; When the straight section of the pipeline needs transverse compensation and it is inconvenient to set the main fixing frame, the upper straight pipe pressure balance type.
There is also a variant-curved pipe pressure balance expansion joint, which is specially used to solve pipes with elbows. If the space is really small, you can also consider the double hinge expansion joint of air-cooled island vacuum pipe. That thing is customized for specific scenes, so don't apply it randomly.
4. The "pit" in the installation: the draw rod, the guide tube and the fixed bracket can't be missed
Choose the right equipment and install the wrong equipment. There are three details in the installation process, and one accident is enough for you to remember for a lifetime.
Adjustment of tie rod nuts.Many installers screw the locking nut on the tie rod tightly, thinking that the tighter the better. SO WRONG! The tie rod of the transverse expansion joint is in a pre-stretched state during transportation. After being installed in place, the adjustment nut must be loosened to let the bellows be at a free length-otherwise, once the transverse displacement occurs, the tie rod will directly resist the limiter, and the bellows can't move at all. Correct practice: First loosen the locking nut, check whether the bellows expands and contracts smoothly, and then readjust the nut position according to the designed pre-displacement. Regarding this, "How to Adjust the Tie Rod Nut of the Expansion Joint" is clearly written in the question and answer, so you can flip through it.
Orientation of the guide tube.The guide tube is a liner tube installed on the inner wall of the bellows, and its function is to guide the flow direction of the medium and prevent the high-speed airflow from directly washing the bellows. But playing backwards is equivalent to playing for nothing. The principle is that the opening direction of the guide tube must be toward the direction of the incoming medium. The steam pipe is okay, but if it is the flue gas pipe, it will wear out in two or three months if the guide tube is installed backwards. Before, one of our customers consulted "the specific function of the expansion joint guide tube", and the focus was on anti-scouring, so don't be careless.
Strength of the fixed bracket.Transverse expansion joints have stricter requirements for fixed brackets than axial types. Because it has to resist not only the pressure thrust, but also the bending moment generated by the lateral displacement. The welds at the joints between supports and pipes must be inspected for flaws, and the specifications of anchor bolts should be calculated for shear strength. I have seen a project in the cement industry, which cut corners with metal corrugated expansion joints and fixed brackets in the cement industry. As a result, the bracket was crooked and the corrugated pipe was cracked after half a year of operation. Do you think it's worth saving that steel money?
V. Practical case: Analysis of lateral displacement compensation scheme of steam pipeline in a power plant
Tell me the truth. At the beginning of this year, I helped a 300MW unit to deal with the vibration problem of the main steam pipeline. Parameters: DN450, PN4.0, temperature 540℃, pipe direction is Z-shaped, and the distance between two fixed brackets is 18 meters. In the original design, two axial expansion joints were used. As a result, the transverse offset of the pipeline reached 45mm, and the expansion joint bellows was directly twisted and scrapped.
When we went to the site, we found that the lateral displacement mainly came from the lateral thrust generated by the thermal expansion of the middle pipe section. Modification scheme:Install a double hinge transverse expansion joint at the bend of the pipe(It just happens that there are double hinge transverse expansion joint products in our station), remove the original axial type, and keep the other axial type as an auxiliary. The hinge spacing of the compound hinge is set to 600mm, and the bellows model is DN450/4.0/4 wave. The calculated angular displacement is about 4.2°, which is within the safe range. The tie rod specification is M30×3, the material is 40Cr, and the treatment is tempered and tempered.
During installation, special attention was paid to the direction of the guide tube-towards the direction of steam flow, and the fixing bracket was re-strengthened, adding two thrust brackets. After a return visit after three months of operation, the vibration decreased from the original 8mm/s to 1.2mm/s, and the lateral displacement was completely absorbed. Customers themselves said that if they had known the energy-saving solution of lateral expansion, they would have taken a detour for two years before.
What does this case say? Do not copy samples when selecting, but measure and calculate on-site for verification. When the transverse displacement of steam pipeline in power plant is large, priority should be given to the transverse type of compound hinge or the pressure balance type of straight pipe, and do not make do with the axial type.
6. Common faults and maintenance: Don't wait for leakage to regret it
The bellows is cracked, the tie rod is broken, and the guide tube falls off. Once the first two occur, they will be shut down immediately; The last one will last a while, but sooner or later something will happen.
Bellows crackingMost of them are caused by stress corrosion. There are many impurities in the steam pipeline, and chloride ions and sulfide are enriched on the surface of the bellows. Coupled with alternating stress, cracks start to drill in from the trough. What to do? Consider corrosion resistant grades, such as 316L and Incoloy825, when selecting materials. In addition, do regular non-destructive testing and magnetic particle testing to detect surface cracks-don't wait until it leaks before changing. The service life of the expansion joint is generally designed to be 100,000 cycles, but it has to be replaced once every three or five years when the actual working conditions are bad.
tie rod fractureIt is usually because the preload is out of control or the hinge is stuck. During installation, the tie rod nut is not adjusted in place, and the tie rod will break when the impact load exceeds the yield point during operation. Prevention is according to "The Function of Expansion Joint Tie Rod". Regularly check whether the tie rod is bent and whether the thread slides. Every time the machine is stopped for maintenance, loosen it and then tighten it to release the accumulated stress.
Guide tube falling offMost of the reasons are poor welding quality, or the medium flow rate is too fast to burst the weld open. Once the guide tube falls into the pipe, not only will the bellows lose protection, but it will also block the pipe. Repairs must disassemble the expansion joint, reweld or replace the deflector. By the way, by the way: the expansion joint and the compensator are the same kind of thing, so don't be confused by the call law. There are also general corrugated expansion joints, external pressure single axial joints, sleeve pipe expansion joints, etc. in our product list. Different structures are selected for different working conditions, but the maintenance ideas are similar.
Transverse metal expansion joints are not installed once and for all. See it once a day in the first week of operation, once a week in the first month, and inspect it at least quarterly afterwards. If you find that the tie rod nut is loose, the guide tube has abnormal noise, and the bellows surface has discoloration or pits, you should deal with it quickly. Don't wait until the leak alarm goes off. That's when you lose more than one expansion joint.