What is the limit device?
Let's make it clear why you need a limit first. How much displacement can the bellows itself withstand? To tell you the truth, it can only withstand limited axial compression or stretching. You let the bellows stiffen for displacement beyond the design range, and the result is instability-the bellows crumbles into a ball like a deflated can, or fatigue breaks, and the medium in the pipe spurts directly out. Therefore, the function of the limiting device (pull rod, nut, limiting ring, guide sleeve, etc.) is to "block this displacement in the safety line". To put it bluntly, it is to install a "brake" on the bellows to keep it from running over the top.
Limit tie rods and transport protection screws are not the same thing. The transportation protection screw is temporarily fixed when it leaves the factory to prevent the bellows from shaking during transportation, and must be removed before installation. The limit rod is a permanent component, which should be retained and adjusted according to the working conditions. Two days ago, I met a customer, and the limit tie rod was completely disassembled as a transportation screw. As a result, as soon as the pipeline heated up, the bellows was directly pushed onto the guide tube. Not only was the guide tube deformed, but the bellows also cracked. Don't make such low-level mistakes.
What are the differences in the design of limiting devices under different working conditions?
Let's go through it one by one according to the product classification of this site.
Universal corrugated expansion joint: Most commonly, the thermal displacement of the pipeline is not large, and it is equipped with a simple tie rod-a few screws plus nuts, and the limit nut is screwed to the design position. For example, for steam pipelines, the temperature is around 200℃, and No.20 steel is enough for the tie rod material.
High temperature axial expansion joint: This is a particular one. The thermal displacement is large, the temperature is always five or six hundred degrees, and the ordinary tie rod will creep-slowly deform over time, and the limit will fail. Therefore, heat-resistant alloy steel (such as Incoloy 800H) should be used, and anti-loosening gaskets should be added to the nuts to prevent the bolts from loosening due to high-temperature vibration. In addition, consider insulation: the tie rod is too close to the bellows, and the heat radiation will make the tie rod soft. Therefore, when designing, the tie rod is generally placed outside the insulation layer.
Large tie rod expansion joint: Used for lateral displacement compensation, such as Z-pipe sections at pipe bends. The limit structure is more complicated, the tie rod is thick and numerous, and it should be matched with hinges or universal joints. When installing, the direction of the tie rod must be consistent with the displacement direction, otherwise it will be stuck-what should I do if it is wrong? Reassembling, labor-consuming and time-consuming, and easy to damage the bellows.
Straight pipe pressure balanced expansion joint: This balance the blind plate force by double bellows and limit tie rods. What is blind board force? The thrust generated by the internal pressure of the pipeline at the end, the larger the pipe diameter and the higher the pressure, the more terrifying this force. The limit tie rod inside the straight pipe pressure balance expansion joint has to bear all the blind plate force, so the cross-sectional area and quantity of tie rod must be calculated clearly, and cannot be estimated by experience.
By the way, mention smoke baffle door and insulation door products. Their restraint devices are biased towards mechanical stops-for example, when the flap door is opened to 90°, a restraint block holds against it to prevent overopening. It is completely different from the logic of the pipe compensator. The compensator allows the pipe to expand and contract but limits the amplitude, while the baffle door allows the door to stop at the designated position and not sway. Two different things, don't mix.
Installation and adjustment: How to screw the tie rod nut correctly?
This is the most frequently asked question at the scene. Refer to the Q&A "How to adjust the tie rod nut of the expansion joint" and "Does the screw of the expansion joint need to be disassembled" on this site. There is only one core step:Loosen all nuts during installation so that the expansion joint is free。 Why? Because the pipeline has not been welded and fixed, if the expansion joint is locked by the tie rod, as soon as the pipeline shrinks after welding, the bellows will be forcibly stretched, and it will be pre-damaged before it runs.
After the pipeline is installed in place and cold tightening is completed, adjust the limit nut according to the design drawing. How to tune it? It's not just tightening by feel. Working Displacement Range and Cold Tight Value are labeled on the design drawings. For example, if the axial compensation amount is 50mm, and the compression amount is 30mm when cold and tight, the limit nut should be locked in the compression position of 30mm, and the margin of working displacement should be left-generally 10% ~15% of the total compensation amount. If it is locked, the bellows can't move, so it loses its compensation function.
In addition, special attention should be paid to the limiting devices of hinge expansion joints (such as double hinge transverse type and air-cooled island vacuum pipe double hinge expansion joints): the hinge axis must be consistent with the displacement direction, and it should be calibrated with a level before installation. In reality, many faults are because the hinge shaft is assembled crooked, which leads to thermal expansion of the pipe, the hinge is stuck, and the bellows is forced to twist and deform.
Troubleshooting and Maintenance of Limit Device
There are three most common types of failures.
- Bending deformation of tie rod: It indicates that the displacement has exceeded the limit, or that the pipe support has failed resulting in additional load. The inspection method is simple: visually inspect whether the tie rod is straight, or use a caliper to measure the concentricity of both ends. If you find bending, check the direction of the bracket and pipe first, and don't rush to change the tie rod.
- Loose nut: Caused by vibration or high temperature. Especially for steam pipelines, the temperature difference is large when starting and stopping repeatedly, and the nut is loose after thermal expansion and contraction several times. Add spring washers or double nuts during maintenance, and check tightening torque every six months.
- Limit block wear: Under frequent vibration conditions, the limit block repeatedly collides with the tie rod or flange, and the groove is worn out. If the wear exceeds 2mm, it should be replaced, otherwise the limiting accuracy will decrease.
If it is found that the guide tube of the expansion joint is deformed or the bellows is cracked, 80% of it is that the limiting device has a problem first. For example: the guide tube holds up but the limit does not work, which means that the limit nut is in the wrong position or the tie rod is broken, resulting in excessive compression of the bellows and deformation of the guide tube. This fault logic should be clarified. Don't change the bellows when you come up. Check the limit first.
Let's talk about non-metallic expansion joints and rubber compensators. Although they do not use metal tie rods, the limiting devices are usually realized by frames and bolts. For example, there are pressure plate bolts around the rectangular non-metallic expansion joint, which tighten the displacement in the direction of limiting the thickness of the expansion joint. The tightness also needs to be checked regularly-rubber and fabrics will age and shrink, bolts tend to loosen, and tightening too tightly may crush non-metallic materials. Therefore, we should operate according to the torque value given by the manufacturer.
Several Details That Are Easily Overlooked in Model Selection
First, the medium temperature and pressure directly affect the material selection of the limiting device. Carbon steel tie rods can be used in ordinary working conditions, but if the temperature exceeds 400℃, heat-resistant alloy steel (such as 1Cr18Ni9Ti) has to be replaced. Otherwise, high-temperature creep will cause the tie rod to slowly elongate and limit failure. In the case of high pressure, the diameter of the tie rod should be calculated according to the blind plate force, so you can't choose it just by looking at it.
Second, the pipeline layout determines the limit form. L-shaped pipe sections are limited by hinges, Z-shaped pipe sections are limited by compound tie rods, and straight pipe sections are limited by axial directions. Once, I met a person from a design institute who used the Z-shaped pipe section as an axial limit. As a result, the lateral displacement of the pipe kept the tie rod from bending. To put it bluntly, the direction of the limiting device must be consistent with the main direction of pipeline displacement.
Third, the blind plate force must be counted. Especially for large pipe diameter (above DN500) or high pressure system (above 1.6MPa), the blind plate force may reach tens of tons. At this time, the cross-sectional area and number of limit tie rods must be able to withstand this thrust, and the safety factor (generally 1.5 times) should be left. There are specific calculation examples in the two questions and answers of "Correct Installation Method of Large Tie Rod Expansion Joint" and "Function of Expansion Joint Tie Rod" in this product, which can be turned out.
When selecting the model, don't only look at the expansion joint itself. The limiting device is linked with the pipe support. If the design of the pipeline bracket is unreasonable (for example, the fixed bracket is not strong enough), no matter how strong the limit device is, it is useless, and the whole pipeline system will still shake. Therefore, it is best to do the pipeline stress analysis together when selecting, so don't save this money.