Anyone who has been to the expansion joint production workshop knows that the protective tie rod looks really inconspicuous. Two screws plus a few nuts are threaded to both sides of the bellows. But you would be wrong to think of it as a normal shipping fixture.
1. The tie rod is not a decoration: first find out the division of labor between it and the limit rod and the lining guide tube
Is this pole to be removed before shipping? This question is asked almost every week. Whether to dismantle or not depends on whether you have a pull rod, a limit rod or a guide tube in your hand-the three do completely different jobs.
The lining guide tube is inside the pipeline, which is responsible for allowing the medium to flow smoothly, reducing turbulence and wear, and by the way, blocking the bellows from being washed by particulate matter. As for the limit rod, its function is to limit the displacement of the bellows in a certain direction, allowing the displacement but not exceeding the design value, which is commonly found in hinge or universal hinge products. The protective tie rod is different. It protects the bellows from being broken by excessive pressure thrust or blind plate force. In the final analysis, it is an insurance for the bellows.
2. What happens without a tie rod? How do these accidents happen, such as blind plate force of pipe system and instability of bellows
Let's talk about a real scene. A DN600 steam pipeline has a design pressure of 1.6MPa and a cross-sectional area of approximately 0.28 square meters. The bellows will produce a blind plate force under the action of pressure, which is close to 45 tons. This force may pull the expansion joint directly off at the pipe elbow or blind plate. What is the concept of 45 tons? A heavy truck pressed on it. The bellows without a tie rod is a thin-walled piece that can bear tension under this working condition, and it can't bear it at all.
Planar instability and column instability. Plane instability shows that one side of the bellows is concave and the other side is convex. When the column is unstable, the whole bellows bends like a column. In both cases, because the axial displacement is too large or the pressure is too high, additional axial force is generated at the effective cross-sectional area of the bellows. At this time, the tie rod becomes the line of defense at the bottom, constraining the displacement within the design range.
3. How to adjust the tie rod nut so that the bellows can deform freely without excessive displacement? Key points of installation and commissioning
The debugging of tie rod nuts, to put it bluntly, is to find a balance point. The nuts on both sides of the tie rod are usually in a locked state when leaving the factory to prevent bellows from shaking during transportation. After installation in place, if it is a cold pipe, loosen the nut on one side to reserve a displacement space for thermal expansion, and keep the other side snug but don't crush it to death.
How much exactly? Don't feel it. Look at the axial compensation or transverse compensation marked on the design drawings, and measure the clearance between the tie rod nut and the ear plate with a gauge or caliper. For example, if the design compensation amount is 30mm, then keep the gap at about 30mm. When tightening the nut, use a torque wrench to ensure that the force on both sides is uniform, otherwise the bellows is deflected by the force, and the compensation amount will be discounted in the slightest case, and the local stress of the bellows will crack in the worst case.
4. Which expansion joints must have protective tie rods? Difference of tie rod configuration between different types of products
This question scores product chat. The general corrugated expansion joint must be equipped with a protective tie rod when the pressure is high or the blind plate force of the pipe system is large, otherwise the corrugated pipe is streaking. High temperature axial type expansion joints and corrugated expansion joints for power station industry, especially large diameter thick wall expansion joints, tie rods are almost standard because of the high medium temperature and pressure.
Compound hinge transverse expansion joint is generally equipped with a combination structure of hinge and tie rod. The hinge bears pressure and thrust, and the tie rod limits the transverse displacement. Straight pipe pressure balance type expansion joint and curved pipe pressure balance type expansion joint have balance bellows inside to offset the blind plate force, and the external tie rod mainly plays an auxiliary stabilizing role. Under vacuum conditions, the external pressure will press the bellows in the double hinge expansion joint of air-cooled island vacuum pipeline, and the tie rod has to bear the thrust generated by external pressure here, and the configuration mode is completely different from that of conventional products.
5. On-site maintenance: whether the tie rod screw should be disassembled, how often to check it, and how to deal with loose nuts
Do not remove the tie rod and screw in normal use. Unless it is a temporary limit screw for transport fixing, it must be removed after installation. Someone asked, can the bellows move if it is not dismantled? Rest assured that the nut on one side of the tie rod leaves a gap, and the bellows can deform freely within this gap range.
Whether the nut is loose, the tie rod is bent and deformed, and the ear plate weld is cracked. If the nut is found to be loose, first relieve the pressure of the pipe, then readjust the gap and lock it tightly. If the tie rod is already bent, don't think about straightening it and continuing to use it. This thing bears a lot of force, and once it yields and deforms, it must be replaced with a new one. Once, an inspector in a chemical plant found that the tie rod was slightly bent and didn't take it seriously. Two months later, the bellows was torn and the whole device stopped for three days.
6. Selection misunderstanding and cost truth: casually installing tie rods will shorten the life of bellows
For the sake of insurance, some design institutes install protective tie rods on all types of expansion joints. It's actually a double-edged sword.
After the tie rod is installed, if the adjustment is improper — such as a nut locking and no clearance reserved — the displacement that should have been absorbed by the bellows can be forcibly transferred to other weak links in the pipe system. More troublesome, the tie rod will change the stress boundary conditions of the bellows. When you give it a lateral constraint in the free expansion and contraction condition in the axial direction, the bellows will generate additional end bending moment instead. The result is that cracks begin to appear before the bellows reach its design life.
Some data show that the fatigue life of the same type of universal corrugated expansion joint is more than 3 times higher than that of the wrong installation of the tie rod when the tie rod is correctly arranged. Therefore, the more tie rods are not the better, it depends on the working conditions.
Think about it, the blind plate force has been offset by balancing the bellows inside a straight tube pressure balanced expansion joint. If you add a row of rigid tie rods to it, it will not only increase the weight of the product by 30%. Which do you choose, expensive equipment or equipment with the right price but accurate tie rod configuration? A good manufacturer will clearly write the configuration of the tie rod, limit rod and guide tube in the design stage, and don't let customers buy a bunch of unused accessories for safety.
The protective tie rod protects not the expansion joint itself, but the safety of the entire pipe system behind the bellows. Find out the division of labor between the tie rod, the limit rod and the guide tube, understand how to calculate the blind plate force, know how to adjust the nut, find out which models must have the tie rod, and finally resist the urge to add accessories indiscriminately. Only by doing these five things right can the bellows work safely in the pipe system for more than ten years.