1. What exactly is the connecting rod metal expansion joint? — Don't confuse it with ordinary bellows
Two days ago, a customer called and asked, "What's the difference between your corrugated pipe with a tie rod and ordinary corrugated pipe?" I said that you didn't call it right-it wasn't "corrugated pipe with a tie rod", it was seriousConnecting rod metal expansion joint。 Structure and the way it is stressed are completely different things.
Ordinary bellows expansion joints (such as our general-purpose corrugated expansion joints) rely on the bellows themselves to carry the axial displacement and pressure thrust. When the pipe is thick and the pressure is high, the wall thickness of the bellows has to be increased, and the cost will rise. And what about the connecting rod metal expansion joint? It uses tie rods and hinges to "unload" the pressure thrust-the bellows only absorbs the displacement, and the thrust is borne by the tie rod system. Think about it, it's like the difference between carrying a sack alone and carrying a sack on a pole, the principle of leverage.
So don't get mixed up. Ordinary bellows is "flexible components alone to resist compression", and connecting rod metal expansion joint is "metal bellows + structural parts work together". In the product catalogue on our site, likeStraight pipe pressure balanced expansion joint、Compound hinge transverse expansion joint、Double hinge expansion joint for air-cooled island vacuum pipelineThey all belong to this big family. Remember one thing: Anything with "pull rod", "hinge" and "pressure balance" in its name is basically it.
2. Structural disassembly: tie rod, hinge, pressure balance, what do they manage?
The structure of the metal expansion joint of the connecting rod is, to put it bluntly, three parts:Bellows(core flexible member),Tie rod/hinge(restraining mechanism),End plate/connector(Transmitting Force). But many people can't even tell the difference between a tie rod and a hinge.
The role of the tie rod is to limit the amount of axial displacement of the bellows-after you install it, once the tie rod nut is locked, the bellows cannot be stretched or shortened at will. But note that this is not a deadlock, but a "limit". There are usually nuts at both ends of the tie rod, and a length will be pre-adjusted when it leaves the factory. After installation in place, the nut needs to be adjusted to set the working stroke. We have it in our FAQ"Function of Expansion Joint Tie Rod"And"How to adjust the tie rod nut of the expansion joint", very detailed.
What about the hinges? It allows the bellows to do angular deflection in a plane. Such asCompound hinge transverse expansion joint, the lateral displacement absorption was achieved with two sets of hinges. The hinge itself is not subjected to pressure thrust and only constrains the direction. The pressure thrust is carried by the pull rod or the pressure balance structure. When it comes to pressure balance, it isStraight pipe pressure balanced expansion jointThat set of design-balanced bellows are symmetrically arranged on both sides of the bellows, so that the pressure thrust is cancelled internally, and the pipe rack is basically unstressed. When you do chemical pipeline design, when you encounter large-diameter, high-pressure steam pipes, you can save a bunch of fixed brackets with this.
3. Select the three details that are most prone to rollover: displacement direction, guide tube, and installation pre-stretching
I've seen a lot of accidents involving model selection rollovers. Three pits, one after another.
The first pit: the displacement direction is reversed.Connecting rod metal expansion joints usually absorb only one direction or one plane of displacement. For example, the hinge type can only absorb angular displacement. If you insist on it absorbing axial expansion and contraction, that is equivalent to asking the screwdriver to twist the nut-using the wrong tool. Before selection, calculate the displacement vector of the pipeline system: How large is it in the axial direction? How big is the horizontal? What is the angular displacement? Then correspondingly selectCompound hinge transverse type、Straight pipe pressure balance typeStillCurved tube pressure balance type。 Don't think of one model to conquer the world.
The second pit: the deflector forgot.The function of the guide tube is to protect the inner wall of the bellows from being washed by high-speed fluid, and at the same time reduce the flow resistance. Our FAQ specifically contains"Specific Function of Expansion Joint Guide Tube", accompanied by pictures. However, many designers think that "if the pressure is not high, it will not be used". As a result, there are particles or cavitation in the medium, and the bellows will be perforated in half a year. Remember: Whenever the flow rate exceeds 10m/s, or the medium contains solid particles, a guide tube must be added. The material can be the same as the bellows, but the thickness is usually 1-2mm thicker.
Third pit: installation pre-stretch not done.When new pipes are installed, the ambient temperature is usually normal, but the operating temperature can be a few hundred degrees Celsius. The thermal elongation of the pipe is calculated according to the formula. For example, a 100-meter steam pipe, with a temperature rise of 300℃, has an elongation close to 400mm. If pre-stretching is not done, the expansion joint will be compressed to the limit as soon as it is installed, and there is no margin at all. Correct practice: Before installation, adjust the tie rod nut to the pre-tension amount (generally half of the total compensation amount), wait for the pipe to heat up and the bellows to return to the middle position. This point is in"Proper Installation Method of Large Tie Rod Expansion Joint"Detailed instructions in.
4. Installation and adjustment: Is the tie rod nut removed or not? — — Common misunderstandings on site
"Do you want to remove the tie rod nut after it is installed?" My answer is: it depends, but most of the time it can't be dismantled.
ForConnecting rod metal expansion jointThe tie rod itself is a permanent structural member, which bears the pressure thrust. You remove the tie rod, the pressure thrust is all on the bellows, and the bellows will soon break from fatigue. Take a real case: the outlet pipeline of the rotary kiln of a cement plant was usedMetal Corrugated Expansion Joints in Cement IndustryThe on-site workers felt that the pull rod was in the way and all of them were unloaded. As a result, less than a week after startup, the bellows bulged and scrapped.
Then when will it be demolished? There is only one case: when the expansion joint is only used to absorb installation deviations or vibrations, and the pipe has independent fixed brackets to withstand thrust, the tie rod can be used as temporary transport protection and needs to be removed when installed in place. However, this type of design is rare in connecting rod metal expansion joints. From our stationUniversal corrugated expansion jointSometimes it is equipped with a transport tie rod, but that one is a temporary piece and must be disassembled after installation. The tie rod of the metal expansion joint of the connecting rod is a "structural part" and should never be disassembled.
Loosen first and then tighten. First loosen all nuts for half a turn to allow the bellows to center freely, and then tighten according to the designed pre-tension amount. Use a torque wrench to ensure that each tie rod is evenly stressed. Otherwise, the bellows will twist and create additional stress. Please refer to the specific steps"Does the screw of the expansion joint need to be removed?"This question and answer.
5. Where is it best used? — — Actual cases of power stations, cement and chemical industries
Tell me a few real scenarios to help you judge whether you should use connecting rod metal expansion joints for your project.
Power station:Air cooled island pipe. The ambient temperature difference is large (-30℃ to 40℃), and the pipeline has to bear vacuum negative pressure. Ordinary bellows are easy to be deflated, so useDouble hinge expansion joint of air-cooled island vacuum pipe, the hinge structure not only absorbs lateral displacement, but also prevents instability. In addition, the main steam pipeline, high temperature and high pressure, withStraight pipe pressure balanced expansion jointIt can save a lot of fixed brackets and save civil construction costs. I have seen a 300MW unit project where the number of pipe racks was reduced by 40% after using the pressure balanced type.
Cement:The raw material mill and kiln tail pipeline have large dust and frequent temperature fluctuations. Recommended useMetal Corrugated Expansion Joints in Cement IndustryEquipped with wear-resistant guide tube. However, note that cement plants often have limited installation space. At this time, the tie-rod type takes up more space than the unconstrained type, so the headroom needs to be measured in advance. Another key point: the cement line starts and stops frequently, and the fatigue life should be designed according to more than 10,000 times.
Chemical:Large diameter, corrosive media pipes. For example, the inlet and outlet of the desulfurization tower, usingDesulfurization flue gas baffle doorCooperateNon-metallic expansion jointCan solve the corrosion problem. However, if the medium temperature exceeds 200℃, the non-metal can not be used and has to be replacedLarge diameter thick wall expansion jointOrExternal pressure single axial expansion joint。 The main advantage of connecting rod metal expansion joint in the chemical field is that it can withstand large pressure thrust, such as methanol synthesis circuit, with an operating pressure of 10MPa, which ordinary bellows can't bear at all, and the pressure balance type is standard.
In short, selection is not a pat on the head. You have to find out the pipeline displacement, pressure, temperature and media characteristics first, and then look for the corresponding model in the product catalog. Each product in our site has detailed parameters, you can compare them if needed"Model and size of expansion joint"And"Metal Expansion Joint Weight Table"To verify.