Metal Expansion Joint Screw Don't Mess! Understand these points, and the pipeline will not "strike"
To be honest, after so many years of operation and maintenance of pipeline equipment, I am most afraid of seeing someone screw the metal expansion joint screw with a wrench. You think those iron sticks are decoration? Tighten the point for peace of mind? Big mistake. Today, let's find out how to serve this screw-get it, your pipeline system won't go wrong for ten years; I don't understand. Just wait for the bellows to be replaced in three months.
What the hell are screws for? Don't use it as a decoration
Metal expansion joint screw, also called tie rod in the industry, its job is to be a "rein". The bellows itself is soft, stretchable and flexible, but if there is no limit, the displacement exceeds the design value due to thermal expansion and contraction-the bellows will either be cracked or deflated. The function of the screw is to tightly frame the displacement within the safe range. Look at our stationUniversal corrugated expansion joint、High temperature axial expansion jointAlmost all of them are paired with this stuff, and they are not given for free.
Then can I not install the screw? You try it. When the temperature of the pipeline rises by 50℃, the axial compression amount of the bellows may reach 20mm. Without screw limit, the bellows directly hits the limit ring, and it will break by fatigue after several rounds. The screw is that "brake" that neither lets it run around nor lets it get stuck.
The Consequences of Wrong Adjustment-Lessons of Real Money Buying
Two days ago, I met a buddy from a cement factory.Metal Corrugated Expansion Joints in Cement IndustryIt leaked after less than half a year of use. When disassembled, the screw was locked tightly, the nut was screwed flat with the end plate, and the bellows had no room to compensate for the thermal displacement-it was forcibly cracked. On the other hand, in another power station project, the screw was loose as if it had not been screwed. When the pipe vibrated, the bellows swung back and forth, and the fatigue life was directly cut in half.
So you see, the elasticity has to be accurate. Not the tighter the better, or the looser the better. To adjust this step, it is to find that "free length" state-the bellows just doesn't bear extra tension or pressure at room temperature.
How to tune it? Just three steps, but not one
The first step is to loosen the nut on the screw to the factory mark position before installation. Generally, a limit block will be engraved or welded on the screw, so don't change it indiscriminately. In the second step, after the pipe is in place, adjust the nut according to the compensation amount on the design drawing-note that it is not screwed to death, but let the bellows just be at the free length. How to judge? Use a ruler to measure the total length of the bellows, which is consistent with the free length marked at the factory. In the third step, after the system runs and heats up, fine-tune it again. After heating up, the pipe is elongated or shortened, and the nut will follow. You take the wrench to recheck it to ensure that there is no abnormal displacement. For specific operation details, you can turn to "How to Adjust the Tie Rod Nut of the Expansion Joint" we wrote before, which is more detailed.
Some people think it's troublesome to skip the third step, but what happened? After heating, the bellows is stretched excessively and directly scrapped. In fact, it doesn't take ten minutes to go down in these three steps, but it can keep your equipment used for two or three years.
Do you want to remove the screw after installation? Don't be one size fits all
This issue has been arguing on Zhihu for several years. And guess what? Scoring occasions. If the expansion joint only absorbs axial displacement and the pipe is attached with a fixed bracket, then the screw can indeed be removed after pressure testing and transport. However, if used in situations where there is lateral displacement or vibration, such as power station pipes, the screw must be retained as a limiting device. OurCompound hinge transverse expansion jointWithout the screw, the hinge structure can't be limited at all, and accidents will occur every minute. AndDouble hinge expansion joint for air-cooled island vacuum pipeline, that screw is the lifeblood, and it is directly misaligned after dismantling the pipeline. Don't listen to some "old masters" saying that they have to be demolished, that is generalization.
Different working conditions, screw selection is very different
High-temperature pipelines, such as steam and flue gas, have a temperature above 500℃, and ordinary carbon steel screws creep at high temperature, which will be wasted in two weeks. Have to use heat-resistant alloy steel screws, ourHigh temperature axial expansion jointThat's what it is standard. In the super-large displacement scenario of large-diameter thick-walled expansion joint or air-cooled island, the screw is under great force, so it is necessary to use a double-headed screw to thicken the nut, otherwise the thread will slide. In the desulfurization system, once the corrosive medium rushes, the surface of the screw must be treated with anti-corrosion, otherwise the rust pit is the starting point of the crack. Flip through the product catalog of this site — —Straight pipe pressure balanced expansion joint、Direct buried (fully buried) type expansion joint…The screw specifications of each model are designed according to the working conditions. Don't make do with general parts.
For daily inspection, don't forget to take a look at the screw
Many operation and maintenance brothers inspect only to see if there is any leakage in the bellows, screw? Not even looking at it. This habit needs to be changed. Twist the nut to see if it is loose, and see if there are rust pits or cracks on the surface of the screw. Especially in the cement industry and power station industry, the vibration frequency is high, and screw fatigue fracture is no joke. It is recommended that each time the machine is shut down for maintenance, take the torque wrench and tighten it again according to the torque value given by the manufacturer. If you find that the screw is bent or the thread slides, replace it with a new one without saying anything-save those dozens of dollars and replace the whole expansion joint later. Which one is cost-effective?
Behind this humble small part of the metal expansion joint screw is the safety logic of the whole pipeline system. Tuned, it is the patron saint; If you smash it, it's a ticking time bomb. Think more about today's article before you start next time, at least don't let the pipeline "strike".