How to fix the metal expansion joint to be firm? Don't wait for a pipeline accident to learn
The pipe was installed and the expansion joint was selected correctly. As a result, it leaked, cracked, and even pulled off the bracket within a few months. I've seen this kind of thing too much. Nine times out of ten, the problem is fixed. Today, let's talk about fixing the metal expansion joint-how to calculate it firmly? It's not as simple as tightening the screws.
To fix the expansion joint, find out why it moves first- -Talking about blind plate force and displacement absorption
When many people hear "fixed", their first reaction is to weld and lock the expansion joint. Wrong! The core function of the expansion joint is to absorb displacement. How does it move if you weld it to death? Then why do you need it?
One is the blind plate force generated by the medium pressure, and the other is the displacement force generated by thermal expansion and contraction. The blind plate force is pushed out along the axial direction of the pipe. If it is not held, the expansion joint can be directly pushed into "pulling noodles". The displacement force is caused by the thermal expansion and contraction of the pipeline itself, and the expansion joint needs to be "eaten".
Hold the blind plate force through the pull rod and the screw rod to prevent it from being transmitted to the bellows; While leaving the bellows to absorb the displacement. Whoever is responsible for carrying, whoever is responsible for sucking, each does his own thing.
Tie rods, screws, limit bolts: what exactly are these fixings carrying?
Tie rod, screw, limit bolt... The name sounds like it, but the function is completely different. If you install it wrong, it will be useless.
- tie rod: for axial type expansion joints, such asUniversal corrugated expansion joint、High temperature axial type expansion joint. Its function is to connect the two end pipes, bear the blind plate force, and prevent the bellows from being elongated or compressed excessively. It is usually a double nut lock when installed, but note-it does not crush the bellows to death, but leaves the design displacement amount.
- Screw (also called adjustment screw): Common inLarge tie rod expansion jointOrDuplex hinge transverse type expansion joint. In addition to carrying the blind plate force, this thing can also do pre-stretch/pre-compression adjustments. How to screw when tuning? Talk about the case later.
- limit bolt: Generally used on hinge or pressure balance expansion joints. Its function is to limit the amount of displacement in a certain direction and prevent the bellows from twisting. Such asThe double straight pipe bypass pressure balance expansion joint, the limit bolt is adjusted wrong, which directly leads to deviation.
Tie rod pipe axial direction, screw pipe adjustment, limit bolt pipe bending radius. Which one is installed wrong, and the expansion joint is wasted.
Can the screw be removed during installation? How to do pre-stretching? You will understand when you turn through the actual case
The expansion joint has arrived. Do you want to disassemble the screw? The manufacturer said it was installed and then disassembled? I said you look at the design temperature first. If it is a normal temperature pipeline, the screw can not be disassembled, but it should be loosened to the designed length. If it is a high-temperature pipe, such as a steam pipe, it must be pre-stretched-otherwise the bellows will be crushed to death as soon as the temperature rises.
Before installation, first screw the nut on the screw to the calculated pre-stretched length (such as 10mm longer than the free length), and then fix the tie rod. After the pipe heats up and expands, the bellows retracts just to 0 position. If pre-stretching is not done, the bellows is in a compressed state from the beginning, and the life is cut in half.
Will the screw be removed or not? And the answer is:Axial type expansion joints that need to absorb axial displacement, the screw must be removed after installation in placeAllow the bellows to expand and contract freely. But if it isStraight tube pressure balanced expansion joint orCurved tube pressure balanced expansion joints, screws or tie rods are permanent fixtures and cannot be removed-because they rely on these pieces to balance out the blind plate force.
Axial type, hinge type, pressure balance type-what is the difference in the fixing ways of different expansion joints?
Different structures of expansion joints, the fixed logic is very different. Don't take one experience set.
Axial type (such asUniversal corrugated expansion joint,External pressure single type axial type expansion joint)
Each of the two end tubes is equipped with a fixed bracket, and a guide bracket is arranged in the middle. The tie rod (if any) is used to limit the maximum amount of expansion and contraction, not to hold the bellows. Adjust the tie rod nut to the design length during installation, and then lock it. The bellows itself absorbs displacement through deformation.
Hinged type (e.g.Compound hinge transverse expansion joint,Air-cooled island vacuum pipe double hinge expansion joint)
The hinge type cannot withstand the axial blind plate force, and must be used in pairs, and the blind plate force is transmitted to the pipe by the hinge pin shaft. When fixing, the bracket on the hinge side must be the main fixing bracket, and the other side must be a guide bracket. The limit bolt is used to control the rotation angle. Don't screw it to death, otherwise the hinge will be stuck and the pipeline will be deformed directly to the weld.
Pressure balance type (e.g.A straight tube pressure balance type expansion joint,Curved tube pressure balance type expansion joint,Compound straight pipe bypass pressure balanced expansion joint)
This self-balanced structure has a working bellows and a balanced bellows inside, and the blind plate force is cancelled out by itself. So it has the lowest requirements for fixing brackets, only a main fixing bracket is needed, and even sometimes the pipe itself can bear it. But the disadvantage is that it is expensive and occupies a large area. When fixed, the tie rod is dead and cannot be disassembled, and the pre-stretching should be strictly operated according to the manufacturer's data.
The distance between the fixed bracket and the guide bracket is not calculated correctly, and the expansion joint is equal to white installation
Speaking of this, it is estimated that many people who have done the scene have suffered losses. The fixed bracket carries the blind plate force and the direction of the guide bracket tube-but how to set the distance?
For axial type expansion joints,Distance of the first guide bracket from the expansion joint ≤4 times the nominal diameterFor example, the pipe of DN200, the first guide bracket is at 800mm. The second guide bracket can be placed to 14 times the diameter (2.8 meters). If the distance is large, the pipeline will be unstable under pressure, and the bellows will be bent, which is also one of the most common failure modes in the field.
For hinged and pressure balanced types, the guide bracket can be looser because the blind plate force is transmitted or balanced. But either way, the fixed bracket must be absolutely rigid-the bracket welded with channel steel on site, sometimes it has to be reinforced. Don't save that little steel, and save the doubling of the rectification cost in the later period.
In addition,Metal rectangular expansion joints andFor non-circular section or heavy load such as large diameter thick wall expansion joints, the force calculation of the fixed bracket should be done separately, and the standard atlas cannot be applied. It is best to let the manufacturer issue a calculation book, or find a professional structural engineer to review.
Therefore, the fixation of metal expansion joints looks like screwing and welding brackets, and behind it is the combination of mechanical calculation and installation experience. Don't wait for the pipeline to shake and ripple before you regret that you didn't spend half a day thinking about the fixed plan.