How to connect a metal expansion joint? Find out which model to use first
I received a phone call from a customer two days ago, saying that the steam pipe leaked only three months after installing the expansion joint. I asked him what model he used, and the answer was "just an ordinary metal expansion joint". Tsk, that's the problem —Method of joining metal expansion jointIt's not a matter of screwing a wrench. If you choose the wrong model, the rest will be for nothing.
The universal corrugated expansion joint is suitable for pipelines with axial displacement as the main; Large tie rod expansion saves energy and carries lateral displacement, but tie rod is the core; Pressure balance type (such as straight pipe pressure balance expansion joint and curved pipe pressure balance expansion joint) is specially designed to deal with high pressure and large diameter, so that the blind plate force does not spread outside. There are also high-temperature axial expansion joints, external pressure single axial expansion joints, double hinge transverse expansion joints... Each structure determines its joining method difference. If you use the high-temperature type as a general-purpose type, the bellows will burn through directly. This is not a joke, but a real thing.
Key checks before installation: Should tie rods and screws be removed or left?
Many masters get the expansion joint, and when they see the tie rod and adjusting screw, they mutter: Is this a transportation protection or a permanent part?
For universal corrugated expansion joints and high temperature axial expansion jointsThe limit tie rod and adjustment screw that came with the factory must be removed or loosened to the designed position before installation. Why? Because their role is to prevent the bellows from being overcompressed or stretched when transported and hoisted. After the pipeline docking is completed, if the tie rod is still screwed to death, the expansion joint will lose its compensation function, which is no different from a section of steel pipe. Two days ago, in a power plant project, the workers didn't dismantle the tie rod, and the bellows bulged directly during the pressure test-the pressure was carried by the tie rod, but the bellows was damaged.
The large tie rod expansion joint and the tie rod on the pressure balance expansion joint are structural parts and cannot be disassembled! Because the big tie rod has to bear the blind plate force generated by internal pressure, you remove the tie rod, and the bellows deforms instantly. Therefore, be sure to look at the nameplate and drawings before installation, and distinguish between "transportation limit" and "working tie rod".
Detailed explanation of installation steps: key points of centering, welding and fixing brackets
Step 1, Align. The pipe flange surfaces must be parallel, and the deviation must be controlled within 1-2mm. If you take the level to hit it, if the bellows is twisted, the life will be directly discounted by 50%. Step 2, welding. If it is a welded expansion joint, fix it by spot welding first, observe the deformation amount, and then weld it in layers, and the thickness of each layer does not exceed the diameter of the welding electrode. The welding current should not be too large to prevent the heat-affected zone from burning the thin wall of the bellows. It is recommended to wrap a damp cloth around the outer wall of the bellows for cooling. The third step is to fix the bracket. Remember one principle: the first bracket at both ends of the expansion joint must be a fixed bracket or a guide bracket to prevent the axial thrust of the pipe from pushing the expansion joint askew. EspeciallyDouble hinge expansion joint of air-cooled island vacuum pipeFor this large-size product, the spacing of brackets must be made according to the design drawing, and one less bracket may lead to hinge failure.
Many on-site diagrams save trouble, and the expansion joint is directly welded to the unfixed pipe. As a result, as soon as the temperature rises, the pipe drifts with the expansion joint, and the compensation amount is all wasted. And guess what? Finally reworked and removed, the bellows has been permanently deformed.
Differences in joining methods of different types of expansion joints
- Universal corrugated expansion joint: Axial displacement, remove the limit pull rod during installation to ensure the free expansion and contraction of the bellows. Pay attention to the direction of the deflector-the arrow must point to the direction of the media flow, otherwise the deflector will be washed off.
- Large tie rod expansion joint: For lateral displacements, such as L-pipe corners. The pull rod cannot be removed during installation, and the nut should be adjusted so that the pull rod is in the force balance position. The FAQ asks "How to adjust the expansion joint tie rod nut"? Generally, it is pre-tightened to 80% of the design length first, and then fine-tuned after the pipeline hydraulic pressure test.
- Straight pipe pressure balanced expansion joint: Working bellows and balancing bellows at both ends with tie rods in the middle. During installation, the three chambers must bear pressure at the same time, so the tie rod should never move. Moreover, this kind of expansion joint usually comes with its own fixed bracket, and only a guide bracket needs to be set on the spot.
- Curved tube pressure balance expansion joint: More complex construction for elbow sections that need to absorb combined axial and lateral displacements. Pay attention to the coaxiality of flanges at both ends when installing, deviation more than 2mm will cause bellows to twist.
- Compound hinge transverse expansion joint: Consists of two hinges and a set of bellows that absorb only the lateral displacement in the plane. The hinge pins must be lubricated during installation and are not allowed to withstand torsional torque.
And there's alsoExternal pressure single axial expansion jointIts bellows is on the outside, subjected to external pressure, and the installation direction has strict requirements. The exhaust hole on the housing must face upward, otherwise the condensate will not drain out-this is where many novices roll over.
Post-installation commissioning and acceptance: displacement inspection and leak test
Measure the length of the expansion joint in the cold state with a dial gauge or vernier caliper and record the initial value. Then introduce the medium (water or gas), slowly raise the temperature to the working temperature, and measure the length again to see if the actual displacement is within the design range. If the actual displacement is larger than the design value, it means that the pipe fixing bracket has failed or the spacing of the guide bracket is too large. If there is no change, congratulations, you may have forgotten to remove the limit rod.
Leak tests must also be done. For corrugated expansion joints used in power station industry or metal corrugated expansion joints in cement industry, it is usually required to hold pressure at 1.5 times the design pressure for 30 minutes without leakage. Small diameter can use compressed air with soapy water, large diameter can use ultrasonic leak detector. Pay special attention to weld seams and bellows crests-that is where cracking is most likely.
Method of joining metal expansion jointIt is not an isolated operation, it has to be matched with pipe design, support arrangement, and media characteristics. The stress analysis of the same model, used in steam pipes and used in flue gas pipes, is completely different. So, don't use "universal type" as a universal plaster. If you are not sure, flip through the product information on the station, or ask the manufacturer's technical engineer directly-it is much more troublesome than thinking about it yourself.