Metal expansion joint assembly, why not simply "screw"?
Two days ago, I met a customer who complained that the metal expansion joint that had just been installed for two months was leaking. When I removed it, the bellows was twisted into a twist. When asked how to install it, the answer was very straightforward: "Isn't it just tightening the flange? It's similar to installing a faucet." Yes, another case of treating expansion joints as ordinary pipe fittings.
The essence of metal expansion joint assembly is to install an "elastic joint" for the pipeline system. It should absorb thermal expansion and contraction, vibration displacement, and even mismovement during earthquakes. If you screw it to death as a rigid part, how does the bellows work? It's like letting people run in sizzling shoes. It's weird if they don't sprain their feet. There is a data in the industry: more than 40% of expansion joint failure cases are rooted in the installation process. So don't underestimate this process, it directly determines whether the equipment can be used for 3 years or 15 years.
Preparation before assembly is more important than thought
Many people get the expansion joint and rush to the nozzle. As a result, the flange bolt hole doesn't match, so they break it hard with a crowbar. Alas, after this, the inside of the bellows has been hurt.
- Check Model Number and Size— — Confirm whether the design pressure, temperature and compensation amount of the expansion joint match the working conditions. Such asUniversal corrugated expansion jointAndHigh temperature axial expansion jointThe wave height and number of layers are completely different, and mixing them means digging a hole for yourself.
- Check the appearance— — See whether the bellows has bumps and scratches; Whether the direction of the guide tube is consistent with the flow direction of the medium. The arrows on the expansion joints are not decorative, but indicate the direction of media flow.
- Clean up the interface— — The sealing surface of the flange should be smooth, and there should be no welding slag and burrs. The pipe must be within the allowable deviation (usually ≤1mm), otherwise additional stress will occur during assembly.
To put it bluntly, preparation is to kill the "hidden danger" before you do it. Don't bother, skip this step, it's all patches behind.
Core step of assembly: butting of bellows and end pipe
This is the highlight of metal expansion joint assembly. Note that you don't just weld the bellows to the end pipe directly-many products have pre-welded the end pipe and bellows when they leave the factory, and all you have to do on site is flange connection or butt welding.
The bolts should be tightened diagonally, and the force should be applied gradually in 2-3 times. The torque value should refer to the parameters given by the manufacturer (different calibers and pressure levels have different requirements). Don't twist the wrench to the bottom, the flange surface will deform and leak.
The expansion joint must be pre-stretched or pre-compressed to half of the designed compensation amount before welding (see drawings for details). Why is it? Because temperature changes during pipeline operation can make the bellows elongate or shorten, if the initial position is wrong, the bellows can easily reach the fatigue limit. When welding, you should also pay attention to isolating heat, and cover the bellows with a damp cloth or asbestos to prevent the intergranular corrosion of stainless steel in the heat-affected zone.
Details decide the success or failure of the debugging of the tie rod and the limit device
Many people forget to adjust the tie rod and limit nut that come with the expansion joint. And the result? Either the tie rod is dead blocking displacement, or the nut is loose causing the bellows to overstretch.
Function of expansion joint tie rodIs to prevent accidental damage of bellows during transportation and installation, and is also used for initial limit. Once installed in place, the locking nut needs to be loosened according to the design drawings, leaving the tie rod in a "free" or "pre-adjusted" state. For example:Large tie rod expansion jointIt is usually used to absorb the lateral displacement at the pipe elbow. If the tie rod nut is not adjusted to the designated position, the lateral compensation amount will be greatly reduced.
How to tune it? Refer to the method in the FAQ of this site: first confirm whether the expansion joint needs to be pre-stretched, and then adjust the tie rod nut according to the distance calibrated in the drawing to ensure that the clearance between the nuts at both ends and the ear plate is consistent. Don't forget to lock the backup nut after adjustment-the nut will loosen itself under vibration conditions.
Inspection and pressure test after assembly, don't rush to put into operation
Finish it when it's done? Don't worry, do three things first:
- Visual inspection: there is no indentation or bulge on the surface of the bellows; The flange sealing surface fits evenly; The pull rod and restraint are in correct position.
- Pressure test: Hydraulic test is usually performed at 1.5 times the design pressure (Note: Remove the limit bolt or leave the tie rod free during hydraulic test, otherwise the expansion joint may be damaged). Hold the pressure for 10 minutes without leakage before it passes.
- Displacement verification: If conditions permit, simulate the thermal displacement direction of the pipeline, and manually push and pull the expansion joint to see whether it is smooth. In some cases, it is necessary to check the specific function of the expansion joint guide tube-the guide tube is not properly installed, and the high-speed airflow directly erodes the bellows, which will perforate in a few months.
If this step is missed, unplanned downtime is waiting for you.
Key Points of Assembly in Different Scenarios: How to Do Power Station, Cement and Chemical Industry
Different scenes, the emphasis of metal expansion joint assembly is very different.
Power station industry: The unit starts and stops frequently, and the temperature changes drastically.Corrugated expansion joint for power plant industryAndDouble hinge expansion joint for air-cooled island vacuum pipeline。 Pay special attention to the direction of the compensation amount when assembling-axial displacement or angular displacement? The limit position of the tie rod must be adjusted strictly according to the value of cold state pre-tension/hot state pre-compression. Moreover, the power station pipeline has a large diameter and thick wall, and the torque of flange bolts should be checked with a wrench, which is unreliable by hand feel.
Cement industry: Dusty, high temperature, commonly usedMetal Corrugated Expansion Joints in Cement IndustryAndRectangular non-metallic expansion joint。 When assembling, the flange sealing gasket should be high-temperature resistant (such as graphite winding gasket), and it is best to install a dust cover on the outside of the bellows to prevent dust accumulation from affecting the displacement. Cement line air duct is usually non-standard size. When assembling on site, you must check the interface size. If it is wrong, it cannot be reworked.
Chemical industry: the medium has strong corrosiveness and large pressure fluctuation,PTFE-lined hoseOrPTFE compensator。 Forced installation is strictly prohibited during assembly, and PTFE lining loses its anti-corrosion ability once it is torn. In addition, chemical pipelines often have vibration sources, so it is recommended to install brackets at both ends of the expansion joint to avoid load-bearing. Guess what? Many leakage accidents in chemical plants are not due to the poor quality of corrugated pipes, but because the expansion joint is used as a pipe bracket during installation.
In the final analysis, metal expansion joint assembly is a delicate job. No matter how good the equipment itself is, it will be scrap metal if it is installed wrong. Next time you see this kind of work, spend half an hour looking at the drawings and adjusting the pull rods, and save enough maintenance expenses for several barbecues.