What is a direct buried expansion joint? What is the fundamental difference between it and ordinary expansion joints
The heating pipe is buried underground. In winter, the water temperature soars to more than 100 degrees, and the pipe can expand and contract by several centimeters. At this time, if a "breathing valve" is not added, the pipe weld will collapse sooner or later. This breathing valve is the expansion joint.
But buried pipes are different from overhead pipes-you can't climb down every once in a while for maintenance, and the moisture, chloride ions, and stray currents in the soil are all staring at the thin bellows. Therefore, the expansion joint of directly buried metal corrugated pipe has done several things in structure: the outer layer is added with an integral anti-corrosion coating (epoxy coal pitch or three layers of PE), the sealing structure at both ends is directly welded to the pipeline, and there is a set of self-sealing sheaths with internal pressure outside the corrugated pipe. To put it bluntly, it is designed for the scene of "basically not touching it when you bury it".
What about regular expansion joints? Such asUniversal corrugated expansion jointIt is used in pipe galleries and brackets, and the environment is dry. It can also screw and paint in the later stage. But if you bury it directly in the soil, the bellows will start to pit as soon as the anticorrosive layer breaks in two or three years-you don't know if it leaks, but it will be too late when the road surface steams.
Direct burial type replaces "maintainability" with "high reliability sealing and anti-corrosion", and the protection is sufficient at once when leaving the factory.
Don't take it for granted: pressure, displacement, soil corrosion, one mistake is useless
"Your directly buried expansion joint has the same parameters as ordinary axial expansion joints, right?" I said it was far from the same. Three things must be true when selecting a model:
First, work stress is not nominal stress.Many projects are nominal to 1.6MPa, but in actual operation, the peak value can reach 2.2MPa due to the impact of water hammer. The fatigue life of corrugated pipe is exponentially related to pressure-pressure plus 20%, the number of cycles may drop by half. Therefore, when selecting the directly buried expansion joint, it is recommended to check according to 1.25 times the working pressure, especially in the scenario where the heating system starts and stops frequently.
Second, the amount of displacement is not only thermal elongation.Buried pipelines also have trace displacements caused by soil settlement and vehicle loads. When these factors are superimposed, the axial displacement may be 30% larger than the design value. When selecting, you must provide the manufacturer with "comprehensive displacement", rather than just looking at the thermal expansion calculation of the pipeline. We have encountered a project in which only 50mm axial compensation was written on the drawing, but in fact, 12mm was added because of the subsidence of the foundation-fortunately, it was usedDirect buried (fully buried) type expansion jointA safety margin was reserved, otherwise the pipe would have cracked.
Third, soil corrosiveness is most easily overlooked.Generally, the resistivity, pH value and chloride ion concentration of soil determine the selection of anticorrosive layer. For example, there are many broken bricks and high moisture content in the backfill soil in the old city, and the ordinary epoxy coating will bulge in a few years. At this time, three layers of PE have to be used to reinforce corrosion protection, or sacrificial anode protection. Don't always think "it's fine if you bury it". The electrochemical corrosion in the soil is silent.
These three things are wrong when installing, and don't cry if you leak them later
The installation link is the hardest hit area. I have seen too many cases-the quality of the expansion joint itself is fine, but it was destroyed by the installer. Remember three key points:
First, it is strictly forbidden to force matchmaking.Both ends of the directly buried expansion joint usually have pipes and grooves. If the misalignment of the pipe exceeds 1mm, don't smash it hard with a sledgehammer. The right thing to do is adjust the trench foundation or fine tune the pipe axial position with a jack. Once, in order to catch the construction deadline at a construction site, workers directly welded the expansion joint to the crooked pipe. As a result, the bellows was pressurized on one side, and it was directly bulged and scrapped during pressure test.
Second, buried depth and protective layer thickness.Many drawings are marked with "buried depth of 1.2 meters", but the bottom of the ditch was not tamped during actual construction. After the pipe was lowered, the soil was backfilled. As soon as the road roller rolled, the pipe sank and the expansion joint was elongated. The correct way is to make 200mm fine sand cushion first, and an extra layer of medium and coarse sand has to be wrapped around the expansion joint, and then tamped in layers. In addition, the depth of covering soil from the outer wall of the sheath of the directly buried expansion joint to the ground is at least 1.5 meters in the northern frozen soil area, otherwise the frost heave force will make the bellows stress abnormally.
Third, drain and seal after pressure test.After the hydraulic test is qualified, the water in the pipeline must be drained clean. If the water is not drained after the pressure test in winter, the water freezes and expands, and the bellows directly cracks-this low-level error occurs every year. After draining the water, don't forget to tighten the end seal fittings. When the directly buried expansion joint leaves the factory, there is a temporary protective cover at the end. Remove it during pressure test, and install the permanent seal immediately after test. Some people are afraid of trouble, so they take a plastic bag and stuff it. When the backfill soil and sediment are poured into it, the seal fails, and the later leakage will be there.
I encountered a heating project two days ago. What is the effect of using a directly buried expansion joint?
Last month, I went to a central heating project in Shanxi for a return visit. The pipe diameter is DN500, the design pressure is 1.6MPa, the temperature is 130℃/70℃, and the length of the buried section is nearly 800 meters. They used 16 unitsDirect buried (fully buried) type expansion jointBoth ends are also equippedRotary compensatorTo absorb the lateral displacement.
An inspection well of expansion joint was dug on the spot. When the sealed cavity was opened, the surface of the bellows was as clean as new, and the anticorrosive layer was not bubbling at all. The project manager told me that it has been in operation for two years, and there has been no leak alarm. Moreover, because the directly buried expansion joint comes with its own sealing sheath, they even save the conventional inspection well-it is buried directly under the green belt, with grass planted on it, and no one knows that there is equipment underneath. This item saves about 150,000 civil construction costs.
The project used crushed gravel soil in backfilling, which caused the local sheath to be pitted by sharp stones. Later, they added a 10cm thick fine sand isolation layer, and the problem was solved. So you see, if you choose the right product, the installation details are still sloppy.
After three or five years of use, do directly buried expansion joints need maintenance?
"Aren't directly buried expansion joints maintenance-free?"
Strictly speaking, it's "low maintenance" rather than "zero maintenance". The design life of directly buried expansion joints is usually consistent with that of the main pipe body, such as 15 to 25 years. But there are a few pits to look at regularly:
- Inspection well (if any)In the seal joint, check once a year before and after the rainy season for any water seepage. If condensed water is found in the sheath, it means that the end seal has failed, and the sealing ring should be replaced in time.
- Cathodic protection potential。 If the pipeline is protected by sacrificial anode or forced current, measure the potential every six months. As soon as potential drift is found, it indicates that the anti-corrosion layer may have damaged points, so it has to be dug up for investigation.
- Pavement settlement monitoring。 If there is obvious settlement of the pavement above the directly buried section, it means that there may be a problem with the foundation, and the displacement of the pipelines on both sides of the expansion joint has exceeded the design range. At this time, it must be excavated for inspection.
In addition, the vast majority of directly buried expansion joints can lie underground for more than ten years under normal working conditions. But don't forget-it is a bellows with fatigue life after all. After the cumulative number of cycles exceeds the design value (generally 1000~3000 times), it is recommended to excavate and replace it during the overhaul of the pipe network. After all, once the buried equipment leaks, the maintenance cost is several times that when it was first installed.