Can the expansion joint be buried in the ground? Don't jump to conclusions yet
Two days ago, I met a customer, and he was in a hurry on the phone: "Our heating pipeline was buried with ordinary expansion joints, and it leaked in one heating season. Can you help me?" Alas, I have seen this too much. In order to save that procurement cost, many Party A directly buried the general-purpose corrugated expansion joint in the soil, feeling that "it's invisible when buried anyway". And the result? As soon as the groundwater bubbles and the soil is pressured, the bellows become unstable, corroded and fractured by fatigue, and the whole set of pipes have to be dug out and reworked.
Can expansion joints be used buried in the ground?The answer is: ordinary ones can't work, but the "directly buried (fully buried) expansion joint" of this station is specialized in this job.
Why can't ordinary expansion joints bear the underground environment?
Standard metal corrugated expansion joints (such as general corrugated expansion joints and high temperature axial expansion joints) are designed without considering external loads at all. It works by default in overhead pipes or pipe trenches with no soil pressure around, no vehicle crushing, and no groundwater infiltration. You bury it, which is equivalent to asking a man in a suit to carry the sandbag-the bellows are directly deformed by earth pressure, and coupled with poor drainage, the rate of corrosion doubles.
In order to save investment, a thermal power company in the north directly buried the general-purpose corrugated expansion joint, and the backfill soil was mixed with gravel. In the first winter of operation, the bellows was pitted by gravel, coupled with water vapor corrosion, and the weld cracked. Finally, the whole pipe section was scrapped, and the excavator dug for three days before replacing it with new parts. It is more than three times more expensive than buying the direct burial type at the beginning.
What is special about the directly buried expansion joint? To put it bluntly, wearing "body armor"
The "directly buried (fully buried) expansion joint" of this station has three layers of protection in structure:
First, the outer sheath is an integral welded carbon steel or stainless steel casing that directly bears soil pressure and ground dynamic loads (such as car rolling).
Second, the interior is filled with insulation material to prevent heat loss, especially suitable for steam pipes and thermal pipes.
Third, the end seal structure can prevent groundwater from seeping into the bellows and avoid electrochemical corrosion.
Note that it is not the same thing as a "sleeve-type pipe expansion joint" or a "rotary compensator". Sleeve type is sealed by packing, which is easy to leak after long-term operation; The rotary compensator absorbs displacement by rotation, but the installation space requires a large amount. Direct buried expansion joints are specially designed for fully buried environments, so don't mix them.
How many of the three easiest pits to step on when selecting models?
Pit 1: Only look at nominal pressure, regardless of external load.Some people think that since the directly buried expansion joint is called "directly buried", just choose a pressure level. SO WRONG! With a buried depth of 2 meters and a buried depth of 0.5 meters, the soil pressure difference is several times. If a car or excavator passes through the ground, the dynamic load is even more terrible. The pressure bearing capacity must be re-checked according to the actual buried depth and ground use.
Pit 2: Ignoring the axial thrust, the fixed bracket is weak.Both ends of buried pipes must be provided with sufficiently strong fixing brackets. In many projects, in order to save costs, the bracket is welded with thin steel plates. As a result, when the pipeline expands thermally, the axial thrust directly pushes the bracket askew, and the bellows is squeezed to be twisted and deformed. Remember: When the directly buried expansion joint compensates for displacement, the axial thrust must be borne by the fixed bracket, and the strength of the bracket cannot be sloppy.
Pit 3: The anti-corrosion grade is lazy, but the insulation layer accelerates corrosion.If there is an insulation layer in the directly buried part, once the moisture penetrates, the insulation material will become a "water absorber", and the risk of electrochemical corrosion is higher than that of exposed metal. Epoxy coating or cathodic protection is recommended, especially in areas with large burial depth and high groundwater table.
If you don't keep an eye on the installation details, no matter how good the expansion joint is, it will be useless
Backfill soil must be tamped in layers, each layer not more than 30 cm thick, and there must be no large pieces of rubble or sharp objects. At least 50cm of operating space should be left on both sides of the expansion joint to facilitate later maintenance-don't bury the bolts in concrete, and you can't screw them even if you want to.
The soil can be covered only after the pressure test is completed. The pressure test pressure is generally 1.5 times of the design pressure, and the pressure can be kept for 30 minutes without leakage before the soil can be filled. The thickness of the covering soil must be according to the design drawings, and it cannot be thickened at will. We have done a project, and the customer buried the directly buried expansion joint 2 meters deep (the design was only 1.2 meters) to save trouble. As a result, the compensation amount was not enough, and the pipeline collapsed directly from the interface during thermal expansion.
Not all buried pipes need to be buried directly, but thermal steam pipes must be honestly selected
For example, the circulating water pipes of power plants and the conveying pipes of cement plants have low medium temperature and low pressure, so it can be done with "rubber compensator" or "polytetrafluoroethylene compensator" with anticorrosion treatment. However, thermal pipelines and steam pipelines-especially those with a medium temperature exceeding 200℃ and a pipeline length of several hundred meters-honestly choose the "directly buried (fully buried) expansion joint" of this station, and don't joke about its service life.
Not sure about the selection? You can ask our technical department for the parameter table of buried working conditions, fill in the soil type, buried depth, ground load, medium temperature and pressure clearly, and we will give you a calculation book. After all, the final answer to the question "Can expansion joints be buried and used?" Depends on whether you choose the right product or not.