Specialized in manufacturing compensators, expansion joints, baffle doors

Two days ago, a customer called and said that the general-purpose corrugated expansion joint arrived, but the flange screw hole didn't match. After asking, he found out that the caliber he measured was the outer diameter of the wave peak, not the diameter. Anyone who has done this kind of thing on the spot knows it-welding the wrong flange, buying the wrong model, the construction period is delayed for a week, the boss scolds and the workers complain. The measurement of the diameter of the expansion joint looks simple, but many of them overturn. Exactly where to measure? Use what tool? What is the difference between different kinds of expansion joints? This article makes the doorway clear.

Find out first: What exactly does the "caliber" of the expansion joint mean?

The caliber in the pipeline industry usually refers to the nominal diameter (DN), that is, the inner diameter of the pipe. However, the structure of the expansion joint determines that it can't directly measure the inner wall-the bellows has peaks and valleys, the flange has stops, and the guide tube extends a section. You take a tape measure and put a card on the crest of the wave, and the data is at least 10-20mm larger than the diameter. That's the margin left for deformation, not the caliber.

Circular metal corrugated expansion jointFor example, general-purpose corrugated expansion joint and corrugated expansion joint for power station industry, the diameter is equal to the nominal diameter of the pipeline connected to it. For physical measurement, measure the diameter between the sealing surfaces of the flange-that is, the diameter of the inner hole after removing the flange boss. Or measure the inner diameter of the connector directly. Remember: Don't measure the crest, and don't measure the outer diameter of the flange.

Rectangular expansion joints and non-metallic expansion jointsHow to measure? Metal rectangular expansion joints and rectangular non-metallic expansion joints, the caliber is actually the length × width of the rectangular channel. Take a tape measure to measure the long and short sides of the inner mouth of the flange, and pay attention to whether the four corners are right or chamfered. The frame of non-metallic expansion joints (fabric fiber expansion joints) is metallic and the caliber is also the inner frame size. Don't count the thickness of the outer skin when measuring, otherwise it will be too large. In addition, such as PTFE-lined hoses, lined with PTFE may affect the inner diameter. It is necessary to confirm whether to measure the diameter of the metal shell or the lining-usually calculated according to the actual diameter after the lining.

Measuring tools and hands-on details

Choose the right tool and save half the detour. The vernier caliper is the most accurate and suitable for small calibers (below DN200). Use a tape measure for large diameter (DN300 or above), but it must be straightened and fit the end face. Measurement position: Select the inner circle of the flange end face to avoid welds and burrs. Measure two to three directions and take the average value-the pipeline may have ellipticity, especially large-diameter thick-walled expansion joints or directly buried expansion joints. Excessive ellipticity will lead to lax seal.

Does the expansion joint come with a deflector? The inner diameter of the guide tube is generally consistent with the diameter, but the front end of the guide tube may shrink. When measuring, it should be measured from the flange surface to the inner wall of the guide tube. Don't be lazy and only measure the outer port, otherwise install it and find that the guide tube holds up against the pipeline, so rework it as well.

So what to do? If conditions permit, it is best to actually remove and measure, or check the original equipment nameplate-the model and size of the expansion joint are generally marked DNxx on the nameplate. Before buying, take a photo of the nameplate and send it to the manufacturer for confirmation, which is more reliable than your own quantity.

It is not enough to have the caliber alone, but to give all these 4 data in the selection

Caliber is just the first step. When buying an expansion joint, you must also report the connection mode (flange or take-over), nominal pressure (PN), working temperature, medium, and compensated displacement (axial, transverse and angular). For example, high-temperature axial expansion joint is used in steam pipeline, and the temperature is not reported, so the consequences of choosing the wrong material are serious. Another example is the expansion joint matched by the desulfurization flue gas baffle door. The medium is corrosive, so the sulfur content, temperature and pressure must be indicated. Also, the number of flange bolt holes, the diameter of the distribution circle, and the form of the sealing surface (convex, flat) must be provided-this directly affects the ability to match your existing pipe flange.

Do you find it troublesome? The price of trouble is that the arrival can't be loaded, the return freight is paid out of pocket, and the construction period is delayed by yourself. Thinking back, it is much more cost-effective to spend 10 more minutes writing all the parameters than tossing for a month.

Finally, let's talk about a few places that are prone to fall into pits

FirstDon't take the mounting size of the non-metallic expansion joint as the caliber. The mounting size is typically larger than the through diameter because the outer edge of the flange is covered. You report the installation size in the past, and the manufacturer produces it according to the diameter. When you install it back, the flange edge is exposed outside, and the sealing gasket is not compacted.

Second, rubber compensator or rubber PTFE compensator is soft, gently press when measuring, do not deadlift, otherwise the data is on the big side. Soft materials deform as soon as they are pulled. You measure 300mm, but in fact, the diameter is only 250mm. It is best to gently lean against it with a caliper, not hard.

ThirdWhen the old equipment is replaced, it is the most hassle-free to check the original nameplate. If you can't see the nameplate clearly, remove it to measure the inner diameter of the flange, or look for the design drawings. If it doesn't work, take the outer diameter of the pipe minus the wall thickness to estimate it-but this method has a large error and is only suitable for emergencies.

Place an order after the quantity is accurate, so as to save returning and exchanging back and forth and delaying the construction period. Remember: the diameter of the expansion joint measures the diameter, not the crest; The inner diameter is measured, not the outer diameter of the flange; Clearance is measured, not hemming size.How to measure the expansion joint diameter?The answer is one: measure the diameter of the inner hole between the end faces of the flange, the diameter of the circle, the length and width of the rectangle, and the attention of the guide tube to avoid the shrinkage. Choose a caliper or tape measure as a tool, and measure three directions to take the average. When the data is complete, the rest will be smooth.

1. Why does the food industry have "special" requirements for expansion joints? It's not just the material.

It's not just any fluid running through the food processing pipeline. Milk, juices, sauces, beer…these media require extremely high hygiene levels. Even the slightest dead angle or trace precipitate in ordinary expansion joints may contaminate the whole batch of products. From the light to the scrapping of the whole pot, from the worst to the start of product recall, the loss is easily hundreds of thousands.

The core of food-grade expansion joints is not "whether it can compensate for displacement" at all-that is a basic function, and even an expansion joint should do it. The real hard indicator is: whether food safety can be guaranteed. Behind this is a series of hard levers involved in materials, construction, surface treatment, cleaning verification, and more.

Two days ago, I met a customer who made condiments, saying that their last batch of tomato sauce was found to exceed the standard of heavy metals. At the end of the investigation, the problem was a stainless steel hose on the pipeline that was not passivated. Tsk, just this one oversight, the whole batch of goods was dumped. So don't think that "stainless steel is food grade", it's far from it.

Second, the material is the bottom line: stainless steel, PTFE or rubber? How to choose to be compliant.

Food contact materials must comply with FDA, GB 4806 series and other standards. What material to choose depends on the media characteristics and process conditions.

• PTFE-lined hose/PTFE compensator: The lining polytetrafluoroethylene (PTFE) is extremely inert, can resist acid and alkali and does not precipitate. Suitable for highly clean or corrosive food media, such as CIP cleaning solution, acidic juice, vinegar. This site'sPTFE-lined hoseAndPTFE compensatorExcellent in corrosion resistance and hygiene.
• Stainless steel metal hose (304 or 316L): Suitable for high temperature steam, dry materials or purified water. However, note that the surface roughness cannot exceed the standard-Ra ≤0.8 μ m is the bottom line, and it is best to achieve 0.4 μ m. Moreover, passivation treatment must be done, and the weld should be polished and smooth. This site'sUniversal corrugated expansion jointAndMetal hoseBoth support custom polish grades.
• rubber compensator: Food grade ethylene propylene monomer (EPDM) or silicone rubber can be used, but no plasticizer migration must be confirmed. For example, when transporting dairy products or oils, if small molecules precipitate in the rubber formula, it will contaminate the product. This siterubber compensatorAndRubber PTFE compensatorAll provide food grade material certification reports.

Some suppliers say that "stainless steel is food grade", but in fact, no passivation is done and the welds are not polished. The surface roughness is large, it is easy to hang on the wall and breed bacteria, and CIP cleaning can't wash it clean at all. So don't just look at the material grade, ask about the surface treatment and dissolved substance test report.

Third, the structural design determines the "dead zone" and "cleanliness". Don't underestimate the guide tube and the dead corner.

What are food grade expansion joints most afraid of? Hiding dirt and evil. If there are steps or gaps at the connection between the bellows and the pipe, bacteria will grow after the material remains, and CIP cleaning will not clean it. What about that? Deflectors are not optional, they are a necessity.

The inner diameter of the guide tube must be consistent with the inner diameter of the pipe, and the transition must be smooth to avoid vortex and deposition. Regarding this, this siteSpecific Function of Expansion Joint Guide TubeThat question and answer said it very clearly-to put it bluntly, it is "straighten out the flow path of the medium so that the dirt has no place to hide".

In addition, it is recommended to use concave and convex surfaces or tongue and groove surfaces for flange sealing surfaces, and cooperate with food-grade gaskets (such as PTFE coated gaskets) to prevent gasket debris contamination. Structurally, dead corners such as narrow slits and exposed threads should be avoided. And guess what? In order to reduce the cost, some manufacturers saved the guide tube. As a result, the pipeline was blocked after half a year of use by the customer. When they were disassembled, they saw that the root of the bellows was full of mold spots.

IV. Manufacturing process and testing: Materials alone are not enough, surface treatment and certification should be implemented.

Just having good materials and no corresponding craftsmanship is equal to nothing. The metal inner walls of food grade expansion joints must be mechanically polished or electrochemically polished, with a roughness Ra ≤0.8 μ m (even more stringent to 0.4 μ m), and the weld must be smoothed. The welding process should adopt argon arc welding, and do acid pickling passivation treatment to restore the passivation film on the surface of stainless steel.

Before leaving the factory, it must pass the pressure test, air tightness test, and the verification of simulated CIP cleaning. In terms of certification, domestic products can issue third-party food contact material testing reports, while export products need FDA or EU 1935/2004 certification.

V. Actual selection: What should be used in different sections of the same food workshop?

The pretreatment section of a dairy factory needs acid and alkali resistant CIP cleaning, the temperature is about 80℃, we recommendPTFE-lined hoseThe lining PTFE is corrosion resistant and does not adsorb milk fat; The filling section is a normal temperature and sterile environment.Food grade stainless steel metal hose (universal corrugated expansion joint)Polished to Ra0.6 μ m and connected with a quick-assembly clamp; The pipeline displacement of steam sterilization section is large, so selectHigh temperature axial expansion jointThe inner liner is designed as a removable structure for easy access.

You see, different locations of the same factory have different requirements. The key is to match the existing product models of this site according to the medium, temperature and cleaning frequency, rather than buying a "food grade" label with your eyes closed. There is no universal solution in the food industry. Choosing the right expansion joint in each section can not only ensure safety but also save maintenance money.

The expansion joint is a "small part" in the pipeline system, but once it strikes, the whole line has to stop. How to judge how old it is to the extent that it should be replaced? Don't wait for it to leak before panicking. Everyone in this industry should know the following methods of judgment.

1. Why do expansion joints age? Understand the aging mechanism first

Expansion joint aging is not metaphysics,Physical and chemical factorsEach and a half. Metals-such as general-purpose corrugated expansion joints and high-temperature axial expansion joints-mainly carry fatigue and corrosion. Repeated action of alternating stress, coupled with chloride ions and sulfide in the medium, cracks will occur in the bellows over time. Non-metallic ones are more troublesome, such as non-metallic expansion joints (fabric fiber expansion joints) and rubber compensators. Ultraviolet rays, ozone and temperature will make rubber hard and fabric fibers delaminate. To put it bluntly, high temperature, high pressure and corrosive media are three "killers". Only by accelerating aging and figuring out these can the judgment have a basis.

The desulfurization flue gas baffle door used in power plant desulfurization system, the non-metallic expansion joint next to it, the medium temperature is high and the acid condensate is also present, and the life time is often less than half of the design value. This is the result of the acceleration of dielectric corrosion superposition temperature.

2. Aging signals visible to the naked eye

No matter how professional the testing is, the first step is always to look. Rust pits on the surface of the metal expansion joint and fine cracks at the trough of the bellows are typical signals. Non-metallic expansion joints are more pronounced: cracked, sticky, bulging surfaces, or layered blistering of fabric layers. In addition, pay attention to whether there is any abnormal displacement in the installation position, such as the deformation of the guide tube and the loosening of the tie rod nut (how to adjust the specific question and answer on how to adjust the tie rod nut of the expansion joint), which may mean that the internal structure can no longer hold up.

Two days ago, a customer reported that the non-metallic expansion joint next to the flue gas baffle door had a large area of delamination in three months, and when it was disassembled, it was seen that the fibers were carbonized. Did you say it was miserable? This kind of thing can be detected by the naked eye, and there is no need to wait for the detection data.

For directly buried (fully buried) expansion joints, the surface cannot be seen, but it can be checked whether there is abnormal uplift or settlement on the ground, and indirectly judge whether the interior fails.

3. Performance data will not lie: measure stiffness and displacement

If your eyes can't see correctly, talk to the data. Once the stiffness of the expansion joint (refer to the article on the stiffness and calculation formula of the bellows) changes by more than 20%, it will basically fail. How to measure? Measure axial stiffness with a simple loading device or directly compare factory reports. The displacement compensation ability is also a hard indicator-if the expansion joint cannot be restored to its original position in actual operation, it means that the bellows has undergone plastic deformation.

The corrugated expansion joints and compound hinge transverse expansion joints used in the power station industry should be regularly tested for displacement rebound. Don't be too troublesome, you can screen out 80% of the hidden dangers with one test. And guess what? Some factories are too troublesome, and after three years of accident, the bellows got stuck, the stress of the whole pipeline exceeded the standard, and the brackets were crooked.

There is also the system stress exceeding the standard caused by stiffness attenuation, which can be known by measuring the stress of the pipe support with a strain gauge. Don't wait for the bracket to deform before checking, then the cost will double.

4. Key points of aging judgment of different types of expansion joints

Different materials and structures have different "dead points" of aging. Don't look for non-metals with the one you use for metals.

• Metal expansion joint(Including direct burial type, straight pipe pressure balance type, curved pipe pressure balance type, etc.): Focus on the thinning of bellows trough thickness,Ultrasonic thickness gauge is the most accurate。 Large diameter thick wall expansion joints also have to check the weld parts, especially the tie rod joints.
• Rubber compensator and rubber PTFE compensator: The hardness tester brushes and brushes, and it is basically wasted if the hardness increases by more than 20 degrees; Press again to see if there is any elastic restoring force. After the rubber is aged, it becomes hard and can't compensate for the displacement, so the hard brace will crack.
• Non-metallic expansion joints (fabric fiber expansion joints, rectangular non-metallic expansion joints): Pull a piece of fiber to see the strength, or use water pressure to check the sealing performance. The non-metallic expansion joint matched by the flue baffle door is even worse. It runs in high-temperature flue for one year, and the aging rate is three times faster than that indoors.
• Special scenario: Desulfurization flue gas baffle door, air-cooled island vacuum pipeline double hinge expansion joint, these should consider medium corrosion and vacuum fatigue, and it is easy to miss judgment by appearance alone. The fine cracks caused by vacuum fatigue can't be seen clearly with the naked eye, so they have to be tested by penetration.

In addition, the rotary compensator, sleeve-type pipe expansion joint and other poor maintainability, once aging, the repair cost is higher than replacement. So judge more decisively.

5. Break when you break: When should you change instead of repair?

If you find aging, don't always think about making do. Several hard indicators are well held:

• The crack depth of the metal expansion joint exceeds the wall thickness by 10%;
• The leakage test of the non-metallic expansion joint is unqualified, or the damaged area of the fabric layer is more than 10%;
• The stiffness attenuation causes the system stress to exceed the standard (it will be known by measuring the force of the pipeline support with a strain gauge);
• The surface crack depth of the rubber component exceeds 2mm.

At this time, don't worry about "how long it can be used", and go directly to the manufacturer to replace it. For example, rotary compensator, sleeve-type pipe expansion joint, maintenance requires dismantling the pipeline, and the labor cost is more expensive than new parts. Remember, the life of expansion joints is usually only 60%-80% of the design life (refer to the Q&A on the service life of expansion joints), so making spare parts plans in advance is the way to save money. Don't wait until the middle of the night to make a call in a hurry. At that time, doubling the spot price will delay the construction period.

The core of the aging judgment method of expansion joint is "eye diligence, hand diligence and data diligence". Regular inspections and records are better than anything else.

Let's tell the truth first. Two days ago, a purchaser from a chemical plant complained to me, saying that their hot water circulation pipeline used the expansion joint for less than half a year, and suddenly it cracked, and the scene was sprayed in a mess. I asked him if he heard a loud noise, if the pipe was shaking violently, and he said yes, and when the valve was closed, it was like someone had smashed a hammer in the pipe. Okay, solved the case- -water hammer.

The water hammer is called "invisible killer" in the industry. It doesn't take its time like corrosion, wear and tear, it is an instant crit. Think about it, the high-speed flowing liquid is suddenly stopped by a valve or pump, and the kinetic energy is instantly converted into a pressure wave, which bounces back and forth in the pipeline. The peak pressure can reach several times or even ten times the normal working pressure. If the pipe can't hold it, just blow it up.

How does water hammer happen? Understand this "invisible killer" first

A long water pipe, the water is running at a speed of 3 meters per second, and suddenly you shut the valve sharply-the water can't stop. Its forward pressure is blocked by the valve, and the pressure wave begins to propagate upward at the speed of sound from the valve. When it hits the elbow, tee and blind plate, it will be reflected back and form an oscillation. How violent is the peak of this pressure wave? The measured data show that the peak water hammer value of DN300 steel pipe can soar to more than 15kg/cm² under the normal working pressure of 3kg/cm². It's weird that the pipe doesn't crack.

What about that? There are two ways to solve the problem from the root: one is to control the operation, such as slowly closing the valve and installing a slow-closing check valve; The second is to rely on equipment to carry it hard-the expansion joint is the protagonist here.

Expansion Energy Saving Holds Water Hammer? The principle is actually very simple

The core component of the expansion joint is the bellows. Whether it is metal or non-metal, it essentially uses elastic deformation to absorb the displacement and vibration generated by the pipe. When the pressure wave rushes over, the bellows will compress or stretch like a spring, converting kinetic energy into deformation energy, thus reducing the peak pressure. The principle sounds simple, but whether you can hold it depends on the selection and design.

For example, the product of this siteUniversal corrugated expansion jointThe design is mainly to absorb the axial displacement of thermal expansion and contraction, and the wall thickness and wave pitch of its bellows are determined according to the thermal displacement conditions. The pressure wave of a water hammer is a high-frequency shock, which is completely different from the slowly changing load of thermal displacement. You take the universal model to carry the water hammer, which is like taking an umbrella to block a bullet-not that it is completely useless, but it is easy to break through.

Type selection door: Not any expansion joint can prevent hammer

Then what should I use? The key lies in three parameters: pressure grade, stiffness and fatigue life.

• Pressure rating: Do not select according to the working pressure, at least 1.5 times the peak pressure of the water hammer. For example, the working pressure is 4kg/cm², and the peak value of water hammer is calculated as 12kg/cm², so the nominal pressure of the expansion joint has to start with PN16.
• stiffness: The smaller the stiffness, the stronger the ability to absorb shocks. But the stiffness is too low and it is easy to become unstable, so we have to find a balance. Generally recommendedExternal pressure single axial expansion jointIts bellows is subjected to external pressure, has good stability, and the stiffness can be made relatively low, which is suitable for absorbing shocks.
• Fatigue life: Water hammer is a cyclic load, with few times but huge amplitude. The ordinary thermal displacement design may only require a fatigue life of 1000 times, but it is recommended to do more than 10000 times for water hammer resistance. This site'sStraight pipe pressure balanced expansion jointAndCompound hinge transverse expansion jointThere are redundancies in fatigue design, which can cope with such shocks.

In addition, the design of the guide tube cannot be ignored. The guide tube can guide the medium smoothly through the bellows, avoid vortex impact, and at the same time, it can also prevent high-pressure waves from directly impacting the root of the bellows. In the product information of this site,Corrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAll of them have been strengthened on the diversion tube, so you can see the specific structure diagram.

If it is a corrosive medium or high temperature working condition, you have to consider the material. Such as usingPTFE-lined hoseOrPTFE compensatorThey are corrosion resistant, but their pressure-bearing capacity is relatively weak, so they need to be installed with a limit tie rod. How to adjust the limit pull rod? You can refer to "How to Adjust the Tie Rod Nut of Expansion Joint" we wrote before. Simply put, loosen the nut after installation, let the bellows expand and contract freely, and tighten it to the set pre-stretching amount.

Installation and configuration: wrong location, wasted effect

If you choose the right product, it will be useless if you can't install it. Under water hammer conditions, there are several iron laws for the installation of expansion joints:

• The fixing bracket must be strong enough: The expansion joint itself does not bear the blind plate force, and the blind plate force should be carried by the fixed bracket. Under the impact of water hammer, if the stiffness of the fixed bracket is not enough, the whole pipe will be pushed away, and the expansion joint will directly exceed the displacement limit and tear. The main fixing point should be set near the valve, elbow and reducing diameter.
• The spacing between guide brackets should be calculated well: In order to prevent the pipe from swinging laterally under the action of pressure waves, the spacing of the guide brackets shall not exceed 80% of the allowable span of the pipe. Especially for horizontal pipe sections, if the spacing is too large, the pipe will swing up like a fish's tail and twist the expansion joint.
• Pre-stretch/pre-compression should be accurate: If there is a large difference between the installation ambient temperature and the working temperature, it is necessary to pre-stretch or pre-compress according to the calculated value. For specific operation, please refer to the steps in "Correct Installation Method of Large Tie Rod Expansion Joint". The core is to loosen the nut → pull or press to the predetermined size → lock the nut.
• Don't reverse the direction of the expansion joint: Many products have arrows on them, which is where the medium flows. If the direction of the arrow is wrong, the position of the guide tube will be reversed, and the high-pressure wave will directly hit the inside of the corrugation, which is easy to tear.

Two days ago, another customer asked me that they had installed the pipelinerubber compensatorIs it more resistant to water hammer than metal? The rubber compensator does have a good vibration damping effect, but its pressure resistance is not as good as the metal bellows, and it ages quickly. If your water hammer has high pressure and high frequency, it is recommended to use itMetal expansion jointRubber is suitable for low pressure and small amplitude applications, such as water pump inlet and outlet.

Comparison of real cases: How big is the gap between putting it right and putting it wrong

Last year, there was a steam condensate recovery system in a paper mill, with a design pressure of 6kg/cm² and an operating temperature of 95℃. They originally used a certain brand of general-purpose corrugated expansion joint, which took about 8 months. After an emergency pump stop, a DN250 expansion joint on the main pipeline exploded, and the corrugated pipe directly tore open a 30cm hole. The scene was filled with water vapor, almost injuring people.

The peak water hammer is at least 20kg/cm², while the nominal pressure of the expansion joint they use is only PN10, and the fatigue life design is only 500 times. Moreover, the spacing between the guide brackets is too large during installation, and the expansion joint is twisted by transverse sliding of the pipeline during impact, and the stress at the root of the bellows is concentrated, which leads to direct brittle fracture.

Later we changed the one we recommendedExternal pressure single axial expansion jointWith a nominal pressure of PN25 and a nominal fatigue life of 12,000 times, a fixed bracket is added on the outlet side of the valve, and the spacing between the guide brackets is reduced from 6 meters to 3.5 meters. I haven't had any accidents for more than two years now. Is that the truth? Choosing the right model and installing the right position doesn't cost much, but it saves a lot of worry.

Another case is the steam turbine bypass pipeline of a power plant, high temperature and high pressure steam, and the risk of water hammer is also high. They usedStraight pipe pressure balanced expansion jointThis structure has its own pressure balance cavity, which can effectively offset the blind plate force. At the same time, the bellows adopts multi-layer thin-wall design, which has good flexibility and obvious effect of absorbing shock waves. After using it for three years, when I opened it and inspected it, I couldn't even find cracks on the surface of the bellows, and there was only slight oxidation on the top. Therefore, don't save that little money, spend more time on the selection, and the maintenance cost can save a lot.

Finally, water hammer protection can not be solved by expansion joints alone, but also by integrating pipe system design, operation process and bracket configuration. However, as the last line of defense, the expansion joint can save your life if you choose it right, but if you choose it wrong, it will be a hidden danger. If there are any specific project conditions, feel free to ask. Let's calculate the parameters together and don't let the pipeline crack again.

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.