1. First understand the core parameters: what do you think of stiffness, pressure, temperature and compensation?
I have been in this business for 20 years, and I am most afraid that I will fill in "high temperature resistance" directly on the selection list. There are only a few core parameters of bellows expansion joints, but each one hides pits.
stiffness— — Don't be fooled by the word "stiffness". Simply put, it is the ability of bellows to resist deformation. The greater the stiffness, the greater the thrust generated at the same displacement. If you choose an expansion joint with too high stiffness, the pipe bracket and fixing point may directly push through. This site has a special "Stiffness and Calculation Formula of Bellows", which gives the theoretical algorithm. However, when actually selecting the model, it is recommended to ask the manufacturer directly for the measured stiffness value on the sample, which is the real guy.
pressureAndTemperatureIt's a pair of conjoined babies. The pressure level is easy to understand. Which one should I choose between 0.6MPa, 1.0MPa and 1.6MPa? But as soon as the temperature rises, the permissible stress of the material drops. For example, the bearing capacity of 316L stainless steel at 400℃ and room temperature is quite different. Many peers only look at the design temperature, but ignore the "maximum working temperature × duration"-short-term overtemperature may be fine, and long-term operation at the critical point will directly cut the life of bellows in half. So chooseHigh temperature axial expansion jointWhen, not only look at the nominal temperature resistance value, but also confirm whether the number of corrugated layers and wall thickness meet the fatigue life requirements.
Compensation amountIt's a gimmick hit area. Some manufacturers mark "axial compensation ± 50mm", but that is the theoretical value calculated by multiplying the single wave compensation by the wave number. In fact, the pre-stretching amount, safety factor and cold tightness amount have to be deducted. The correct way is to first calculate the thermal elongation of the pipeline, then multiply it by the insurance factor of 1.2~1.5, and finally check it against the "allowable compensation curve" given by the manufacturer. Don't expect an expansion joint to overcome the world. If the compensation amount is too large, consider using itCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointTo share.
2. Don't pat your head when selecting models: high-temperature axial type, general-purpose type, straight pipe pressure balance type, working conditions determine the model
I met a customer two days ago. The temperature of the steam pipeline is 350℃ and the pressure is 1.0MPa. The direction of the pipeline is simple, but it is necessary to go upStraight pipe pressure balanced expansion joint。 Why? Because I heard that "the pressure balance type is good, and there is no blind board force". However, the pressure balance type has complicated structure, high cost and large installation space requirements. For the working condition that there is no strict thrust limit in the straight pipe section, it is purely a knife for killing chickens.
- High temperature axial expansion joint— — It is suitable for straight pipe sections with high temperature and large axial displacement. The temperature resistance can reach above 600℃, but it can only absorb axial displacement and cannot bear lateral or angular displacement. Note that its guide tube is standard to prevent high-temperature media from directly washing the bellows.
- Universal corrugated expansion joint— — It is often called universal type, which can absorb axial, transverse and angular displacement, and is suitable for pipelines with small and medium diameter, low pressure and temperature no more than 300℃. High cost performance, but the fatigue life is relatively short, so don't use high-frequency vibration occasions.
- Straight pipe pressure balanced expansion joint— — It is mainly used in places where the fixed bracket cannot bear the blind plate force, such as the inlet and outlet of steam turbine and the outlet pipeline of pump. It can balance the thrust generated by internal pressure, but it needs to reserve enough space when installing, and the two bellows need to act synchronously, which requires high welding and assembly accuracy.
Apart from that, there areCurved tube pressure balance expansion jointFits at the elbow,Double straight pipe bypass pressure balance typeSolve the problem of large compensation amount. I just want to tell you that there is no standard answer to model selection, only the working conditions determine the model. Don't save trouble, calculate the design parameters clearly, and then flip through the product library of this site. Each model has the corresponding application scenario description.
3. The three most common pits at the installation site: the direction of the guide tube is reversed, the pre-tension is forgotten, and the tie rod nut is randomly adjusted
If you choose the right model, it will still be useless if you mess up the installation. There are countless pits on the scene, and these three are the most typical ones.
The first pit: the direction of the guide tube is reversed.The function of the expansion joint guide tube is to protect the bellows and avoid direct scouring of the medium. However, many masters think that it is a flow direction sign when they look at the arrow-wrong, the direction of the arrow is the flow direction of the medium, and the open end of the guide tube must face the direction of the incoming flow. If it is installed backwards, the guide tube becomes a gas collecting chamber, and the medium forms a vortex in the tube, which intensifies the vibration, and the bellows is worn out in minutes. Regarding this point, there is a diagram in the question and answer of "Specific Function of Expansion Joint Guide Tube" on this site. It is recommended to take a look before installation.
The second pit: forgot to do the pre-stretch.For axial bellows expansion joints, if pre-tensioning (or cold tightening) is not done during installation, the expansion joints may be directly pulled to the limit after the pipeline heats up, and the bellows may be plastically deformed and scrapped. The amount of pre-stretch is generally half the amount of design compensation. For example, the design compensation amount is 80mm, and the pre-stretching amount is 40mm. How to pull? Use the pull rod and the nut to cooperate, don't pull hard, it is easy to hurt the ripples. Note: Remove the limit screw for transportation after pre-tensioning, otherwise the expansion joint will not work. Some expansion joints (such asExternal pressure single axial expansion joint) Comes with pre-stretching device, confirm before installation.
The third pit: the tie rod nut is randomly adjusted.What is the function of the tie rod of the expansion joint? It is clearly written in the question and answer-limit the excessive expansion and contraction of the bellows and prevent instability. However, some on-site masters think that tightening the nut means fixing, and loosening it means releasing. As a result, the tie rod is adjusted to a dead limit, and the expansion joint completely loses its compensation ability. The correct way is to adjust the nut to the design position according to the drawing before installation (usually with a gap), and then fine-tune it according to the actual needs after installing the pipe. EspeciallyLarge tie rod expansion jointThe tie rod is not a load-bearing part, but a limiting part. Don't use it as a hard support. There are also expansion joints with screws, asking "Do the screws need to be removed"? The answer is: the transport screw must be disassembled, and the tie rod screw remains but the position is adjusted.
4. Industry application comparison: how to set up corrugated expansion joints for power stations, metal corrugated expansion joints for cement industry and desulfurization flue gas baffle doors
Different industries, working conditions vary greatly, and the emphasis of type selection and installation is completely different.
Power station industry: The main steam pipeline, reheating pipeline and water supply pipeline are all large-diameter, high-pressure and high-temperature expansion joints, such asCorrugated expansion joint for power station industryAndDouble hinge expansion joint for air-cooled island vacuum pipeline。 The calculation of pipeline thrust in power stations is extremely strict, and compound hinges or pressure balance types are often used to release the thrust on steam turbines and boilers. Special attention should be paid to when installing: preheating before welding, and slow cooling after welding to prevent stress cracking in the heat-affected zone. Moreover, power station pipelines often have vertical sections, so guide brackets should be set to prevent the expansion joint from twisting.
Cement industry: The high-temperature fan outlet, kiln tail smoke chamber and decomposition furnace inlet and outlet of cement production line, the temperature is 350~500℃ all the year round, with large dust content and serious medium wear. You can't use ordinary stainless steel bellows at this time, you have toMetal Corrugated Expansion Joints in Cement IndustryIn particular, attention should be paid to the lining wear-resistant layer and multi-layer corrugated design. In addition, such occasions are usually associated withRectangular non-metallic expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)Used on flue gas pipelines, non-metallic materials are corrosion resistant and have high fault tolerance.
Desulfurization flue gas system: The flue gas after wet desulfurization has low temperature but acid corrosion, so it must be usedDesulfurization flue gas baffle doorMatching expansion joint. The most common thing here isDouble-sealed single-axis circular baffle doorAndRound Flap Door (Double Seal), cooperate withrubber compensatorOrPTFE-lined hoseTo isolate corrosive media. Note: The rubber compensator is not resistant to high temperatures and can only be used for clean flue with smoke temperature below 80℃; If the flue gas temperature fluctuates greatly, it is recommended to usePTFE compensator。 The length of the connection between the baffle door and the expansion joint should be sufficient, otherwise the expansion joint will be squeezed and deformed after closing.
5. There is a way to maintain: What is the impact on the service life of the expansion joint? Replacement cycle of rubber compensator and metal hose
Expansion joints are not permanent pieces, but they vary greatly in life. Main factors affecting longevity-by importance:
- Large temperature cycle-frequent start-stop process harms the bellows the most, because the number of thermal stress cycles directly determines the fatigue life.
- Medium corrosion-the chloride ion content exceeds 100ppm, 316L can't bear it, must be replacedPTFE-lined hoseOr higher grade stainless steel.
- Installation error-the initial stress caused by misedging and forced matching of the pipeline is equal to overdrafting the life every day.
- Vibration-Pipe vibration will accelerate the stress concentration at the crest of the bellows, which is prone to cracking.
thatrubber compensatorHow often do you change? Depending on working conditions. The rubber compensator used for desulfurization flue is subject to acid-alkali corrosion and ultraviolet aging, and generally needs to be inspected in 2~3 years. If you find that the rubber surface is hardened, cracked, or even delaminated, replace it immediately. WhileMetal hose(includingSpecial hose for vacuum) The replacement cycle is slightly longer, and it can be used for 5~8 years when there is no obvious corrosion, but the premise is to regularly check the outer surface of the bellows for pitting or wear marks.
Every overhaul cycle, open the insulation layer and visually check the bellows for pits, cracks and scratches; Touch the guide tube with your hand to see if it is loose; Whether the loosening spacer for the tie rod nut is still there. Don't wait until a leak is replaced, by which time you may have damaged nearby brackets and equipment.
To put it bluntly, the bellows expansion joint is worth ten years if it is selected correctly, but it will be scrapped if it is installed wrongly for ten days. I hope these experiences can help you step on fewer pits at the scene.