What exactly can metal expansion joints solve? Understand its core role first
Anyone who does pipeline design knows that thermal expansion and contraction are hard bones that can't be avoided. Once the steam pipe heats up, the pipe of tens of meters can eject several centimeters of displacement. Hard to carry it? Pipe frame deformation, cracked welds, flange leakage, which is enough for you to drink a pot. Metal expansion joints do the job-it uses the structure of a bellows to absorb displacement while ensuring that the pipe is not stressed extra.
So don't think of the expansion joint as an ordinary connector. The core role is "compensator". Compensate for thermal displacement, compensate for installation deviations, compensate for equipment vibration, and even reduce noise. For example, the ones commonly used in the power station industryCorrugated expansion joint for power station industryWith hundreds of degrees of steam at every turn, the whole thermal system can't run without it. Another exampleDirect buried (fully buried) type expansion jointSpecially dealing with the soil extrusion and temperature change of buried pipelines. Think about it, the pipe is buried three meters deep underground. Once there is a problem, it will be dug out and repaired, and the cost will be enough to buy several expansion joints.
To put it bluntly, choosing the right expansion joint is equivalent to buying a "flexibility insurance" for the pipeline.
How to select the type under different working conditions? Universal type, high temperature type, direct burial type... Don't choose the wrong one
Two days ago, I met a customer who installed a general-purpose expansion joint into the desulfurization flue, and it leaked in three months. Why? The corrosiveness of media is different, and the material is not on the right path. The matter of model selection has to be looked at from several dimensions:
Temperature is the first switch
Ordinary steam pipe, for use below 350℃Universal corrugated expansion jointThat's enough. Go up, for example, the kiln tail flue gas in the cement industry, if the temperature can soar above 500℃, you have to ask outHigh temperature axial expansion joint。 Thermal insulation layer will be added inside this product, and the corrugated pipe material will be upgraded to heat-resistant alloy, otherwise the corrugated pipe will creep and fail in minutes.
Media and environment determine the "skin"
Corrosive media For example, desulfurization slurry and chlorine-containing waste gas must bePTFE-lined hoseOrPTFE compensator。 PTFE lining can carry almost all strong acids and alkalis, but it should be noted that the temperature cannot exceed 200℃. If the medium contains particles and has strong wear, such as ash pipelines in power plants, you have to chooseLarge diameter thick wall expansion jointThe wall thickness of the bellows is thickened, and a guide tube has to be equipped to prevent erosion.
The type of displacement determines the structure
There are many axial displacements, so use axial type; When lateral and angular displacement is required, useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 Directly buried pipelines are preferredDirect buried (fully buried) type expansion joint, there is an anti-corrosion layer and insulation layer on the outside, which can be buried directly in the soil. Then what judgment? Look at the amount of displacement on the design drawings, don't pat your head.
Pits that are easy to step on in installation and commissioning: the direction of draw rod, guide tube and arrow should not be careless
Choose the right product, install one wrong, all dry for nothing. The most outrageous thing I have ever seen is that the worker installed the expansion joint backwards, and the direction of the arrow was opposite to the direction of the medium flow. As a result, the guide tube was overturned by the airflow, and the inner wall of the bellows was directly exposed to the high-speed fluid, which wore out in two months.
Look at the direction of the arrow before installing. The expansion joints are marked with flow direction arrows pointing in the direction of the media flow. This arrow is not just a hint, it determines the orientation of the internal guide tube. If it is installed backwards, the guide tube will not play the role of drainage and protection, and the flushing will all be washed on the bellows.
Let's talk about the pull rod. Many expansion joints come from the factory with tie rods and nuts, such asUniversal corrugated expansion jointCommon onexpansion joint tie rodDon't screw it to death during on-site installation. The function of the tie rod is to bear the blind plate force of the pipe and prevent the bellows from over-stretching, but the adjustment amount of pre-stretching or pre-compression should be allowed when installing. How to tune it? Refer to the installation instructions given by the manufacturer, generally adjust the nut to the specified scale. If you forget to adjust it, it will be welded to death directly. After the pipeline heats up, the expansion exceeds the bellows stroke, and it will be pulled out immediately.
The guide tube can reduce the direct flushing of the fluid to the bellows, and also reduce the flow resistance. Therefore, when installing, ensure that the inner diameter of the guide tube matches the inner diameter of the pipe. Too much misalignment will still cause vortex wear.
Metal Expansion Joints vs Non-Metal Expansion Joints vs Rubber Compensators: Which One Should I Use?
There is no universal compensator, only the right one.
Metal expansion joint(Corrugated expansion joint) has high pressure resistance and high temperature resistance (the highest can be above 800℃), but it can't withstand large angle twisting displacement, and it is expensive. It is suitable for scenes such as high temperature and high pressure steam, oil and gas pipelines and power station boilers. This site'sHigh temperature axial expansion joint、Universal corrugated expansion jointAll fall into this category.
Non-metallic expansion joint(Fabric fiber expansion joint) is mainly made of fiber material plus rubber/silicone coating, and the temperature resistance can reach about 300℃. The advantages are large compensation, good vibration reduction and light installation. It is especially suitable for large-diameter flue gas pipelines, such as desulfurization system and dust removal system. The disadvantage is that it cannot withstand high pressure and is easy to age. OurRectangular non-metallic expansion jointAndNon-metallic expansion joint (fabric fiber expansion joint)Is a typical representative.
rubber compensator(also called rubber joint) has the best flexibility, can absorb vibration and noise, and is easy to install. However, the temperature resistance is only about 100℃, the pressure resistance is also low, and it is not resistant to oil and organic solvents. Suitable for pump inlet and outlet, air-conditioning pipeline, fire-fighting pipeline and other low-pressure and normal-temperature systems. This site'srubber compensatorAndRubber PTFE compensatorDifferent media can be dealt with.
Then how to choose? A simple and rude principle: the temperature exceeds 200℃ or the pressure exceeds 1.0MPa, preference is given to metals; Large-diameter flue gas pipes (such as flue ducts of power plants), non-metal are more cost-effective; Use rubber where there is vibration. Keep this in mind, 80% of the selections will not go wrong.
Service Life & Maintenance: When Should You Replace? How to extend life?
Metal expansion joints are not lifetime pieces. Generally, the design life is 10~15 years, but in actual use, due to the influence of temperature, corrosion and fatigue, it may have to be replaced in 5~8 years. How do you judge that it's time to change? Observe whether there are cracks, corrosion pits and obvious plastic deformation on the bellows surface; Or whether there is any abnormal noise or vibration aggravated when the pipeline is running. If you find that the guide tube falls off and the tie rod is loose, you must deal with it quickly.
- Select the right materialFor example, heat-resistant alloy is used for high temperature working conditions, 316L stainless steel or even Hastelloy is used for corrosion working conditions. Don't choose ordinary 304 in order to save thousands of dollars, and scrap it in the last two years.
- Specification installation: The aforementioned arrow direction, tie rod adjustment, and guide tube matching are all basic skills.
- Regular inspection: Especially for pipelines with frequent start-and-stop, disassemble the insulation to check the state of the bellows once in each overhaul cycle, and immediately replace the early cracks.
Rotary compensatorAndSleeve type pipe expansion jointFor this kind of product, the sealing packing needs to be replaced regularly, otherwise leakage is a matter of time. While likeStraight pipe pressure balanced expansion jointIn this kind of structure, once the internal bellows is damaged, it is difficult to repair, so simply replace the assembly directly.
Well, the knowledge of metal expansion joints has been reviewed from principle to selection, installation, comparison and maintenance. What weird problems have you encountered in actual projects? Let's take a look. Most pits can actually be avoided in advance.