1. Don't be fooled by the word "high quality"-first find out where it is expensive
The price of metal expansion joints on the market ranges from several hundred to tens of thousands. With such a big price difference, is it just a brand premium? Of course not. High-quality metal expansion joints are valuable in three cores:Material, corrugation molding process, end structure。 In terms of material, the price difference between 304 stainless steel and 316L stainless steel is very big, let alone inconel625, a high-temperature nickel-based alloy. Some suppliers take 201 stainless steel as 304, and after one year, intergranular corrosion begins, corrugated pipes crack, and pipelines directly leak media-this kind of thing is too common in power stations and chemical industries.
Look at corrugation again. Hydroforming and roll forming, the cost gap is not small. The fatigue life of hydroforming corrugated pipe can be increased by more than 30% with uniform wall thickness and good stress distribution. However, some low-priced products are molded with different wall thicknesses, and the corrugated roots are prone to stress concentration, which will not take long to crack. Where do you think it's so expensive? It is valuable in the solid materials and craftsmanship.
2. Selection and rollover site: material, number of corrugated layers and end structure, which is the most pitted?
Two days ago, I met the purchase of a cement factory, saying that the expansion joint they used always leaked, and it didn't work after several changes. Working conditions: temperature 600℃ +, medium containing dust, pressure 0.3MPa. He used a general-purpose corrugated expansion joint, made of single-layer 304 material. Can this not roll over?
- Material: Heat-resistant alloys must be used for high-temperature operating conditions, such asHigh temperature axial expansion jointinconel or heat resistant stainless steel is usually used. Consider 316L or PTFE-lined solutions for corrosive media, e.g.PTFE-lined hoseIt is specially designed to deal with strong corrosion.
- Number of corrugated layers: Multi-layer bellows can withstand higher pressures, but the fatigue life will be reduced. Choose single layer or double layer for high voltage, and consider multi-layer for low voltage and large compensation-don't blindly pursue more layers.
- end structure: Welded end pipe or flanged connection? Do you want to add the deflector?expansion joint guide tubeThe function is to guide the flow direction of the medium and avoid the direct washing of the bellows by high-speed fluid. The medium flow rate in the power station pipeline is high and contains particles. Without the guide tube, it is equivalent to letting the bellows streak naked, and the life of the power station will decrease by a cliff.
Which do you say is the most pitful? I think the wrong choice of material is a fatal injury, and it can't bear the working conditions at all. However, the design error of the end structure is chronic suicide, which will go wrong sooner or later.
Third, stiffness, fatigue life and compensation amount-what do you think of these three figures?
Many purchasers got the parameter table, and when they saw the three figures of stiffness, fatigue life and compensation amount, they were directly dumbfounded. I'll teach you how to see it.
stiffnessDetermines the thrust of the expansion joint on the pipe. The greater the stiffness, the greater the thrust, and the higher the requirements for the bracket. When selecting a model, focus on axial stiffness and transverse stiffness, not just total stiffness-because the actual displacement is a combination of axial, transverse and angular directions. Such asCompound hinge transverse expansion jointIt mainly absorbs lateral displacement, and its lateral stiffness is the key.
Fatigue lifeIt is usually calibrated by the number of cycles, such as 1000, 5000. But note: This number is measured under standard conditions, and the actual working conditions should be discounted. Experienced practice is to choose fatigue life not less than 3 times the actual number of displacement cycles. For example, the pipeline is expected to expand and contract 300 times a year, and the design life is 10 years, so the fatigue life is at least 9,000 times.
Compensation amountIt's also easy to misunderstand. The compensation amount is divided into axial direction, transverse direction and angular direction. Each type of expansion joint has different compensation ability. Such asUniversal corrugated expansion jointmainly compensates for axial displacement, whileDouble straight pipe bypass pressure balance type expansion energy saving simultaneously compensate axial and transverse direction, suitable for pipelines with limited space.
These three numbers must be looked at in combination. Low stiffness, large compensation and high fatigue life-this kind of product does not exist, it can only be a balance after trade-offs.
4. What kind of metal expansion joint should be selected under different working conditions of power stations, cement and chemical industries?
Different industries, the working conditions are hugely different, and the selection ideas are completely different.
Power station industry: The temperature of the main steam pipeline is as high as 560℃ and the pressure is above 10MPa, and must be usedCorrugated expansion joint for power station industryOrHigh temperature axial expansion joint。 The material is heat-resistant steel, and the structure usually has a guide tube and heat insulation layer. And power station pipelines tend to be large diameter, likeLarge diameter thick wall expansion jointIt is specifically designed to deal with this kind of working condition. In addition, air-cooled island pipeline is commonly usedDouble hinge expansion joint for air-cooled island vacuum pipelineTo absorb the displacement of the vacuum pipe.
Cement industry: High temperature, high dust, low pressure. The temperature of the kiln end pipe is above 800 ℃, and the dust concentration is high, so it is suitable for useMetal Corrugated Expansion Joints in Cement Industry, often used in combination with non-metallic expansion joints. Special attention should be paid to the design of the guide tube in high dust environment, otherwise it will wear out quickly.
Chemical industry: Corrosive media are the core issue. Sulfuric acid, hydrochloric acid and sodium hydroxide are carried in the pipeline. Ordinary stainless steel can't bear it, so it has to be usedPTFE-lined hoseOrPTFE compensator。 It can also be considered when the pressure is not highrubber compensatorOrRubber PTFE compensator。 In addition, the chemical pipeline often has vibration, and vibration reduction should be considered when selecting the type,Metal hoses have better flexibility.
Flue gas baffle doorAndDesulfurization flue gas baffle doorIt is used in desulfurization system with expansion joint. The medium contains acidic corrosive substances, and the temperature is not high but the humidity is high. It is more safe to choose stainless steel 316L with coating.
5. The most easily overlooked details in installation and maintenance: pull rod, guide tube and screw
If the equipment is selected correctly, it will still be scrapped if there is a problem with the installation.
Let's startexpansion joint tie rodFunction: The tie rod is used to withstand the internal pressure thrust and prevent the bellows from over-stretching. Do not screw down the tie rod nut during installation, and leave a margin for pre-tensioning or pre-compression. When installing the expansion joint of the large tie rod, be sure to make sure that the direction of the tie rod is consistent with the displacement direction of the pipeline-install it backwards not only has no compensation effect, but may break the bellows. aboutHow to adjust the tie rod nut of expansion joint, usually adjusted to the specified length in the cold state, and then fine-tuned while running.
guide tubeThe direction is also particular. The direction of the arrow on the deflector must point to the direction of the media flow-if installed backwards, the deflector can hinder the media flow, generating turbulence and vibration, and in severe cases, fatigue fracture of the bellows.
AndDo you need to remove the screw of the expansion joint？ A lot of people ask. The answer is: the screw is used for transportation fixation before installation, and it must be loosened or even removed after installation, otherwise the expansion joint cannot be freely expanded and contracted. In many accidents, workers forget to remove the screw, and the expansion joint is held when the pipe expands thermally, and finally the bellows breaks.
6. To judge whether suppliers are reliable or not, focus on these three points
There are too many suppliers on the market that call themselves "high quality". How to Screen? Keep an eye on three points:
- First, see if the technical parameters are complete: Reliable suppliers will provide stiffness curves, fatigue life curves and compensation calculation books, instead of just giving one model and price. Such asMetal expansion joint weight table、Metal hose size comparison tableThese basic information will be disclosed by regular manufacturers.
- Second, see if the case analysis is specific: Ask the other party for actual cases, such as "What type of steam pipeline is used for the 1000MW unit of XX Power Station? How many years has it been used? Have you changed it?" The more specific the question, the more you can see the other party's strength. Experienced suppliers in the industry can casually tell the selection scheme of various working conditions.
- Third, look at the after-sales service process>: Installation guidance, on-site commissioning, regular inspections-these services are more important than the product itself. The problem of some expansion joints lies in the installation, which can avoid most of the failures.
Oh, yeah,Is the expansion joint the same as the compensator？ It's actually the same thing, but it's called differently. However, when selecting a model, be sure to find the corresponding product series according to the actual working conditions. Don't use the general-purpose model to fight high temperature and high pressure-that is to lay mines for yourself.
After all, choosing high-quality metal expansion joints is a technical job. Material, design, selection, installation, every step has pits. I hope this article will help you avoid detours. If you are selecting a model, you might as well look at the working condition parameters. What points have not been confirmed yet? Hurry up and ask the supplier.