Inner layer of metal expansion joint: the right material is chosen, and the life is doubled
Two days ago, I met a customer who was a power plant. The equipment was used for less than a year, and the bellows was perforated. When I took it apart, the inner layer had long since been eroded into potholes. He asked me:Inner layer of metal expansion jointIsn't it just a bushing? Won't you just pick the cheaper one?
Alas, this is like confusing a guide tube with an inner liner, which is going to be a big problem.
What exactly does the inner layer of a metal expansion joint mean? Don't confuse the deflector with the liner
Let's make the concept clear first.Inner layer of metal expansion jointUsually refers to two parts: one is in direct contact with the mediuminner liner(e.g. PTFE lining, rubber lining, spray coating); The other isguide tube。 Many people think of the deflector as the inner layer, or think that the liner is the deflector-these are two different things.
The inner lining layer is an anti-corrosion layer attached to the inner wall of the bellows, which plays an isolation role. The guide cylinder is a metal cylinder installed inside the bellows near the inlet end of the medium. Its main function is to guide the flow direction of the medium and reduce erosion. The two have different structures and complementary functions. Therefore, when selecting the type, the lining material and the guide tube material should be considered separately, and cannot be generalized.
Inside our stationPTFE-lined hoseThe inner liner is PTFE, and the guide tube is made of stainless steel. If the corrosive media scour the PTFE liner directly, the life will fall off by a cliff. The existence of the guide tube is to help the liner block the first wave of impact.
How to choose the inner material? 304, 316L or coating? Working conditions make all the difference
There is no standard answer to choosing materials. You take it to treat high-temperature flue gas, and it's okay to use 304; However, if it is a chemical medium containing chloride ions, 304 will be chloride brittle and cracked in less than half a year.
- 304 stainless steel: Universal type, resistant to neutral media, water, steam. The upper temperature limit is about 450℃. If you exceed this number, don't put it hard.
- 316L stainless steel: Contains molybdenum, and its resistance to chloride ion corrosion is stronger than 304. Suitable for seawater, weak acid and sewage treatment. However, the chloride ion concentration is too high. If it exceeds 500ppm, you still have to consider a higher grade.
- Coating/lining: likePTFE compensatorRubber lining, temperature resistant coating. It is used for strong acid and alkali, high temperature corrosion, but it has poor wear resistance, so it needs to be protected by a guide tube.
- Heat-resistant alloy: For example, Inconel and Hastelloy are used in high temperature, high pressure and strong corrosive environments. High price but long life.
How to choose? lookTemperature, media composition, flow rate, particle contentThese four parameters. Such asCorrugated expansion joint for power station industryThe flue gas contains sulfur and the temperature fluctuates greatly. Usually, 316L plus anti-corrosion coating is used, and 304 is used for the guide tube. WhileMetal Corrugated Expansion Joints in Cement IndustryThe medium contains a large amount of dust particles, and the erosion is serious. The lining needs wear-resistant materials, and the guide tube is thickened.
Can you go directly to the highest grade material? Of course, is Party A willing to pay for the doubling of the cost and the lengthening of the delivery period?
Deflector is not an option: How its structure and inner layer work together to protect bellows
Many people think that the deflector is dispensable, and it can be installed or not. This is the typical money-saving idea, and often the result is a small saving and a big loss.
- rectification: Turn turbulent flow into laminar flow and avoid high-speed fluid going straight to the bellows.
- anti-scouring: The granular medium is first hit on the wall of the guide cylinder, and does not directly collide with the bellows.
- river diversion: Ensure the smooth passage of the medium and reduce the pressure loss.
Deflector andInner layer of metal expansion jointHow does it work together? Like inHigh temperature axial expansion jointIn, the medium enters from the inlet, first passes through the guide tube to decelerate and rectify, and then contacts the inner wall of the bellows. If the medium is corrosive and the inner layer is lined with PTFE or coated, the guide tube is equivalent to a "pioneer", helping the inner liner to withstand mechanical wear. In turn, if the deflector is damaged, the bellows is directly exposed to the high-velocity particle flow – it is only a matter of time before the perforation occurs.
Therefore, when designing, the material, thickness and length of the guide tube should be calculated clearly. Especially the length, it can't be too short, otherwise the medium all rushes onto the bellows. It should not be too long, otherwise it will affect the amount of compensation. For specific values, you can refer to the product manual on the site, such asUniversal corrugated expansion jointThe length of the guide tube is usually 0.8~1.2 times of the nominal diameter of the pipe.
Three killers of inner layer failure: erosion, corrosion, and temperature differential stress
And guess what? MostInner layer of metal expansion jointThe scrapping is not that the material itself is not up to the standard, but that the selection does not match the working conditions. The three most common causes of failure:
- Erosion: The medium contains solid particles, such as dust from cement industry and fly ash from power station. When the flow rate exceeds 15m/s, the particles hit the inner layer and can be worn out in a few months. At this time, light thickening is useless, so you have to use a wear-resistant guide tube, or even add a layer of wear-resistant ceramics on the outside of the lining.
- corrosion: The most common in chemical and desulfurization industries. Acid mist, chloride ions and hydrogen sulfide are a test for the inner material. Like inDesulfurization flue gas baffle doorIn the system, the flue gas contains water vapor and SO2, and 304 can't bear it, so 316L or even bidirectional stainless steel has to be used. If it doesn't work, it will be lined with PTFE scheme.
- Temperature differential stress: Frequent start-up and shutdown, the temperature soars from normal temperature to 500℃ and then drops back, and the thermal expansion coefficients of the inner layer and the bellows are different, which will generate stress. In light case, the coating cracks and falls off, and in worse case, the inner lining deforms and sticks the guide tube. In this case, a liner material matching the thermal expansion coefficient of the base metal is selected, or a gap is reserved in the structure.
Which of these three killers have you met? To be honest, many projects exist at the same time. Such asDouble hinge expansion joint of air-cooled island vacuum pipe, the medium temperature is high, the pressure is low, and there is condensate corrosion-the inner layer is not well selected, and it is fast to change it once a year.
Practical Selection Suggestions: From Power Station to Cement Industry, How Should the Inner Layer Configuration Differ
Something practical. Different industries,Inner layer of metal expansion jointThe configuration ideas are completely different.
Power station industry: Mainly flue gas and steam, with high temperature (300-600℃), containing sulfur and ash. Recommended scheme: 316L or heat-resistant steel for bellows, without anti-corrosion coating on the inner layer (because the coating will fail if the temperature exceeds), but it must be addedguide tubeThe material is the same as the bellows, and the thickness is not less than 4mm. If it is the clean flue gas after wet desulfurization, the temperature is low and the corrosion is strong, it is recommended to use PTFE-lined scheme and Hastelloy for the guide tube.
Cement industry: Extremely severe working conditions-high temperature (400-800℃), high dust (concentration can reach 50g/m³), fast wind speed (20-30m/s). The core of inner layer selection is wear resistance. The bellows can be made of 2520 stainless steel, the guide tube is thickened to more than 8mm, and even the wear-resistant layer is surfaced. Inner liner The cement industry basically doesn't need coating, which is too easy to wear out. If you really want to prevent corrosion, you can only do itNon-metallic expansion joint(the kind of fabric fiber), but has limited temperature resistance.
Chemical industry: The temperature is not high but the corrosion is serious. For example, hydrochloric acid, sulfuric acid, lye. The optimal solution isPTFE-lined hoseOrPTFE compensator, lined with PTFE, 316L for guide tube. Note that PTFE is afraid of negative pressure, and anti-negative pressure rings should be added in vacuum conditions.
Steel smeltingHigh temperature (800 °C or higher), containing a reducing gas such as CO. The inner layer is made of heat-resistant alloy, and the deflector is best designed to have a replaceable structure-because it is the one that wears out the fastest.
Are there any materials that are both wear-resistant and anti-corrosion? Yes, such as duplex stainless steel and nickel-based alloys, but the price is there. When the project budget is limited, you have to make a fuss about the guide tube: it is better to have a thicker and replaceable guide tube than to scrap the whole expansion joint.
Inner layer of metal expansion jointSelecting materials is not a matter of patting the head. When you are unsure, throw the working condition parameters (temperature, pressure, medium composition, particle size and speed, start-off frequency) to the manufacturer and let the technical support help you calculate.