Why is skin bonding the "Achilles heel" of non-metallic compensators? — — Start with structural failure cases
In their factory, a rectangular non-metallic expansion joint that has just been running for half a year, the skin falls off directly from the flange interface, and the dust is sprayed everywhere. When removed, the fabric fibers of the skin itself are intact, but the adhesive layer is thoroughly carbonized and delaminated. Anyone who has worked on pipeline compensators in this scene is familiar with it-the core sealing component of non-metallic compensators (that is, we often call non-metallic expansion joints or fabric fiber expansion joints) is the skin, and whether the skin can stay where it should be depends on that layer of glue.
Structurally, a non-metallic compensator consists of a metal frame, a skin (typically a multi-layer composite fabric), and a compression bolt. The metal frame provides rigid support, the skin is responsible for absorbing thermal displacement and vibration, and the key to connect the two is the adhesive layer. Once the glue fails, the compensator is equal to an air leak. You said it wasn't "the heel of Achilles." Who was?
Common skin bonding failure modes in the industry: temperature, medium and stress, which is the real murderer?
Let's first dismantle three common cases of failure. The first is that the temperature exceeds the limit. The upper limit of temperature resistance of most general-purpose silica gel or neoprene is about 200℃, but the high-temperature hot air in the flue of power plants is often above 350℃, and even some high-temperature axial expansion joints can reach above 600℃-ordinary glue will be brittle and cracked in two hours. Second, medium corrosion. The flue gas passage behind the desulfurization flue gas baffle door and the desulfurization flue gas baffle door contains a large amount of sulfide and condensed acid, and ordinary glue is like paper when exposed to acid. The third is stress fatigue. The pipeline expands and contracts repeatedly, and the skin keeps stretching and shrinking. If the adhesive layer is not elastic enough, it will be degummed sooner or later.
So which one is the real murderer? Truth be told, most of the time it's a three-pronged approach. Temperature makes the glue lose elasticity, medium corrosion accelerates aging, and stress finally tears this layer of crispy skin apart. When selecting glue, only a single factor is considered, and there is a high probability of rollover.
Step-by-step teach you to choose the glue for skin bonding: temperature resistance grade, bonding strength and construction window period are all indispensable
When picking the glue for bonding the skin of the non-metallic compensator, you can't just look at the publicity data given by the manufacturer. There are three hard indicators that must be measured or reported by a third party:
- Temperature resistance grade: Can the glue with nominal temperature resistance of 300℃ run continuously at 300℃ for 1000 hours? The "temperature resistance" of many glues refers to the short-term peak value, not the long-term operating temperature. Let the shear strength after heat aging speak for themselves.
- Adhesion strength: How much does the draw test have to be done? The peel strength between the skin and the metal frame should be at least 5 N/mm (refer to JB/T 12235-2015 standard). Moreover, it depends on the retention rate at high temperature. No matter how high the room temperature index is, it will be useless if the heating drops to zero.
- Construction window periodThis is the easiest thing to ignore. Some two-component epoxy glues gel half an hour after mixing, and it is too late to paint a large area on the spot. The open time is best more than 40 minutes, otherwise the glue will dry in the bucket halfway through brushing, and it is useless for you to scold your mother.
Also, don't be superstitious about imports. Several domestic manufacturers who have done well, such as the high-temperature modified silicone glue customized for non-metallic expansion joints, have the same comprehensive performance as foreign brands, and the price is half cheaper. The key is to find a supplier who really understands the working conditions, not a trader who sells general glue.
Construction technology is the "invisible killer": surface treatment, glue thickness and curing conditions are all wrong
Everything will be fine if you choose the right glue? Beautiful thought. I have seen too many cases: the glue I bought was fine, but the construction team wiped it twice with a brush, and it still leaked after three months. There are three iron laws in the construction technology of skin bonding:
First, the surface treatment should not be perfunctory.Oil stains, rust and old glue residues on the metal flange frame must be thoroughly removed with acetone or special cleaning agent, and then bristled with sandpaper until the natural color of metal is revealed. Some construction teams plan to save trouble, and wipe it with a rag. This kind of surface bonding strength is at least 30% off. The bonding surface of the skin itself should also be lightly ground with sandpaper to increase the physical anchor point.
Second, the glue thickness is particular.If the glue layer is too thin, it will be easy to lack glue, while if the glue layer is too thick, it will solidify and shrink to produce internal It is usually controlled between 0.5 and 1.5mm, and evenly coated with a toothed scraper. When the compression bolts are tightened, the glue will overflow a little. That overflow should be wiped off before it is cured, otherwise it will become a hard lump after solidification, and the next maintenance will be troublesome.
Third, the curing conditions cannot be made do.Many glues require a certain amount of temperature and humidity to fully cross-link. In winter, when the on-site temperature is lower than 5℃, the curing time of glue may be extended to more than 3 days, or even it can't be cured at all. If conditions permit, use a heating blanket or infrared lamp to assist in heating, keep it between 15~25℃, and the relative humidity is less than 70%. At the time of acceptance, press the glue layer with your finger. If there is still an indentation, it means that it has not been cured thoroughly. Continue waiting.
Suggestions on glue selection for different working conditions: How to match flue gas desulfurization, high temperature hot air and dust environment respectively?
Different application scenarios have different requirements for glue. Let's talk about the three most typical working conditions:
Working condition 1: Flue gas desulfurization (FGD) system.This kind of working condition is accompanied by the subsequent desulfurization flue gas baffle door or flue gas baffle door. The medium temperature is generally 60~120℃, but the humidity is extremely high and contains acid (sulfuric acid, sulfurous acid). Ordinary organic silicone has poor acid resistance, so special adhesives based on fluororubber or polytetrafluoroethylene (PTFE) must be selected. If the compensator itself is lined with PTFE hose or PTFE compensator, the glue must have good adhesion to the fluoroplastic surface. It is recommended to treat it with surfactant before applying glue.
Working condition 2: High temperature hot air duct.For example, the hot air temperature of non-metallic expansion joints used in cement industry or power station industry can reach above 350℃. At this time, you can only choose ceramic fiber cloth composite skin with high-temperature inorganic glue (such as phosphate-based or silicate-based glue). Note that this inorganic glue is brittle and cannot withstand too much shear deformation. Therefore, at the same time, it is necessary to ensure that the compensator has sufficient displacement margin in design, and don't let the glue hard carry deformation.
Working condition three: dust environment.In pulverized coal transportation, dust removal pipeline and other occasions, dust particles will penetrate into the gap between the skin and the flange, and wear the adhesive layer. Under this working condition, in addition to using polyurethane glue with high wear resistance, it is recommended to add a sealing strip on the inside of the flange to keep the dust out. In addition, the compensator near the manual plug-in plate isolation door or the electric plug-in plate isolation door must have the characteristics of dynamic fatigue resistance due to the frequent valve action and large impact vibration, and the tear strength of the glue must be paid attention to when selecting the type.
Summary: Don't Let Glue Ruin the Entire Compensator-A Practical Pit Avoidance Checklist from Procurement to Acceptance
In the final analysis, the glue for non-metallic compensator skin bonding may account for less than 5% of the cost of the whole compensator system, but it determines 80% of the reliability of the compensator. A list of avoidance pits, no thanks for taking it away:
- Before purchase: Suppliers are required to provide the aging test report of glue at the corresponding temperature and medium. Don't just look at the normal temperature data.
- On arrival: Check the glue batch number and production date, and direct return if it exceeds the shelf life of half a year. For two-component glue, confirm whether the proportioning tool is complete.
- Before construction: Make a small sample to test sticking. After curing at room temperature for 24 hours, pry it with a blade to test the peel strength, and have a bottom in your heart.
- During construction: Take pictures to record each process-surface treatment, gluing, and curing. Follow-up problems are well documented.
- During acceptance: Use a micrometer to measure the thickness of the glue layer, and use a hardness meter to measure the hardness of the glue layer after solidification (Shore A hardness is preferably between 50 and 70).
- After installation: After 48 hours of first start-up operation, stop and check once to see if there are bubbles and cracks in the bonding area. If there is any, rectify it immediately, and don't wait for the compensator to be scrapped before regretting it.
Accessories are small and life is at stake. The next time you select glue, it will take half an hour to confirm the working condition parameters, which can save tens of thousands of downtime losses.