How to choose between non-metallic expansion joint and metal expansion joint? First distinguish the working conditions and then talk about the skills
Are nonmetals or metals more expensive? Actually, it's the opposite question. Choosing a non-metallic expansion joint or a metal expansion joint depends on the working condition, not the price.
The biggest advantages of non-metallic expansion joints (fabric fiber expansion joints) are high temperature resistance, large compensation amount and low stiffness. In such situations as the flue duct of power station boiler and the flue gas system at the tail of cement kiln, the temperature is always five or six hundred degrees, and the metal bellows can't bear it at all-even if heat-resistant alloy is used, the cost is frightening, and the fatigue life of metal is there. The non-metallic skin is made of ceramic fiber as the heat insulation layer, and the temperature resistance is above 1000℃, and the displacement compensation ability is particularly large.
On the other hand, if it is a high-pressure steam pipeline or a liquid pipeline with corrosive medium, then honestly use metal, such as universal corrugated expansion joints or double hinge transverse expansion joints. No matter how strong a non-metallic structure is, it can't stand high pressure. It is not for that job.
Look at the medium, the temperature, the pressure and the direction of displacement. After these four parameters are set, it is basically clear what type of expansion joint to choose. Don't shoot your head and choose. If you shoot it wrong, you will be in trouble.
Most common mistakes when installing non-metallic expansion joints: tie rods, deflectors, and directional markers
Installation is the hardest hit. Let's talk about the tie rod first-many buyers forget to dismantle it when they use it. The tie rod that comes with the non-metallic expansion joint when it leaves the factory is used to locate and prevent transportation deformation, not for you to hold it forever. After installation in place, the tie rod nut should be loosened to the displacement specified in the design, so that the expansion energy saving can expand and contract freely. You have seen the pipe without removing the tie rod after installation, and there is no place for the heat expansion and contraction of the pipe, so the top of the bracket next to it is deformed. Alas, this situation has been seen many times.
The direction of the guide tube is more particular. The function of the guide tube is to drain the medium to the inside of the corrugation, so as to avoid the air flow directly washing the skin. When installing, the guide tube must be faced in the direction of medium flow. Installing it backwards is equivalent to adding a grinding machine to the skin, and the fabric layer will be worn out in a few months. Directional signs have arrows when they leave the factory, so just dress. However, some project sites bumped off the logo when the expansion joint was transported, and did not confirm it with the manufacturer, so they were hard-fitted, and the result was waiting for rework.
Non-metallic expansion joints are divided into different types such as single hinge, double hinge, duplex, etc. The drawings must be checked on site to confirm the installation position and direction. And guess what? Some customers installed rectangular non-metallic expansion joints on square flues, but as a result, the bolt hole spacing didn't match, so they took gas cutting to expand the holes on the spot-this destroyed the sealing structure at the edge of the skin, and it was strange that there was no air leakage.
The life of fabric fiber expansion joints in high-temperature flues is hidden in details
Many people think that the short life of fabric fiber expansion joints is due to poor materials. In fact, 80% of them are due to the installation and use environment.
Stainless steel wire mesh, ceramic fiber blanket, polytetrafluoroethylene film, silicone rubber coated fiberglass cloth, each layer performs its duty. Ceramic fiber insulation, tetrafluorine membrane anti-corrosion and anti-penetration, glass fiber cloth as bearing layer, steel wire mesh anti-scour. This layered structure is not random, it can't even be missing a layer.
Let's talk about the working conditions. When the dust concentration in the high-temperature flue is high, the airflow speed is high, and the wear on the windward side of the skin is very severe. At this time, if the guide tube is made short, or it is too far from the skin when installed, the dust will hit the skin directly. The correct way is to keep the guide tube as close to the inner wall of the skin as possible, leaving a thermal expansion gap but not too many gaps. This degree, experienced designers will tell you: generally 10~30mm, according to the smoke temperature and expansion amount, not randomly determined.
In addition, positive or negative flue pressure will also affect the life. In the negative pressure system, the skin is depressed inward, which is easy to accumulate dust in the insulation layer; In a positive pressure system, the air flow pushes outward, and the skin bears more stress. These two working conditions have different requirements for skin material and layer number.
Don't wait for leakage before replacing-routine inspection and maintenance skills of non-metallic expansion joints
Many factories' maintenance ideas are still broken. But the non-metallic expansion joint is an accident if it leaks, not a small matter. If high-temperature smoke leaks out and people hurt and equipment is damaged, the loss is not the price of an expansion joint.
Daily inspection, the frequency depends on the working conditions. What to look at every day: whether there is bulging, delamination and cracking on the skin surface; Are the fasteners loose? What to look at every quarter: whether the skin is abnormally deformed and whether the color changes (whitening, yellowing and scorching are all signals of abnormal temperature). If possible, scan the temperature distribution of the skin surface with an infrared thermometer. If the temperature difference exceeds 30℃, check whether the insulation layer has sunk.
Insulation settling. When the expansion joint is installed vertically, the ceramic fiber blanket will slowly sink under the action of gravity, causing cavities in the upper part, and the local temperature of the skin will soar. This problem can be prevented at the design stage-choosing non-metallic expansion joints with internal support structures, or adding thermal insulation nails for fixation.
If there is a small pinhole leak in the skin, emergency treatment can be applied with high-temperature repair tape, but this is only a stopgap measure. Replace the skin while the plan stops. Don't change the skin yourself. The manufacturer has special plates and tools, and it can be screwed with self-tapping screws, which makes the flange surface impossible to seal.
By the way-the integrity of the tie rod and the guide tube is also a handy look. When the tie rod is bent, it means that there may be abnormal stress in the pipeline system. If the skin of the guide tube falls off, it will wait to be worn out.
From power stations to cement plants: Adaptation hacks for non-metallic expansion joints in different industries
Power stations, cement plants, metallurgy, desulfurization... The demand for non-metallic expansion joints in each industry is different, and the selection logic should change accordingly.
In the power station industry, the focus is on the boiler smoke duct system. Rectangular flue is usually equipped with rectangular non-metallic expansion joint. The air duct size is large and the displacement is large, so the skin is required to be temperature resistant, corrosion resistant and low leakage. The desulfurization system of the power plant should also be equipped with the desulfurization flue gas baffle door, and the expansion joint should be resistant to acid corrosion-choose the skin material of PTFE coating. Air-cooled island vacuum pipeline system uses vacuum special hose or double hinge expansion joint. In that scenario, vacuum negative pressure is the key parameter.
In the cement industry, the kiln tail flue gas temperature is high, the dust concentration is large, and the alkali-containing metal corrosive medium. Metal corrugated expansion joints in cement industry are commonly used in the cooler system of kiln head and kiln tail, but the hot flue gas channel still depends on non-metal. This kind of working condition requires the wear-resistant layer of the skin to be thickened, the guide tube to be lengthened, and the insulation layer to be thickened.
Incinerators that burn waste gas in the chemical industry have both corrosive gases and dust in the flue gas, as well as temperature fluctuations. Under this working condition, it is recommended to choose a structure with double-layer guide tube, and reserve temperature measurement holes to monitor the temperature of the heat insulation layer in real time. Don't be reluctant to let go of these temperature measuring ports. In case the flame inside penetrates the skin and can't be seen outside, the thermometer will tell you.
Speaking of which, let me tell the truth again: the non-metallic expansion joint is not a high-precision black technology, but it is a systematic project-only when there are problems in every link of selection, installation and maintenance can it work honestly. It is much better to spend more time thoroughly understanding the working conditions, implementing the installation specifications and setting up the inspection system than making up leaks afterwards.