Why do you have to use expansion joints in flue? To put it bluntly, thermal expansion and cold contraction cause trouble
Our brothers in power plants, chemical plants, or boiler rooms all know that flues are not fuel-efficient lamps. The temperature of the smoke is always two to three hundred degrees, and some even rushes to more than five hundred degrees. Look at that metal pipe. From cold to hot, the length can change by several centimeters or even ten centimeters. If there is nothing to let it stretch and contract freely, the stress is all held on the pipes and supports, from leaking flanges to cracking the chimney, do you think it is scary?
So this is what the expansion joint does — specifically to absorb the displacement of the pipe due to temperature changes. You understand it as a "spring joint" on the pipe, let it squeeze when it's hot, let it pull when it's cold, don't let it hit hard anyway. Metal expansion joints were used much earlier, but later it was found that the sulfur-containing and acid-containing flue gas in the flue corroded the metal too hard, and it took less than two years to perforate. Therefore, the non-metallic expansion joint became popular. In the final analysis, it is because it can fight better than metal in resisting corrosion and absorbing complex displacement.
The core secret of non-metallic expansion joints: "overcoming rigidity with softness" by flexible materials
Have you ever seen what a non-metallic expansion joint looks like? Generally, it is a ring made of rubber cloth or fiber cloth, sandwiched in the middle of the flange, and it looks soft. The principle is actually not complicated: since the pipeline is going to move, let a soft thing bear the move. Soft materials can stretch, compress, twist, and even deflect without the huge reaction forces that metal does. This is called "overcoming rigidity with softness"-how strong your pipe is, the more soft my flexible material will be, absorbing the energy.
Specifically, the main body of the non-metallic expansion joint is a flexible band, which is composed of several layers of cloth with different functions. The belt is enclosed in a tubular shape, and its ends are fixed to the flange with plates or wire rings. When the pipe expands and contracts axially, the belt bulges or deflates with it; When the pipe is displaced laterally or angularly, the belt twists like a body. Isn't it a bit like the gimbal you played with as a kid? It is more durable though, because there is a doorway in the material.
How does it withstand heat and corrosion? There is a doorway hidden in the material combination
Soft materials are afraid of high temperature, won't they melt when baked? Don't worry, non-metallic expansion joints are not as simple as a layer of cloth. It is usually a composite sandwich: the outermost is fluororubber or silicone rubber, which can withstand the temperature to 250℃. If the temperature is higher, polytetrafluoroethylene (PTFE) or ceramic fiber cloth can be used, which can hold up to above 600℃. A layer of insulation, such as high-silicone fiber or aerogel felt, is sandwiched in between to keep the heat from passing to the sealing layer inside. The innermost side in contact with the smoke should also be coated with an anti-corrosion coating, such as polytetrafluoroethylene or acid-resistant cement. Combined in this way, it is soft and carryable.
In addition, it has a unique skill-corrosion resistance. Sulfur dioxide and hydrogen chloride in the flue are corrosive to stainless steel, but they are itchy to fluororubber. Therefore, on many flues of desulfurization and denitrification, non-metallic expansion joints are almost standard. Think about it, it is difficult to replace the metal one once when it is broken, while the non-metal one usually only replaces the ring inside when it is broken, and the cost is much lower.
Compared to metal expansion joints, where does it lose and where does it win?
Let's talk about where you lost. Metal expansion can save energy and bear pressure, up to tens of MPa, while non-metallic ones can usually only carry a few thousand Pa, at most 10,000 to 20,000 Pa, so it can't be used for high-pressure pipelines. In addition, non-metals have a short life span, and they usually have to be replaced every three to five years, while metals take ten or eight years or even longer. Also, non-metallic ones cannot be used in places with open flames or extremely high temperatures (over 800℃), but metallic ones can be carried with heat insulation liners.
But it has more to win. First, the amount of compensation is large. For the same expansion joint, the axial compensation amount of non-metal can be several times that of metal, and the lateral and angular displacement are the advantages of rolling stage. Second, it is easy to install. The non-metal ones are light and can be carried by one person, while the heavy metal ones have to be served by a crane. Third, vibration and noise reduction. Soft materials naturally absorb vibration, and the low-frequency vibration in the flue is transmitted to the non-metallic expansion joint, which is directly eaten by more than half. Fourth, cost. Your metal expansion joint can easily cost tens of thousands, and the non-metal one can be done for hundreds to thousands. If it is broken, it will not be distressed to replace it. So which one to choose depends on your working conditions-choose metal for high temperature and high pressure, low pressure and large displacement are also corrosive, and use non-metal with closed eyes.
Some of the most prone pits to rollover when installing, how many have you encountered?
Alas, no matter how good the equipment is, it is useless if it is not installed properly. I've seen too many people install non-metallic expansion joints backwards. Note that its guide tube must be facing the direction of airflow, otherwise the smoke directly washes the flexible belt, and it will be perforated in a few days. Also, do not twist the expansion joint when installing-although it can absorb torsional displacement, it must be installed in a natural state and cannot be forcibly twisted. Otherwise, before it even runs, the internal stress will tear the seal ring apart.
Pre-stretched. When the non-metallic expansion joint is installed in the cold state, if the amount of pre-stretching is wrong, it will easily bulge or tear when it is hot. You have to calculate how long to pull in the cold state according to the design displacement of the pipe. Many workers try to save trouble and install it directly according to the original length. As a result, there will be problems as soon as they are baked. One more detail: bypass protection. If there is condensate or rinse water in the flue, be sure to set a drain under the expansion joint, otherwise the water will accumulate in the corrugation and corrode much faster than you think. Tsk, you can remember these pits after stepping on them once. How many have you encountered?