What exactly is the scouring problem? Find out how it came about first
To put it bluntly, the high-speed medium in the pipeline-steam, smoke, dusty gas or particulate liquid-rushes against the inner wall of the metal bellows. Over time, the ripples and troughs are thinned and worn out. Especially in the working conditions of power stations, cement and desulfurization, with high temperature, fast flow rate and ash in the medium, erosion is not a "possible" thing at all, but it will come sooner or later. And guess what? I have seen an expansion joint in the main steam pipe of a power plant, which has been running for less than two years, and a hole has been worn out in the trough, and the steam comes out like a whistle. Tsk, that commotion, everyone in the scene heard the scalp numb.
The particles in the medium are like sandpaper, polishing the bellows over and over. The higher the flow rate, the harder the particles, and the scour speed doubles up. Some data say that when the medium flow rate exceeds 20m/s, the erosion rate can be multiplied several times. So don't expect the bellows to carry it on its own-it was designed to absorb displacement, not as a wear plate.
Can the deflector block it? Yes, but it depends on how you pretend
The guide tube is the most direct anti-scouring means in the expansion joint, which guides the high-speed medium open and prevents the bellows from carrying it hard. But here comes the problem: the material of the guide tube is not selected correctly, the thickness is not enough, or the installation gap is large, and erosion will still come to your door. Two days ago, another customer asked what to do if the deflector was washed through-that can only be changed, nothing else. The deflector itself is a consumable part, but at least carried in front, protecting the bellows. By the way, remember to align the direction of the arrow of the guide tube with the flow direction of the medium. If it is installed backwards, it is equal to displaying, and the medium directly attacks the bellows-this question has also been mentioned in the question and answer. The direction of the arrow of the expansion joint refers to the flow direction of the medium, so don't reverse it.
In addition, the gap between the guide tube and the bellows is also particular. If the gap is too large, the medium will rush in through the gap and wash the inner wall of the bellows, just like not installing it; If the gap is too small, it may get stuck, causing the expansion joint to fail to expand and contract properly. Generally, the design clearance is controlled between 2~5mm, depending on the caliber and working conditions. There are also materials. Heat-resistant stainless steel has to be used for high-temperature flue gas conditions, and ordinary carbon steel will burn out after one month.
In the selection stage, you should be on guard. Don't wait for the equipment to be installed before saying "I knew"
In the face of high risk of scour,Universal corrugated expansion jointIt certainly won't work hard. Cement industry to go upMetal Corrugated Expansion Joints in Cement IndustryWave pattern, wall thickness and material are all optimized for dust-containing gas. It needs to be used at the power stationCorrugated expansion joint for power station industryHigh temperature resistance and fatigue resistance. The working condition of desulfurization flue gas is more troublesome-the flue gas after wet desulfurization has acidic condensate, which is a double blow of corrosion and washing. At this time, it is necessary toDesulfurization flue gas baffle doorProtected with PTFE lining, or directlyPTFE-lined hose、Rubber PTFE compensator。 The materials, wave patterns and wall thicknesses used in each working condition are different. Such asLarge diameter thick wall expansion jointThis thing is specially designed to carry scouring and high pressure. The wall thickness can be 8mm or even thicker, and the trough is thickened, so the life span can be three times that of ordinary expansion joints.
Don't forget to include the wear allowance when selecting a model. Some design diagrams save trouble, and they directly copy the standard model. As a result, it is better to spend more money on heavy duty as if it were changed once every two years. The cost account can't only look at the purchase price, but the operation and maintenance cost is the big head.
There are many pits hidden in installation and maintenance
For example, the direction of the arrow of the expansion joint refers to the flow direction of the medium. Installing the guide tube backwards is equal to displaying, and the flushing directly hits the bellows. For example, if the tie rod nut is too dead and the screw should not be disassembled, these operations will change the stress state of the bellows and accelerate the local scour failure. I remember a user who screwed the tie rod nut tightly, thinking it was safer. As a result, the bellows couldn't expand and contract normally, and the impact force of the medium was all concentrated in the trough, which cracked in three months. Don't underestimate these details. Half the difference in equipment life is considered small.
Also pay attention to pipe alignment when installing. The pipe is crooked, the expansion joint is subject to additional bending moment, one side of the bellows is elongated and the other side is squashed, and the erosion and wear are more uneven. Also, don't overpressure during pressure test. Overpressure may lead to plastic deformation of the bellows, and the subsequent anti-scour ability will drop linearly. In terms of maintenance, check the wear of the guide tube regularly. If you find that there is thinning at the trough, quickly change the guide tube or add repair welding. Don't wait until it leaks before replacing it, by then the bellows may have been scrapped.
What should I do if I really can't bear the scouring? Composites and sacrificial layer ideas worth looking at
Some working conditions are too bad, such as pulverized coal pipeline, which has high flow rate and hard particles, and the guide tube can't hold it for half a year. At this time, composite materials or sacrificial layer design can be considered.PTFE-lined hoseAndRubber PTFE compensatorIt performs well in corrosive media, but pure PTFE is afraid of high temperature and mechanical impact, and begins to soften when the temperature exceeds 200°C. Some working conditions will engage in a "double-layer structure"-the inner layer is made of thick-walled metal as a sacrificial layer, and the outer bellows is responsible for bearing pressure. This is more expensive, but it is much more cost-effective than replacing an expansion joint in two or three years. For example, a cement factory used a set of metal expansion joints designed with sacrificial layer on the kiln tail pipeline. The sacrificial layer was made of 304 stainless steel with a thickness of 5mm, and the outer bellows was made of 316L. It took five years to change the sacrificial layer, and the bellows were still as new.
Another idea is to add wear-resistant coating, such as spraying tungsten carbide on the inner wall of the guide tube, which has high hardness and erosion resistance. However, the coating process requires high, and peeling is troublesome. Generally speaking, there is no new way to deal with erosion, and the scheme has to be selected according to the medium temperature, flow rate, particle hardness and corrosiveness. If you are not sure, find a manufacturer to do erosion simulation analysis, and don't pat your head.
Metal expansion joint scouring problems, prevention is always more cost-effective than repair afterwards. Choose the right product, install the right direction, and check regularly. These three points are achieved, and the life of the equipment is easily doubled. Don't wait until the pipeline leaks, shuts down, or fines, before you remember that you should have chosen the model carefully.