How harsh are the working conditions of the boiler smoke duct? Why are metal expansion joints prone to problems?
To put it bluntly, the boiler smoke duct is pressed by three mountains of high temperature, dust and corrosion. The temperature of the flue gas at the outlet of the furnace is always eight to nine hundred degrees, and although the tail flue is low, it is still three to four hundred degrees. In addition, when the sulfide and nitrogen oxides in the flue gas are tossed, the metal expansion joint is like being baked in acid rain.
The metal expansion joint absorbs displacement by the deformation of the bellows, but the bellows is most afraid of two things: one is stress concentration, and the other is corrosion. The dust particles in the flue are washed at a high speed, and the wall thickness of the bellows is only a few millimeters. It is a matter of time before they wear out. What's more troublesome is that metal bellows are prone to fatigue cracks at alternating temperatures. Once cracks appear, the air leakage rate rises linearly and the boiler efficiency drops sharply.
According to some statistics, the average life of metal expansion joints in the boiler flue duct is only three to five years, and if you are unlucky, you will leak in one year. You calculate the cost of shutting down to replace the expansion joint-shutdown loss, manual hoisting, disassembly and assembly of surrounding pipelines, which is not money? Therefore, many power plants and boiler plants have long stopped using metal in smoke ducts.
Structure and working principle of non-metallic expansion joint
The core structure of non-metallic expansion joint (fabric fiber expansion joint) is a flexible fabric band, which is usually made of high-temperature resistant ceramic fiber, glass fiber, polytetrafluoroethylene film, etc., and then wrapped with stainless steel wire mesh or silicone rubber coating. It has no bellows and absorbs pipe displacement and vibration by the flexible deformation of the fabric loop.
The ring belt is sandwiched between the upper and lower flanges. When the pipeline expands and contracts thermally, the ring belt stretches or compresses, and the displacement is all eaten by the ring belt, so the stress of the pipeline will naturally be small. Moreover, the fabric fiber itself has low thermal conductivity, much better thermal insulation performance than metal, small heat dissipation loss and low surface temperature, so the maintenance personnel will not burn off a layer of skin if they accidentally touch it.
It should be mentioned separately hereRectangular non-metallic expansion joint。 Boiler smoke ducts are basically rectangular cross-sections, and circular expansion joints can't be directly installed. Rectangular non-metallic expansion joints are born to do this. It has rounded transitions at the corners to avoid stress concentration, the circle band can be customized according to the flue size, and several meters by several meters of flue can be wrapped.
How to choose non-metallic expansion joint in different parts of boiler?
The working conditions of each position on the boiler are different, and the selection ideas are also different.
Furnace Outlet:Here, the temperature is the highest, and the dust content of flue gas is large. Non-metallic expansion joints with high temperature resistance must be selected. The ring belt material must be made of high aluminum ceramic fiber, and the temperature resistance should exceed 1000℃. In addition, dust washing is very fierce, so wear-resistant lining should be added to the inner layer, or deflector should be designed to guide the airflow away, otherwise the ring belt will be worn out quickly.
Tail flue:The temperature dropped, but the acid dew point corrosion problem was more prominent. The sulfide in the flue gas will condense into dilute sulfuric acid at low temperature, and even metal parts can't bear it. The PTFE film layer of non-metallic expansion joint is used to carry corrosion. When selecting the model, pay attention to this anti-corrosion layer, and the flange and surrounding metal parts must also be treated with anti-corrosion.
Desulfurization tower inlet:In this position, the wet smoke, slurry and chloride ions are all gathered together, which is more corrosive than the tail flue. Moreover, the sedimentation of the desulfurization tower is large, and the displacement to be absorbed by the expansion joint is also large. It is best to choose non-metallic expansion joints with integral PTFE lining, and the number of coil belt layers should be sufficient to ensure that no deflation is sucked under negative pressure conditions.
Easy mistakes during installation: pre-stretching, deflectors, insulation
When installing non-metallic expansion joints, the most common mistake is not to do pre-stretching. The operating temperature of the pipeline is much higher than the installation temperature, and the expansion joint will have an installation length mark when it leaves the factory. During installation, the ring belt must be pre-stretched or pre-compressed to the specified size according to the requirements of the manufacturer. If you don't pull, the displacement after running will exceed, and the ring belt will tear directly. At that time, it will be too late to cry.
The second pit is the deflector. If the high-speed dusty flue gas in the flue directly washes the gap between the ring belt and the flange without a deflector plate, the dust will drill into the interlayer of the ring belt, and the ring belt will be worn out over a long time. The correct method is to measure and install a guide tube or guide plate in the expansion joint to let the airflow pass smoothly and protect the ring belt.
The third pit is the insulation layer. Many people think that non-metallic expansion joints themselves have heat insulation effect, so they don't add heat insulation, but this thing is temperature resistant enough, and the surface temperature is not low. The outer insulation layer can reduce the heat dissipation loss and prevent the aging of the ring belt from accelerating-ultraviolet rays, rain and hail are all enemies of the ring belt. Adding a stainless steel protective cover or outer insulation layer can last several years longer.
Daily inspection and maintenance: several hard indicators that should be changed or not
Non-metallic expansion joints are not installed once and for all. The frequency of inspection is recommended at least once per maintenance cycle, focusing on several locations.
First, see if there are cracks, delaminations or bulges on the surface of the ring belt. If the crack exceeds 2mm, you have to pay attention to it. The bulge indicates that the inside of the ring belt has been flooded or delaminated, not far from the perforation.
Second, measure the leakage rate. Use a flue gas analyzer or infrared temperature measuring gun. If the temperature around the expansion joint is obviously high, or there is a sizzling air leakage sound, it means that the seal is broken. The air leakage rate of normal non-metallic expansion joints should be less than 1%.
Third, see if the flange bolts are loose. The expansion joint will be accompanied by vibration when it runs, and the bolt will easily loosen after a long time, especially the position of the four corners. Loosening can exacerbate band pulling and cause the flange seal to fail.
Fourth, observe the outer sheath and the deflector. If the outer sheath is corroded and peeled off, it should be repainted and replaced. If the deflector is worn seriously, the airflow will rush directly to the ring belt. At this time, it is more economical to replace the deflector than to replace the expansion joint.
To be honest, non-metallic expansion joints are more worry-free than metallic on the boiler smoke duct. If you really encounter a metal expansion joint that leaks after one year's operation, it's better to replace it with a non-metal one while it is shut down for maintenance. If you replace it once, you won't have to toss it in the next few years.