Why do flues prefer rectangular non-metallic expansion joints-the pain point of high-temperature large-section pipes in power plants and steel mills
People who do desulfurization in power plants and dust removal in steel plants have probably suffered from flue leakage. The high-temperature flue gas carries corrosive particles, the pipeline is always hundreds of degrees Celsius, and the cross section is always several meters wide. Under this working condition, the metal rectangular expansion joint often can't survive a maintenance cycle-stress concentration cracking, weld corrosion and perforation, and it is changed again and again. Do you think it's upsetting or not?
The flue rectangular non-metallic expansion joint is precisely aimed at these pain points. It has no metal fatigue problem, can absorb displacement in axial, transverse and angular directions, and is temperature-resistant, corrosion-resistant and low-cost. Two days ago, I met a customer. The metal rectangular expansion joint originally used in the desulfurization flue of the power plant leaked three times in half a year and was replaced with a non-metallic one. So far, there has been no accident for more than two years. It is not unreasonable that such scenarios are becoming more and more common in the industry.
Take a look: How does the "multi-layer cloth + temperature-resistant cotton" structure of rectangular non-metallic expansion joint hold hundreds of degrees?
Pick open a flue rectangular non-metallic expansion joint and you will see that it is not as simple as a cloth at all. A typical structure is divided into several layers from the inside to the outside: the innermost layer is temperature-resistant cotton-ceramic fiber cotton or glass wool, tens of millimeters thick, responsible for heat insulation; The outer layer is a fabric layer, usually coated with fluororubber or silicone rubber, but also reinforced with stainless steel wire mesh; There may be a sealing film and waterproof layer in the middle. This composite structure, to put it bluntly, relies on layer-by-layer thermal insulation to keep the high temperature out.
The internal flue gas temperature is 650℃, and it can basically drop below 60℃ on the outer surface. It's just warm when you touch it with your hands. How does it do it? The thermal conductivity of ceramic fiber cotton is extremely low, and with multiple layers of air gaps, the heat transfer is crushed to death. Moreover, the fabric itself is elastic. As soon as the pipe stretches and contracts, it follows, unlike metal bellows. Speaking of which, do you think this thing is quite clever?
Don't compare it to a metal rectangular expansion joint-it's not the same thing at all
Which is better, flue rectangular non-metallic expansion joint or metal rectangular expansion joint? This is actually the wrong question. Metal rectangular expansion joint depends on the deformation of stainless steel bellows to compensate the displacement, and the stress concentration at the corner of rectangular section is a natural shortcoming, which encounters frequent start-stop and high-temperature creep, and its life is pitifully short. The non-metallic expansion joint is flexible compensation. The structure of fabric with cotton can naturally absorb multi-directional displacement, and the upper limit of corrosion resistance and temperature resistance is high (up to more than 1000℃).
Metal is suitable for high pressure, small diameter and clean media; Non-metal is specialized in treating low-pressure, large-diameter and corrosive flue gas. If you use metal to treat the desulfurization flue, it is not only expensive, but also easy to break-this is not asking for trouble. Therefore, find out before selecting: whether your pipeline is low-pressure large section or high-pressure small diameter, don't mix the ratio.
The three pits that are most prone to rollover in model selection: temperature, medium corrosiveness and displacement, all accidents will happen if one is less
Some people only reported a "300℃", but the peak value could rush to 500℃ in an instant, and the cotton layer burned through directly. It is necessary to distinguish between the continuous operating temperature, which determines the material grade, and the instantaneous peak temperature, which determines the safety margin. The second pit is media corrosivity: SO₂, HCl, Cl⁻¹ in the smoke are chronic poisons to fabric coatings. Choose fluororubber coating to deal with acidic environment, choose silicone rubber to deal with alkaline, and choose the wrong one for half a year to powder. The third pit is the displacement: the thermal expansion calculation of the pipeline should leave sufficient margin, and the non-metallic expansion joint has the maximum compensation amount. If it exceeds it, the cotton layer will be cracked. According to JB/T 12235 standard, a displacement margin of at least 20% shall be considered in design. Do you say one less can prevent the car from rolling over?
Advice from the master at the installation site: direction of the guide tube, pre-tension, bolt torque, air leakage and smoke run in a wrong step
The installation thing looks simple, but in fact it's all details. Let's start with the guide tube-this thing is inside the expansion joint, and its function is to guide the airflow to wash away the dust and protect the fabric. The direction must be toward the fluid, and the smoke directly hits the fabric if it is installed backwards, and it will wear out in less than three months. What about pre-stretching? Many installation teams save trouble and directly go up and pull it down. When installing non-metallic expansion joints, they usually need to be pre-stretched or pre-compressed, and the cold displacement is preset, otherwise the fabric will be stressed too much in the hot state. Finally, the bolt torque: the flange bolts should be tightened symmetrically and evenly, and the torque value should be controlled within the product description range. Eighty percent of leakage accidents are caused by unevenly tightened bolts. And guess what? Once at the scene, three bolts were tightened to slippery wire, and the other was loose enough to turn by hand. The smoke leaked like a smog day. So don't be too long-winded, you have to keep an eye on every step.
How many years will it last? Life Truth and Maintenance Cycle of Non-Metallic Expansion Joints
To be honest, the design life of flue rectangular non-metallic expansion joints is generally 3 to 5 years, but there are also those that can be used for seven or eight years. Life depends on three things: temperature profile, corrosion strength, mechanical damage. If the working condition is stable, there is no accidental overburning, and the external surface is regularly checked without bulging, scratching or deformation, then it is no problem to continue using it.
Maintenance cycle It is recommended to inspect the appearance every six months, focusing on whether there are any air leakage traces on the flange sealing surface and whether the fabric layer is partially hardened or discolored. Measure the external surface temperature once a year, and compare it with the factory data. If the deviation exceeds 20℃, it indicates that the insulation layer may be damaged. You can consider the overall replacement in 3 to 5 years. Don't hold on until the air leaks before replacing it-sudden failure causes shutdown and production, and the loss is much greater than replacing an expansion joint.
After so much, the core is one sentence: choose the right model, install the right details, and maintain it on time. This thing is quite reliable. Does it make sense?