Anyone who has worked on denitrification projects knows that the inlet and outlet flue of SCR reactor is never a worry-free matter of "going to the end with a straight pipe". Temperature fluctuations, flue gas loads, equipment vibrations, and settlement of civil foundations are all forcing the pipeline to displace. If you go head-on and weld the equipment to death with rigid connections, the result will be stress concentration, cracking the desulfurization tower wall, and even pulling the reactor flange to deform.
So something has to come out and "soften" it. This thing is the SCR non-metallic expansion joint.
From flue gas baffle door to non-metallic expansion joint: What is the difficulty of pipeline compensation in SCR denitrification system
The flue of SCR denitrification system has one characteristic: large cross section, low pressure and not small temperature fluctuation. You find a metal corrugated expansion joint to compensate for the rectangular flue that is three or four meters wide at every turn? Let's not talk about the cost. Just such a large-diameter metal corrugated structure has its own stiffness, which can't absorb much lateral displacement, and the thrust requirements for the bracket are frighteningly high.
What's more troublesome is that the flue gas of SCR system comes from the tail of the boiler, and the temperature sways back and forth between 300℃ and 420℃, and the temperature change rate may exceed 5℃/min when the boiler is started and stopped. Metal corrugated expansion joints are prone to fatigue cracking under such low frequency and large displacement conditions. However, SCR non-metallic expansion joint has low structural stiffness and can absorb large three-way displacement with small reaction force-this is the first reason why it becomes just needed for denitrification flue.
In addition, the flue section around the flue gas baffle door (such as desulfurization flue gas baffle door and circular baffle door) often needs to be disassembled and overhauled. The skin of non-metallic expansion joints can be replaced as a whole, unlike metal bellows, which have to cut the whole section of pipe.
Material and Structure Disassembly: How to Match Fabric Fiber Layer, Skin and Metal Frame
Non-metallic expansion joints are not as simple as "one cloth at both ends and one clip". It has at least three layers of structure: the outer skin, the middle fabric fiber layer, the inner guide tube, plus the metal frame at both ends.
The skin assumes the sealing function. The denitrification flue gas contains SO₂, NO and fly ash, and the skin material must be resistant to acid and alkali corrosion, and it must be able to withstand the continuous operation temperature above 300℃. The commonly used combination is fluororubber + polytetrafluoroethylene film + ceramic fiber cloth, which is bonded with high temperature resistant glue between layers, and then fixed to the frame by pressing strips and bolts.
The middle fabric fiber layer is the main force that absorbs displacement. It is made of layers of ceramic fiber cloth or glass fiber cloth laminated and uses slippage between the fibers to digest the thermal expansion of the pipe. The metal frame is responsible for connecting the whole expansion joint to the flue flange. The stiffness of the frame determines whether the expansion joint can work stably as a whole.
How can these three cooperate so that nothing goes wrong? The key sentence: the skin is responsible for "not leaking", the fiber layer is responsible for "being active", and the frame is responsible for "holding up". Which layer is missing, or which layer is selected, and it should leak after half a year's operation.
Temperature and corrosion resistance are not the only indicators: How to calculate low-frequency vibration and displacement compensation of SCR flue
What degree does your expansion joint withstand? It seems like everything will be fine as soon as the temperature is satisfied. The real situation is far from that simple.
The vibration source in the SCR flue is mainly the airflow disturbance at the induced draft fan and the denitrification and ammonia injection grille. The fan speed is generally 600~1000 rpm, and the vibration frequency is not high, but the amplitude is not small. Non-metallic expansion joints happen to have a natural damping effect on low-frequency vibrations, which metal corrugated expansion joints can't compare.
The calculation of displacement compensation amount can't be slapped on the head. You should calculate the three-way displacement separately: axial displacement depends on the thermal elongation of the pipeline, lateral displacement depends on the settlement of the equipment foundation and wind load, and angular displacement depends on the moment at the corner of the flue. When selecting, superimposing these three items and multiplying them by a safety factor of 1.2~1.5 is the final compensation amount. Note that the greater the safety factor, the better. If it is too big, the size and cost of the expansion joint will expand, and the cost/performance ratio will become worse.
Pits that are easy to step on during installation and maintenance: the direction of the guide tube, the amount of pre-tension and the torque of the bolt cannot be saved
Let's take a look at a few common rollover cases on the spot.
The deflector is set in reverse direction. The function of the guide tube is to guide the flue gas to the inside of the expansion joint, so as to reduce the erosion of the skin by the vortex. It must be installed in the direction of flue flow. Once installed backwards, the fly ash will hit the gap between the fiber layer and the skin directly, and it will wear out in a few months.
The pre-stretch amount was not done. Non-metallic expansion joints, like metallic expansion joints, need to be pre-stretched or pre-compressed when installed according to the direction of displacement during operation. If you think about it, the pipe will elongate when heated. If you don't pre-stretch it, the expansion joint will already be at half the design displacement in the cold state, and it will directly exceed the limit in the hot state.
And bolt torque. The skin is compressed by beading, and the torque of the beading bolts must be uniform. Some people try to save trouble, and they beat them hard with wind cannons, and the torque suddenly increases and decreases. And the result? Where the torque is high, the skin is fractured, and where the torque is low, the air leaks. The correct approach is to tighten diagonally in three passes, and the final torque is controlled within ±5% of the design value.
Look at these three points before quoting: Flow area, media composition and operating conditions determine which non-metallic expansion joint you use
Don't ask for the price as soon as you come up. The quotation of non-metallic expansion joints is by project, and the three parameters directly determine the price range.
First, the flow area. The cross-sectional area of the flue determines the cross-sectional dimension of the expansion joint. Rectangular flue is conventionally 2m ×3m to 4m ×6m. The larger the area, the amount of skin and frame steel will go up, and the price will naturally increase exponentially.
Second, a medium component. The sulfur content, dust content and humidity in the flue gas directly determine the grade of skin material. For power plants burning high-sulfur coal, the concentration of SO₂ in flue gas is high, SO the skin should be covered with fluororubber composite layer with better acid resistance; For working conditions with high dust content, the inner lining layer should be thickened or wear-resistant guide tubes should be installed.
Third, operating conditions. Continuous operation or frequent start-stop? Is the design pressure ±5kPa or ±10kPa? These parameters determine whether the product needs to reinforce the frame structure or need to add a stainless steel liner to resist erosion.
Only when these three conditions are reported clearly can the manufacturer match the specific structure and material for you. If the conditions are unknown, it is necessary to quote, and there is a high probability that there will be problems in the later period.
How to select the boundary of non-metallic expansion joint and metal corrugated expansion joint in SCR system
No one completely replaces the other, and both have their own niches in the SCR system.
For scenarios requiring large-diameter rectangular cross-section compensation, such as SCR reactor inlet and outlet, economizer outlet flue, choose SWF type non-metallic expansion joint (i.e. non-metallic expansion joint (fabric fiber expansion joint) or rectangular non-metallic expansion joint). This kind of position has high temperature, large cross section and complicated displacement, so it is difficult for metal corrugated expansion joint to handle it.
However, small-diameter pipelines with high temperature and high pressure, such as denitrification steam pipelines and compressed air pipelines, still honestly choose metal corrugated expansion joints. The pressure-bearing capacity of non-metals is limited, and the positive pressure exceeds 30kPa, which is a little overwhelming.
Two kinds of expansion joints are used in series on the same section of flue. Metallic corrugated expansion joints at the front end of the flue gas baffle door absorb axial displacement, and non-metallic expansion joints near the reactor outlet absorb lateral and angular displacement. Each manage a section, and cooperate.
In the final analysis, the boundary of type selection lies in: large cross section, medium temperature, low pressure, complicated displacement, and preference is given to non-metals; Metal corrugated expansion joint is more suitable for small pipe diameter, high pressure and high temperature.
The reason why SCR non-metallic expansion joint has become an "invisible need" is not because it is conspicuous in BOM, but because it silently shoulders the most troublesome displacement compensation problem in denitrification flue. Temperature resistance, corrosion resistance, low-frequency vibration resistance, and three-way displacement absorption-these things are superimposed together, which is not the icing on the cake, but a timely charcoal.