Why steel mill pipes can't be separated from non-metallic expansion joints-talk about those choices forced out by high temperature and corrosion
Dry steel plant equipment maintenance brothers know that the most troublesome thing in the pipeline system is thermal expansion and contraction and corrosive media. Blast furnace gas pipeline, sintering flue gas pipeline, coke oven waste gas pipeline-which is not the double critical strike of high temperature and corrosion? Under this working condition, the metal corrugated expansion joint is either corroded by acid dew point or cracked due to thermal fatigue, and its life is so short that people want to curse.
At this time, the non-metallic expansion joint (also called fabric fiber expansion joint) comes up. It is not afraid of acid and alkali, can withstand several hundred degrees of high temperature, and can also absorb multi-dimensional displacement. Think about it, the pipelines in steel mills are hundreds of meters long at every turn, and the thermal expansion is small in millimeters. The non-metallic compensator eats the stress through the flexible ring belt, so that the pipelines and equipment can run safely. To put it bluntly, it is not how advanced the non-metallic expansion joint is, but the optimal solution forced out by the working conditions.
What exactly does the structure of a non-metallic expansion joint look like? Don't be confused with metal corrugated expansion joints
When many people hear expansion joints, they think of that stainless steel bellows. Brother, that's a metal corrugated expansion joint, which is completely different from a non-metal expansion joint. The core component of the non-metallic expansion joint is the loop belt, which is made of multiple layers of materials: the outermost layer is a corrosion-resistant fluororubber or silicone rubber coating, the middle layer is a reinforcement layer such as glass fiber cloth and aramid cloth, and the inner layer may also have thermal insulation cotton. The loop belt is fastened to the flange at both ends by pressing plates and bolts, and the whole structure is like a large pocket with soft connections.
It has no ripples and does not rely on elastic deformation of the metal to compensate for displacement. It relies on the flexion and tensile deformation of the loop itself. So its compensation capacity is much greater than that of metal, and it is especially suitable for large diameter pipes and complex displacements. For example, the rectangular non-metallic expansion joint commonly used on the flue gas pipeline of steel mills can absorb displacement in axial, transverse and angular directions at the same time. If the metal expansion joint is so dry, it will be fatigued early.
By the way, there are rubber compensators and rubber PTFE compensators in this station. Although these are also non-metallic materials, their structures are different from fabric fiber expansion joints. Don't get confused when selecting, depending on the working conditions.
The three most headache parameters in model selection: temperature, medium, displacement, one than the other pit
Let's start with the temperature. The temperature span of steel mill pipelines is extremely large, ranging from normal temperature to above 1000℃. The temperature resistance rating of non-metallic expansion joints depends on the band material. Ordinary silicone rubber coating can only carry to about 250℃, and fluororubber is also 300℃. Even higher, it will be a ceramic fiber composite layer, such as the structure used for high-temperature axial expansion joints. Where is the pit? Many people only look at the design temperature of the pipeline, ignoring instantaneous excessive temperature and local thermal radiation. For example, near the outlet of the hot blast stove, the measured temperature may be 50-80℃ higher than the design value, and the ring belt will burn through directly.
Sulfur-containing flue gas, chlorine-containing waste gas, alkali mist, water vapor…Each medium is different in its corrosiveness to materials. For example, the pipeline behind the desulfurization flue gas baffle door is very acidic, and ordinary glass fiber cloth will be crisp in a few months. A PTFE compensator or PTFE-coated belt is required. However, PTFE has low mechanical strength and is not suitable for high pressure. How to find this balance? Generally, composite structure is used: PTFE is lined, and the outside is reinforced with aramid cloth.
Finally, talk about displacement. Displacement calculation is a delicate job. Pipe thermal expansion, installation error, equipment vibration, wind load-all factors superimposed. The allowable displacement of non-metallic expansion joints is different according to the structural design. Usually, the axial compression amount can reach 15%-20% of the pipe diameter, and the transverse displacement is also about 10%. However, some people choose the standard product with a large compensation amount for cheapness. As a result, the actual displacement is only 5%, and the ring belt is in a relaxed state for a long time, which is easy to accumulate dust and age. Alas, this degree is stuck.
Those little doorways in installation and routine maintenance that no one tells you
Directions. The direction of the arrow on the non-metallic expansion joint refers to the direction of media flow, not the direction of displacement. Never install it backwards, otherwise the guide tube (if any) will pocket and cause vibration. In addition, the tightening torque of flange bolts should be uniform, and do not tighten and loosen, otherwise the ring belt will be twisted and the life will be cut in half.
In terms of maintenance, many people think that non-metal expansion joints are maintenance-free. Bullshit. Check at least twice a year: see whether there are cracks, bulges and leakages on the surface of the ring belt, whether the pressure plate bolts are loose, and whether the guide tube has fallen off. In addition, when parking in winter, the water accumulates in the pipeline and freezes. If water is stored inside the expansion joint, the ice expansion stress can directly deform the flange. Therefore, be sure to drain the standing water before parking.
Expansion joint tie rod. Some models come with tie rods for pre-compression and transport protection during installation. The tie rod nut should be adjusted in place after installation, but it does not need to be removed. The adjustment method refers to what is said in the FAQ of this site-first loosen the locking nut, adjust to the design displacement, and then lock it. Don't overscrew, otherwise the tie rod becomes a rigid support, and the expansion joint is equal to uninstalled.
When it comes to longevity and cost, are non-metallic expansion joints worth it? Take the actual working conditions of the steel mill to calculate the account
Just say the numbers. The original metal corrugated expansion joint of the head pipe of the sintering machine in a steel mill has an average life of 8 months, and the cost of each set is 2,500 yuan, including the cost of shutdown and labor replacement, which is about 12,000 yuan each time. It is changed 1.5 times a year, with a total cost of 23,000 yuan. Switch to non-metallic expansion joints (fabric fiber type), the cost of a single set is 3500 yuan, but the life span reaches 3 years. Equivalent to the annual cost of 1,167 yuan, plus the three-year replacement time fee of 4,000 yuan, with an average of 2,500 yuan per year. A saving of 87%.
Moreover, the non-metallic expansion joint is corrosion resistant and will not leak due to pitting corrosion, which reduces unplanned shutdown. How much does a steel mill lose by shutting down production in one day? You do the math yourself. On the surface, the unit price of non-metallic expansion joints is high, but the whole life cycle cost is a few blocks away from metal. According to JB/T 12235-2015 standard, the design life of products of regular manufacturers is 5-10 years, provided that the selection is correct and the installation is standard.
Of course, there are also occasions where non-metals are not suitable: such as high-temperature and high-pressure steam pipelines (exceeding 2MPa) and occasions where impact resistance is required. These places also rely on metal corrugated expansion joints or double hinge transverse expansion joints. Selection is a game.