Two days ago, I met the project manager of a steel mill, and talked about the hot air duct in their factory recently-the weld cracked, the bracket deformed, and the machine was shut down every three days for emergency repair. I asked him: What compensator is installed in your pipeline connection section? He was stunned for a moment and said, It's just an ordinary metal expansion joint. I told him directly, switch to a non-metal one. In fact, many people have similar confusion: Why do steel mills use non-metallic expansion joints? Isn't it just a cloth bag? Really not. Today, let's make it clear what this thing is used for in steel mills, how to use it, and how to choose it so that it won't step on pits.
1. How "tossing" is the pipeline system of steel mills? — — Temperature, displacement and corrosion
Steel mill pipeline is not an ordinary pipeline, it is in "deep water" every day. Take the blast furnace hot blast stove as an example. The supply air temperature is often above 1000℃, and the temperature difference can reach 800 or 900 degrees when hot and cold alternate. Pipe thermal expansion and contraction, a pipe section more than ten meters long, the thermal displacement may exceed 100 mm. That's not the end, what's running in the pipeline? High-temperature flue gas, sulfur-containing waste gas, dust particles, and some media are also highly corrosive. In this environment, ordinary carbon steel pipes are riddled with holes by corrosion in a few months. Not to mention the additional displacement caused by the settlement of the pipe support foundation and the vibration of the fan. Think about it, in such a torturous environment, if there is no reliable compensation means at the pipeline connection, the whole system will fall apart.
Second, what is the use of non-metallic expansion joints in steel mills? — — Dismantling three core functions
What can non-metallic expansion joints (also called fabric fiber expansion joints, we have a special product introduction in our station) do? Break it apart and say three things:
1. Absorb multi-dimensional displacement and do not produce thrust.Although the metal expansion joint can also compensate for displacement, it will generate elastic reaction force, which requires high pipe support. The band of non-metallic expansion joint is made of multi-layer flexible material composite, which can easily absorb axial, transverse and angular displacement, and has almost no resilience. For example, a section of hot air duct expands by 80mm due to temperature increase, and the non-metallic expansion joint directly "swallows" this displacement without transmitting force to the equipment flange.
2. Vibration isolation and noise reduction to protect equipment.The fans and dust collectors in steel mills vibrate heavily, and the metal hard connection will directly transmit the vibration to the pipeline. After a long time, the bolts will loosen and the welds will crack. The non-metallic expansion joint itself is a flexible body, which is equivalent to a soft joint, and can filter out 70% to 80% of high-frequency vibration. In a previous case, a steel mill installed a non-metallic expansion joint at the outlet of the induced draft fan, and the vibration amplitude of the pipeline was directly reduced by 60%, and the life of the equipment was obviously extended.
3. Corrosion resistance, high temperature resistance, adapt to harsh media.The ring belt material of the non-metallic expansion joint can be customized according to the working conditions-fluororubber is resistant to strong acids, silicone rubber is resistant to high temperatures, and PTFE is resistant to corrosion. For the sulfur-containing wet flue gas in the desulfurization flue, the temperature is not high but the corrosion is strong, and the rubber PTFE compensator is much more reliable than the metal expansion joint. Metal expansion joint Once the bellows is corroded and perforated, the whole system has to leak. Even if the non-metal ring belt is partially damaged, it is easy to repair and does not need to be replaced as a whole.
III. Comparison of metal expansion joints: Why prefer non-metals in flue gas and hot air ducts of steel mills?
Hey, are metal expansion joints useless? Of course not. Metal expansion joints (such as the general corrugated expansion joints and high-temperature axial expansion joints in our station) are still the first choice in high-pressure and high-purity media applications. However, in the low-pressure, large-diameter and high-temperature flue gas pipelines of steel mills, the advantages of non-metals are too obvious.
For the same caliber compensator, the price of non-metal tends to be only 1/3 to 1/2 of the price of metal. Look at the installation again: the metal expansion joint requires high installation accuracy, and the direction of the guide tube and the adjustment of the tie rod nut (refer to the content of how to adjust the tie rod nut of the expansion joint in our Q&A) are wrong. The non-metal expansion joint is simple to install, just tighten the flange connection, and be careful not to twist when installing the ring belt. Another point: once the metal expansion joint is welded to the pipeline, it must be hot cutting for maintenance and replacement, while the non-metal expansion joint only needs to remove the flange bolts to pull out the ring belt and replace it with a new one. Steel mill maintenance time is money, which makes people in the equipment department love it.
Of course, non-metals also have shortcomings: the pressure bearing capacity is low, generally no more than 0.1MPa. The flue gas pipeline of steel mills is originally slightly positive or negative pressure, which is just the right way. If you use it to connect to the steam pipe, it won't work. It must be emphasized here that the selection should not be messed up, but should be done according to the working conditions.
4. Typical application scenarios of steel mills: from blast furnace hot blast furnace to desulfurization flue, one is indispensable
Let me list a few of the application scenarios of non-metallic expansion joints in steel mills:
- Blast furnace hot blast furnace area: On the hot air main pipe and branch pipe, the temperature is 800~1200℃, and high-temperature resistant non-metallic expansion joints (such as aluminum silicate fiber + ceramic fiber cloth composite band) are used to compensate for thermal displacement and insulate heat at the same time.
- Desulfurization and denitrification flue: The flue gas temperature after wet desulfurization is 50~80℃, but it contains a large amount of SO₂ and HCl, which is extremely corrosive. At this time, with rubber tetrafluorine compensator or polytetrafluoroethylene compensator, the fluoroplastic layer can withstand strong acid corrosion, and has good flexibility, which can absorb the displacement of flue due to temperature difference.
- Dust removal system pipeline: The inlet and outlet of the bag dust collector, the flue gas has high dust content and large temperature fluctuation. The inner wall of the non-metallic expansion joint is smooth, does not accumulate dust, and can also absorb fan vibration. Some steel mills use rectangular non-metallic expansion joints for large rectangular flues, specifically matching square sections.
- Gas pipeline: Blast furnace gas and converter gas pipelines, the temperature is not high but contains CO, and the sealing requirements are high. The multilayer sealing structure of non-metallic expansion joints has extremely low leakage rate and will not be damaged by water hammer.
Lo and behold, from the hot end to the cold end, from the dry process to the wet process, non-metallic expansion joints cover nearly all nodes of a steel mill's flue gas system. Isn't it a bit of a surprise?
5. Pay attention to the selection: temperature grade, media composition and installation position directly affect the life span
If you choose the wrong model, no matter how good the product is, it will be useless. Focus on three parameters when selecting:
First, temperature.The non-metallic expansion joints have limited temperature resistance. Conventional silicone rubber belt can withstand 200 DEG C, fluorine rubber 250 DEG C, and ceramic fiber layer can be above 1000 DEG C. However, it should be noted that the actual operating temperature should be at least 50℃ lower than the material limit, otherwise the aging will be accelerated. For example, a steel mill used a ring belt with a temperature resistance of 400℃ on a flue gas pipeline at 450℃, and as a result, it was brittle and cracked in three months. Degraded usage is a common mistake.
Second, a medium component.Oily smoke can not use rubber ring belt, oil will swell the rubber. Flue gas containing chloride ions must be isolated by a fluoroplastic layer. The desulfurization flue has a large amount of moisture, so it is necessary to choose a PTFE compensator that resists hydrolysis. In the product information of our station, there are desulfurization flue gas baffle doors and matching compensators specifically for desulfurization conditions, which can be used in combination.
Third, the installation location.If the expansion joint is mounted on a horizontal pipe, consider whether dust accumulates. On the vertical pipeline, it is necessary to prevent the ring belt from bearing and cracking. Some positions need to install a guide tube (refer to the specific function of the expansion joint guide tube in the question and answer) to prevent high-speed airflow from directly washing the ring belt. In addition, the layout of pipe brackets is also critical-fixed brackets must be set on both sides of the expansion joint, and the spacing of guide brackets is also required. These details are in our Q&AAs mentioned in, the tie rod is mainly to prevent over-stretching, and it is also applicable to non-metallic expansion joints.
6. Just put it on and you're done? Those pits for installation and routine inspection
When installing non-metallic expansion joints, there are several pits that must not be stepped on:
- Pit 1: Forced counterpart.Pipe installation deviation is large, and expansion joints are used to "level". Isn't that the same as hard twisting the loop tape into a twist? Correct practice: Adjust the pipe bracket to ensure that the expansion joint is installed in the free state and the amount of pre-deformation does not exceed 10% of the design value.
- Pit 2: The loop belt is installed backwards.Some non-metallic expansion joints have directionality (for example, the guide tube faces the flow direction of the medium). If the guide tube is installed backwards, it can't play a protective role, and the airflow directly washes the inside of the ring belt.
- Pit 3: The bolts are tightened too tightly.The torque of the flange connecting bolt is too large, which will press the ring belt out of dents and destroy the sealing surface. It should be evenly tightened diagonally, and the torque should be according to the manufacturer's requirements.
- Pit 4: Neglect insulation.The exterior of the non-metallic expansion joint on the high-temperature pipe must be wrapped with an insulation layer, otherwise the outer surface temperature of the ring belt will be too high, which will accelerate the aging. The insulation layer should also leave an inspection port to facilitate daily inspection.
What do you see for daily inspections? Look at the surface of the ring belt for bulging, cracking, aging and powdering. Second, see if the flange bolts are loose. Third, see if there is any abnormal noise-the ring belt rubs against the pipe wall or the displacement of the guide tube will make a sound. Four, see if there is a drip. The maintenance cycle of the steel mill is long, and the inspection frequency is recommended once a week. Use an infrared thermometer to measure the surface temperature of the ring belt. Compared with the initial record, we should be vigilant if the temperature difference exceeds 20℃.
After so much verbose, it is actually a sentence: steel mills use non-metallic expansion joints not because they are cheap, but because they are indeed more suitable than metallic ones in the scenarios of flue gas, hot air and corrosive media. If you choose it right, install it right, and maintain it well, it will not be a problem to use it for three or five years. If you are still unsure which one to choose, flip through the product pages of rectangular non-metallic expansion joints, rubber compensators and PTFE compensators in our station, or ask the technology directly.