1. The pain point of pipeline compensation under high temperature conditions: why can't the metal expansion joint bear it?
Two days ago, I met a customer, and the temperature of the flue gas pipe rushed to 750°C. At first, he chose an ordinary stainless steel corrugated expansion joint, which cracked after three months. This is not unusual at all-metal materials will creep, oxidize and plummet in fatigue strength at high temperatures. Especially when the medium still carries corrosive gases (such as wet flue gas after desulfurization), the life of metal hoses and high-temperature axial expansion joints will be greatly reduced. So what do we do? Nonmetallic compensators come in handy at this point.
To put it bluntly, metals start to "soft feet" above 600°C-the yield strength of stainless steel drops like a cliff, and grain boundary oxidation and thermal fatigue cracking follow one after another. If there were more sulfur and chlorine in the smoke, the corrosion rate would almost double. And guess what? Many engineering companies carry it hard, and as a result, the expansion joint is changed twice in three years, and the downtime loss is much more expensive than the equipment itself.
2. What does the nonmetallic compensator rely on to carry high temperatures? Material and Structure Disassembly
All the answers are in the fabric fibers. The non-metallic expansion joint (also called fabric fiber expansion joint) of this station adopts multi-layer composite structure: the innermost layer is usually temperature-resistant silicone gel cloth or fluororubber cloth, with ceramic fiber blanket or glass fiber blanket sandwiched in the middle for heat insulation, and the outer layer is protected by PTFE film or corrosion-resistant coating. This structure can withstand the continuous high temperature of 600°C ~1200°C (depending on the material selection). And because it is non-metal, there is no problem of metal fatigue at all.
There are also rectangular non-metallic expansion joints, which are specially matched to square flues, such as the outlet of power station boilers. Rectangular flue with metal expansion joint is difficult to make, non-metal scheme directly fabric molding, and the installation is also light. Tsk, if you've seen the row of giant square tubes at the outlet of the power station boiler, you know how worry-free non-metals are.
3. Core performance indicators: don't just look at temperature resistance, but also pressure, compensation amount and leakage rate
It's not enough to carry the heat. Do you have to know if it is under positive or negative pressure? The pressure difference between the front and rear of the desulfurization flue gas baffle door varies greatly, and the non-metallic compensator must be designed with sufficient pressure-bearing skeleton-such as stainless steel wire mesh or reinforcing ring. In terms of compensation amount, the non-metallic compensator can absorb axial, transverse and angular displacements, and the compensation amount is much larger than that of the metal expansion joint of the same size. Leakage rate is also critical-double seal design or high airtightness coating can be controlled below 0.5% to meet environmental requirements.
At the outlet of the preheater of a cement plant, the flue gas at 800°C has a pressure of only a few kPa, but the displacement of the pipeline exceeds 50mm. They tried metal bellows and it leaked in two months. Replaced to a non-metallic expansion joint (fabric fiber plus ceramic fiber lining), which has not been replaced after four years of use. Is that the truth? When selecting non-metals, you can't only look at temperature resistance, but the compensation amount and leakage rate are the real efforts.
4. Selection and installation: Don't take the hard sleeve of metal expansion joint
Many engineers have the habit of selecting the type according to the method of metal expansion joint-calculating the thrust, the number of corrugations and the pre-stretching. Nonmetallic compensators don't have to do this at all. It has a light weight and almost negligible thrust on the pipe bracket. But there is a pit: you must confirm the flow direction arrow during installation (it was said in the Q&A of the site that "the direction of the arrow of the expansion joint refers to the flow direction of the medium"), especially if the one-sided flange is fixed reversely, the fabric layer will be torn.
In addition, do you want to add the deflector? If the medium contains dust (such as the working conditions of cement industry), a guide tube must be added, otherwise it will wear too quickly. We have a question and answer devoted to the role of the deflector, which makes it clear: the deflector can protect the fabric layer from high-speed particle erosion. Don't save this money, saving is joking about longevity.
5. Typical application scenarios: power stations, cement, desulfurization and air-cooled islands
According to the product classification of this station, although there are many corrugated expansion joints used in power station industry, non-metallic compensators are almost standard in the high-temperature section of boiler flue outlet, the inlet and outlet of induced draft fan and the inlet of desulfurization tower. Metal corrugated expansion joints are also common in the cement industry, but non-metal is the correct answer in places exceeding 800°C, such as preheater outlet and kiln head cover. There is also air-cooled island vacuum pipeline-although the temperature is not high (about 100°C), the metal bellows is easy to become unstable under vacuum negative pressure, so it is safer to use non-metal (refer to the double hinge expansion joint of air-cooled island vacuum pipeline in this station, which is a metal scheme, but non-metal can also be designed for vacuum).
The temperature and corrosiveness of the flue gas baffle door at the entrance of desulfurization tower are extreme. The circular baffle door (double seal) of this station with non-metallic expansion joint can effectively isolate, and the leakage rate is below 0.1%. If you are doing a desulfurization project, this set of combination boxing is worth considering.
6. Maintenance life: Don't think of "maintenance-free for life"
Non-metallic compensators typically have a shorter life than metallic, but are cheaper to replace and quicker to construct. According to the national standard JB/T 12235-2015, the design life is generally 5 to 10 years, which actually depends on the frequency of temperature fluctuation and media corrosiveness. It is necessary to regularly check the outer layer for cracks and the inner layer for bulging. If you find that the fluororubber layer is aging and hardening, change it quickly. Don't wait for an air leak to find out-the loss of whole plant shutdown is much more expensive than the compensator.
Well, maintenance is actually not complicated: scan the surface temperature with an infrared thermometer to see if there are any abnormal hot spots; Then take a flashlight to see if there is any smoke coming out of the flange connection. Once every three months, it takes less than half an hour. If you wait for the smoke to blow out before changing it, it will be a torment.