Why must flue systems use non-metallic expansion joints? -Thermal displacement, corrosion and vibration, none of them can be missed
The flue system of the power plant is, to put it bluntly, a huge "exhaust gas channel". The flue gas comes out of the boiler, goes through desulfurization, denitrification, dust removal, and finally enters the chimney. The temperature changes drastically along the way-from several hundred degrees to tens of degrees, and the displacement of thermal expansion and contraction of the pipe is considerable. And guess what? If there is no expansion joint, the pipeline itself will suffer internal injuries, and weld cracking and flange leakage are common phenomena.
Then why use non-metal expansion joints instead of metal ones? Because the flue is not only hot, but also corrosive. The flue gas after wet desulfurization contains a large amount of acidic condensate, and the metal bellows will rust through in this environment for several months. Coupled with the low-frequency vibration brought by the fan, metal fatigue came soon. Non-metallic expansion joints (especially fabric fiber expansion joints and rubber compensators) are inherently corrosion-resistant, vibration-absorbing, and compensate for displacement without the stress concentration problems of metal bellows. To put it bluntly, it is only when "soft" deals with "hard" environments can it be held up.
Non-Metal vs. Metal: Who Carries More in the Flue? — — The true performance of fabric fiber expansion joints and rubber compensators
"Are non-metallic expansion joints better than metallic ones?" Not absolutely. Metal expansion joints (such as metal rectangular expansion joints and general-purpose corrugated expansion joints) do have advantages in high-temperature and high-pressure steam pipes, but when it goes into the flue system, that's another story.
Let's look at the specific data. The flue temperature is generally 80℃ ~200℃, and the pressure is usually within ±5kPa (mostly negative pressure). Under this working condition, the non-metallic expansion joint wins completely. The non-metallic expansion joint (fabric fiber expansion joint) of this station adopts multi-layer composite structure: fluororubber lining for corrosion resistance, ceramic fiber insulation layer for heat insulation, and silicone rubber cloth for sealing. The measured service life is 3~5 years, while the pitting corrosion of metal expansion joints under the same working conditions often starts to appear in 1~2 years.
Let's talk about the rubber compensator. It is more suitable for large bore, low pressure scenarios that require large displacements. For example, the inlet and outlet of desulfurization tower, with a diameter of three or four meters, is common for the pipeline flange to mismatch. The rubber compensator easily absorbs axial and lateral displacement at 1/3 the price of metal. Of course, it is not resistant to high temperatures (the long-term working temperature should not exceed 120℃), so it depends on the specific location. The fabric fiber expansion joint can withstand the temperature above 400℃, which can bear the waste heat at the outlet of the induced draft fan.
The three easiest pits to step on when selecting the model: temperature, pressure and media composition. Don't wait for it to blow up before regretting it
When choosing non-metallic expansion joints, you are most afraid of three things: misjudgment of temperature, wrong pressure and neglect of media composition. I have seen a project with my own eyes, in which a rubber compensator with a temperature resistance of 150℃ was installed on a pipeline with a flue gas temperature of 200℃. As a result, it burned through three days after it was put into production, almost affecting the nearby equipment.
Temperature. The temperature distribution in the flue is uneven-it may instantly rush above 300℃ near the boiler side, while the outlet of the desulfurization tower is only 50℃. The maximum continuous operating temperature and the short-time peak temperature must be known when selecting the model. How to match the lining layer and insulation layer in the fabric fiber expansion joint directly determines the life span. It is recommended to use ceramic fiber + aluminum silicate wool in high temperature zone; PTFE or fluororubber is enough for low wet areas.
Stress. Many people think that the flue is negative pressure and ignore the positive pressure impact. In fact, short-term positive pressure may be generated when the induced draft fan fails. If the design pressure of the expansion joint is only calculated according to negative pressure, the positive pressure will bulge. The conventional practice is to select the model according to 1.5 times the maximum working pressure of the system, and install the limit rod at the same time. The rectangular non-metallic expansion joint and circular baffle door (double seal) of this station are equipped with tie rods as standard, just to prevent this.
Media composition. The flue gas contains SO₂, NO, HCl, and dust. If the dust particles are coarse and the wear is serious, the outer layer of the expansion joint should be added with a wear-resistant layer. If there are many acidic droplets, the liner must be made of perfluorinated material. The rubber PTFE compensator in our station is specially designed for this working condition-fluoroplastic lining + rubber outer envelope, which is both acid-resistant and toss-resistant.
Installation and operation and maintenance: 99% of people don't pay attention to those fatal details
Tighten the bolts before adjusting the pipe. The expansion joint must be installed in the free state of the pipeline, otherwise it will hold its strength as soon as it is turned on, and it will be a matter of time before it tears. The correct way is: after the equipment is in place, do not screw the flange bolts first, let the pipeline be naturally centered, and then tighten it diagonally with an even torque. In addition, the tie rod nut is often in a pre-tensioned state when it leaves the factory, and it must be loosened to the designed displacement after installation (does the screw of the expansion joint need to be removed? The answer is: it should be adjusted according to the instructions, not completely removed).
In operation and maintenance, the most common problem is that the inspection only looks at the surface. The damage of non-metallic expansion joints often starts from the inner layer. The outer layer looks good, but in fact the inner layer has cracked. Therefore, daily inspection must be tapped with tools to listen to sounds, or look at the inner wall with an endoscope. In addition, dust accumulation is a big hidden danger. The dust in the flue accumulates thicker and thicker in the folds of the expansion joint, not only adding weight, but also causing local overheating. It is recommended to clean it quarterly and purge it with compressed air during shutdowns.
Never weld anything on a non-metallic expansion joint. As soon as the spark splashes, the fabric layer burns out holes. If you need a temporary shelf, fix it with clamps and don't weld it.
Expansion Joint + Baffle Door: A Complete Partnership Scheme for Flue System Compensation and Partition
The flue system not only compensates for displacement, but also has to be able to cut off. For example, during maintenance, a branch should be isolated. That's when you need flapper doors. The flue gas baffle door and desulfurization flue gas baffle door of this station are specially used with the expansion joint. The baffle door is responsible for partitioning, and the expansion joint is responsible for absorbing the displacement caused by temperature changes, both of which are indispensable.
In the desulfurization island of a 600MW unit, a non-metallic expansion joint is installed at the outlet of the induced draft fan, and a double-sealed single-axis circular baffle door is installed at the inlet of the absorption tower. Close the baffle door during maintenance, and the temperature of the pipe under the expansion joint drops from 80℃ to normal temperature. If this shrinkage is not compensated, the pipes on both sides of the baffle door will be hard pulled, resulting in the deformation of the sealing surface. With expansion joints, it's all done.
In addition, for situations where complete partition is required (for example, people can enter for maintenance), it is recommended to use manual plug-in insulation doors or electric plug-in insulation doors. This kind of insulation door adopts plug-in plate structure, no dead angle, good sealing performance. With the non-metal expansion joint, it can achieve a complete "soft connection + hard partition".
At the end of the day, the design of a power plant flue system is not a slap on the head. Expansion joint selection, installation, operation and maintenance, each step hides details. Do you want to spend a small amount of money to buy a good one for a few more years, or do you want to change it every three days cheaper and weigh it yourself.