First, find out the position of the internal baffle in the expansion joint: it is not the same as the guide tube and bellows
Many people confuse the internal baffle of the metal expansion joint with the guide tube, thinking that it is all about adding a piece of iron sheet to the tube. Wrong. The bellows is that telescopic, corrugated, thin-walled shell responsible for absorbing heat displacement; The guide tube is close to the inner wall of the bellows, which is mainly used to reduce the direct scouring of the bellows trough by high-speed fluid. The internal baffle, which is located more inward, is usually welded to the connection pipe or end pipe, and lies in the middle of the medium flow channel. It does not contact the bellows, nor does it play a role in diversion.
Let's say that the deflector is used as a "personal guard" for the bellows, and the internal baffle is more like a "roadblock" or "diverter", which abruptly changes the flow direction and velocity distribution of the medium. The baffle is not installed to make the pipe smoother, but to deliberately create a little "obstruction".
2. The actual functions of the internal baffle: protect the bellows, control the flow direction of the medium, and reduce erosion and vibration
Its first function is to block. Solid particles such as dust particles, catalysts and coal ash run with the airflow. If there is no baffle, they will directly polish the inner wall of the bellows like sandpaper. The wall thickness of bellows is usually only 0.5~2mm, so it is a matter of time to wear out.
The second function is steady flow. After the fluid enters the expansion joint, due to the abrupt change in the cross section of the bellows, vortex and bias flow will be generated. The baffle can comb out the turbulent airflow, allow the medium to pass more evenly, and reduce the lateral impact and induced vibration on the bellows. In some high-pressure pipes, baffles also reduce the risk of fluid resonance.
The third function, which many people may not think of, is that the baffle can reduce the "dead zone" in the bellows. When the medium stays in the trough, it is easy to accumulate ash and liquid, and it will cause intergranular corrosion over a long time. The baffle holds the center of the mainstream guide pipe and the trough area relatively clean, and the corrosion problem can also be alleviated.
3. Under what working conditions must the internal baffle be installed? The scenes of dust, high temperature and high flow rate are described one by one
Let's start with the dust. Cement industry, ash pipeline of power plant and blast furnace gas pipeline of iron and steel plant must be installed as long as there are hard particles in the medium. Taking the metal corrugated expansion joint in the cement industry as an example, the dust concentration in the kiln tail waste gas often exceeds 50g/Nm³, and the particle size ranges from tens of microns to hundreds of microns. Without baffles, the bellows can't last for one year. With a baffle, the life can be stretched to three to five years.
Let's talk about the heat. When the flue gas temperature exceeds 800℃, the radiant heat and high-speed airflow act together, and the bellows is prone to high-temperature creep and oxidation. The baffle can block a part of direct heat radiation, and at the same time avoid high-temperature air flow directly washing the trough. Note that at this time, the baffle itself has to be made of heat-resistant alloy, not ordinary carbon steel.
High flow rates cannot be ignored either. The flow rate of steam pipeline is often 30~50m/s. If the medium is wet steam, water droplets will impact the bellows and produce water erosion. The baffle stops the water droplets and flows away along the pipe wall, so that the bellows will not be "punched" out of numbers. Desulfurization flue gas baffle door and corrugated expansion joint used in power station industry have similar working conditions, so the baffle should be taken into account in design.
4. What will happen if the baffle is not installed: wear and perforation, bellows instability, system shutdown, real cases are put out
Two years ago, there was a thermal power plant customer. The expansion joint at the outlet of the induced draft fan was not equipped with an internal baffle. After running for less than eight months, two thumb-sized holes were ground at the trough of the bellows, and the air leakage was serious. The fan efficiency dropped by 10%, and finally it was forced to shut down for emergency repair. It is only a few thousand yuan to replace an expansion joint, and the loss of stopping production for one day is hundreds of thousands.
There is also a catalytic cracking unit in a chemical plant. The expansion joint of the regenerator outlet pipe has catalyst dust in the medium, and there is no baffle. After the bellows is worn out, the catalyst leaks, which almost causes a safety accident. Later, it was rectified and a double-layer baffle was added inside the expansion joint, and the problem never appeared again.
In addition to wear, not installing a baffle may also cause the bellows to become unstable. High-speed airflow directly shocks the wave peak, generating lateral forces, and the bellows will twist and deform, losing elasticity and compensation ability. This damage is irreversible and can only be replaced as a whole.
In the final analysis, the baffle is a "scapegoat", which carries all the wear and impact on itself. If it breaks, the baffle can be changed, and the bellows can still be used.
5. Key points of design and selection of internal baffle: material, thickness, clearance and connection mode. Don't wait for the equipment to break before patting your head
In terms of material selection, Q235B or 304 can be used for non-corrosive media at room temperature; 310S or Inconel 625 for high temperature flue gas; Consider duplex steel or higher grade corrosion resistant alloy for sulfur-containing wet flue gas. It's not the more expensive the better, it has to match the medium.
The thickness is generally 3~6mm. If it is too thin, it can't stop the particles. If it is too thick, it will expand greatly and easily crack the connecting weld. Note that the temperature of the baffle is higher than that of the bellows, and the amount of thermal expansion should be reserved. Therefore, a gap must be left between the baffle and the bellows, usually 10~30mm on one side, so as to avoid the baffle resisting the bellows after thermal expansion.
One is welded and fixed on the end pipe, the structure is simple, but it has to be cut during maintenance; The other is a detachable bolted connection for easy replacement. It is recommended to use block baffles for large-diameter expansion joints, leaving expansion joints in the middle, otherwise the thermal stress of integral baffles is too great.
The mounting direction of the baffle cannot be reversed. Generally, the baffle faces the flow direction of the medium. If it is installed backwards, it will not only play a protective role, but will increase the local resistance and even lead the medium to the bellows. So when installing on site, be sure to check the flow direction arrow on the expansion joint.
Alas, to put it bluntly, the internal baffle is usually invisible and invisible, but whether it is installed or not, the life of the equipment is several times worse. When you select the model next time, don't just stare at the material and wavenumber of the expansion joint. Ask "Is there a baffle inside", which can save a lot of maintenance costs.