What exactly are metal expansion joints installed on fan ducts?
Once the fan is turned on, the duct is not stationary. In the hot state, the pipe should be elongated, and the vibration of the fan itself is also transmitted in. You forcefully weld the pipe to death and tighten the flange, and the result is a cracked weld and a leaky flange, all of which is a matter of time. The metal expansion joint is installed in order to absorb thermal displacement and mechanical vibration, and remove the stress in the pipe. General-purpose corrugated expansion joints and high-temperature axial expansion joints are mostly used in the supporting fan of this station, which can adapt to flue gas and air media from medium to low temperature to high temperature, and the axial displacement and a small amount of lateral displacement can also be compensated together, which is the standard configuration of fan inlet and outlet pipelines. To put it bluntly, it is to install a joint to the pipe that can breathe.
Look at the picture first to find the guide tube, the wrong direction is all in vain
A typical fan metal expansion joint illustration is in front of you, and most people are staring at the bellows at a glance – that's right, the bellows are the core. But what really determines whether it can be installed or not are the end connectors, deflectors, tie rods and nuts. Here, the role of the guide tube is most easily overlooked. The wind speed at the outlet of the fan is high, and dust particles are often sandwiched in the medium. The high-speed scour directly hits the bellows wall, which is not blocked by the guide tube, and the life of the bellows drops from a cliff. The deflector is the shield of the inner wall of the bellows, and it must be installed along the flow direction of the medium. When looking at the picture, look for the direction of the arrow of the guide tube first. If it is installed backwards, the compensation amount will be discounted at least, and the bellows will be directly worn out at worst. There is a special article on this site about the role of the deflector, which will not be expanded here. You remember a sentence: If the direction is wrong, the rest will be in vain.
Every number on the size chart is a lesson for money
The diameter, wave number, wall thickness, length and pressure level marked in the diagram are not written casually. They directly determine the rigidity of the bellows and the amount of compensation. The most typical mistake in fan duct selection is to only look at the diameter, regardless of whether the wavenumber is enough or not. The displacement of the same fan in cold state and hot state can differ by tens of millimeters. With less wave number, the bellows is hard-pulled, the stiffness is large, the reaction force is large, and the pipe support also suffers. What about that? Before the selection, the axial displacement, transverse displacement and angular displacement are estimated, and then checked against the stiffness calculation formula of the bellows. When encountering multi-directional displacement or high pressure and thrust, don't hard-set the general-purpose type. This station's compound hinge transverse expansion joint and straight pipe pressure balance expansion joint do this. Mark these types when illustrating, so that the selection will not run into the ditch.
The Tie Rod Nut Thing: It's Not Reinforcement, It's Tethering
The most misunderstood accessories in the diagram are tie rods and nuts. Many people think that the tie rod is used to reinforce, for fear that it will not be strong. In fact, the role of the tie rod is just the opposite. It is to restrain the bellows and prevent it from being pulled off beyond the limit displacement. How to adjust the tie rod nut during installation? Generally, there is a pre-adjustment amount when leaving the factory, and the site cannot be spun to death at will. The direction of the arrow on the expansion joint represents the medium flow direction or installation direction. The adjustment of the tie rod nut should be according to the design drawings. Blind adjustment will cause the bellows to become unstable or abnormal reaction force. Take the fan system as an example. If the tie rods of the inlet and outlet expansion joints are adjusted too tightly, there will be no place for thermal expansion when the fan is started, and the force will all be pressed on the casing. The consequence will be the casing deformation and the coupling alignment deviation. Do you think this is a big deal?
When should I change? Don't wait until you miss it before you regret it
The metal expansion joint of fan has no permanent life, and the bellows will be fatigued under alternating stress. The material and working condition determine the service life. Under normal working conditions, if there are cracks, corrosion pitting on the surface of the bellows, or the compensation amount is obviously insufficient, it should be replaced. Both corrugated expansion joints for power station industry and metal corrugated expansion joints for cement industry in this station pay attention to temperature resistance and corrosion resistance, but no matter how good the material is, it can't withstand long-term overtemperature operation. You usually stare at the trough of the bellows during inspection, where dust and corrosion are the easiest. If you hear abnormal deformation of the bellows or the sound of the guide tube falling off, don't hesitate to stop the machine for inspection. It is not as simple as replacing the expansion joint after the leak is dealt with. The whole fan system has to stop working. Can this account be settled?