Let's start with a problem that is often overlooked on the scene
It is true that the metal expansion joint itself is pressure-resistant and temperature-resistant. But if you look carefully at those bellows that leak after three or five years of operation, nine times out of ten are not corroded by the medium, but damaged by the external environment.
Mechanical damage, foreign body impact, splash wear. Clinker dust at the end of the cement kiln, gypsum slurry droplets at the entrance of the desulfurization tower, and fly ash particles in the flue of the power plant-these things run with the airflow at a speed of tens of meters per second, and hit the corrugated recess like a small grinding wheel. The wall thickness of bellows is originally 1 to 2 mm. If 0.1 mm is worn off, the fatigue life may drop by 20%; Wear-out means direct leakage, and the whole expansion joint is scrapped.
And stress corrosion cracking. During the installation and operation of the bellows, there is residual stress and working stress. If the surface is scratched and small pits are bumped, chloride ions and sulfide will have a place to stay. Over time, the crack spreads outward from the bottom of the pitting pit. This kind of situation is not very visible on the outside, but by the time it is discovered, it is already clear.
Therefore, the metal expansion joint protective cover is not an optional accessory, it is the key to decide that the expansion energy saving can't run out according to the design life. Don't wait until the bellows leaks before removing it and looking at it, then it's too late for anything.
How to choose the material depends on the media and environment
There is no uniform answer for protective cover material. Generally, stainless steel sheets can be used on high-temperature pipelines. 304 and 316L are common, and the cost is not high, and they can also block splashes. But if you use it in a flue gas desulfurization system, it will be different. After desulfurization, the flue gas temperature is not high, but the humidity is high, the chloride ion concentration is high, and the acidic condensate lies on the protective cover, and the ordinary stainless steel is still rusty. This kind of working condition is often used with desulfurization flue gas baffle door, and the protective cover must be made of thicker weather-resistant steel or alloy material, or even lined with non-metallic anticorrosive layer.
Cement kiln tails are the other extreme. The dust concentration is high, the particles are hard, and the temperature is high. The ordinary sheet protective cover was installed, and it was pierced by sandblasting in half a year. The protective cover of metal corrugated expansion joint in cement industry is usually made of double-layer structure: the outer layer is thick steel plate to block mechanical impact, and the inner layer is high-temperature resistant alloy or ceramic fiber for heat insulation and wear resistance. Selection logic is completely different from flue gas desulfurization.
You see, the same metal expansion joint protection cover, one to solve corrosion, the other to solve wear, can the material and thickness be the same?
Structural design is more easily overlooked than material
If the material is selected correctly, the structural design is unreasonable and the car will still roll over.
Let's talk about the integral sleeve or split flap assembly first. The integral sleeve for small-diameter pipeline has no fault, easy installation and good sealing performance. However, large-diameter corrugated expansion joints, especially those with DN2000 or above used in power station industry, are difficult to transport as a whole sleeve, and on-site hoisting is more headache. Although the flap assembly is troublesome to install, it can be opened to check the corrugation state during maintenance, which can't be given by the whole sleeve.
Do you want to leave the heat dissipation hole? Many people think that the protective cover is fully covered and airtight. In fact, under high-temperature working conditions, if the air between the protective cover and the bellows does not flow at all, the heat accumulated in it cannot be dissipated, and the surface temperature of the bellows may be higher than the design value, and the creep speed of the material will be accelerated. But with the holes left, the dust may get in again. It depends on the working conditions here: if the flue gas has a large dust content, it is better to have a high temperature than to open a large hole; For pure gas-phase media, opening a few small heat dissipation holes is protection instead.
Fewer people knew about the inner wall deflector. The bellows groove is the easiest place to accumulate dust. If a circle of deflectors is added to the inner wall of the protective cover, the airflow can go along the axis, and the dust accumulation and wear can be reduced a lot without directly pouring it into the groove.
The gap between the protective cover and the expansion joint body is also calculated. When the gap is too small, the protective cover stuck the bellows when the heat expanded, and the compensated displacement all became additional stress; If the gap is too large, the airflow will form a vortex in the cover, and the vibration noise will become louder, which will also aggravate the local wear. The fixing method and bolt material can't be careless either-ordinary carbon steel bolts in high temperature conditions will start to oxidize in two months. If you use heat-resistant alloy steel, it can be used for several years.
Installation and overhaul are the high incidence areas of rollover
Many projects are installed quite regularly. After one year of operation, the protective cover loosens, shifts, and even wears out the bellows. What's the problem? There is a high probability that a mine is buried in the installation process.
The matter of alignment is particularly critical. The protective cover and the expansion joint are not centric, and the vibration is superimposed during operation, and the edge of the cover directly rubs the bellows. If you think about it, the protective shield was originally designed to protect from external damage, but it turned out to be a source of wear.
The deformation of welding connection should be controlled, and the weld seam should be checked whether it is continuous after welding; The pre-tightening force of bolted connection should be calculated-if it is tightened too tightly, the bellows will be deformed by pressure, and if it is tightened too loosely, it will be loose after several months of operation. Some on-site diagrams save trouble. Use an ordinary wrench to screw it according to your feeling, which is to bet on your life.
See if the protective cover has obvious displacement; Second, listen to whether there is any abnormal friction sound or vibration; Third, check the fixtures for any signs of loosening. I met a customer two days ago, saying that the expansion joint leaked after less than two years of use. As a result, when I disassembled it, the protective cover had long been deviated, and the bellows had been rubbed out of a deep trench. So keep in mind: the protective cover is not a once-and-for-all installation, it needs to be checked regularly like the expansion joint.
The most practical trick of model selection: make requirements by using the protective cover as part of the expansion joint
Many engineers only give a length and diameter when ordering, and the rest depends on the manufacturer to guess. That's not gonna work.
The selection of metal expansion joint protective cover should be regarded as a part of the expansion joint to put forward requirements. In addition to the interface size, at least give the following information to all:
- Medium temperature range, including normal operating temperature and temperature fluctuations at start and stop
- Dust concentration and particle hardness-this directly determines whether to thicken or liner
- Is there any risk of acid dew point corrosion? What is the content of chloride ions and sulfide in flue gas components
- Whether there is a risk of vehicle collision at the site, such as whether the protective cover is installed next to the passage will be touched by the forklift
- Whether frequent maintenance and disassembly is required, decide whether to choose the split-flap structure
Only by giving these parameters to the manufacturer can people design a protective cover that truly fits the working conditions. Otherwise, what you buy back is a tin tube. Whether it can hold up to the scene environment can only depend on luck.
By the way, put together a selection checklist, and compare it when ordering: Does the material match the temperature and corrosiveness of the medium? Can the wall thickness withstand dust scour? Is the heat dissipation hole open correctly? Does the clearance and fixation take into account thermal expansion? Are bolts resistant to high temperatures? Is it inconvenient for the maintenance side to split or overall? These points have been confirmed, and basically there will be no stepping on pits.