What exactly does the roaster metal expansion joint endure?
Don't hurry to ask for the price yet. The expansion joint in the roaster flue, which looks like a bellows, actually has to carry much more than an ordinary pipe compensator. The flue gas in the furnace contains sulfur and dust, and the temperature always fluctuates by seven or eight hundred degrees, which fluctuates greatly. The expansion joint not only compensates for thermal displacement, but also has to survive the double pinch attack of acid corrosion and dust abrasion.
I met a customer two days ago and asked me to quote a price when he came up. I asked him what the temperature of the flue gas was, what the dust content was, what the sulfide concentration was, how many directions of displacement was, and what the diameter of the pipe was, and he said, "Just give me a rough idea". How do you give this? The working parameters are incomplete, the quotation is empty talk, and the signing of the contract is also a mine.
In the selection of metal expansion joint of roasting furnace, the core is three words: temperature, medium and position. Temperature determines material grade, medium determines corrosion resistance strategy, and displacement determines structural form. If these three are not clear, the rest is all guessing.
How to choose the material of corrugated pipe? Don't just look at the upper temperature resistance limit
316L, Inconel 625, 254SMO, these are common bellows materials in roaster flue. Many people think that 316L has enough temperature resistance, but the problem is that the corrosion of 316L by sulfur-containing flue gas at high temperature is not a joke. Sulfide begins to react with chromium above 400℃, and the grain boundary is chromium-depleted, followed by high-temperature vulcanization corrosion. If you take a universal corrugated expansion joint hardtop, it is only a matter of time before it cracks.
Inconel 625 does play in high-temperature long-lasting strength and sulfur corrosion resistance, but the price is also impressive. 254SMO is good for chloride ions and sulfides, and is suitable for scenarios where chlorine and sulfur in the medium are not low. The key is not to choose the most expensive one, but to match the materials with the working conditions. In the same working condition, others used the high-temperature axial expansion joint for five years without any problems, but you used the general-purpose corrugated expansion joint for half a year and it leaked. It is not that you bought it cheaply, but that you chose the wrong type at all.
To put it bluntly, the material depends on the durable strength at high temperature, not just check a temperature resistance meter. The thinning amount of bellows after forming and the influence of welding process on corrosion resistance should also be considered. These data may not be given to you by the design institute, but the person who makes the selection has to know it well.
Structural design is the key
Single or multi-layer? Do you want a deflector? How big is the deflector clearance? These problems directly determine that expansion energy saving can't survive the first maintenance cycle.
Look at the corrugated expansion joints used in the power station industry of our station and the metal corrugated expansion joints in the cement industry, which are obviously different in structure. The flue gas of the power station is relatively clean, and the gap of the guide tube can be larger; The cement industry has heavy dust, so the wear-resistant design of the guide tube has to be strengthened. The working conditions of the roasting furnace are more severe, and non-standard design is often required-such as surfacing wear-resistant layer on the outer wall of the guide tube, or changing the bellows waveform into a multi-layer thin-walled structure to reduce the stress of the single layer.
Some users say, can I buy a standard expansion joint? I can't do it. There is SO₂, dust and temperature fluctuation in the flue of the roaster. The standard product is for ordinary working conditions, and it is premature aging when you get it here. Non-standard is not cheating money, but saving lives.
Failure case review: The problem lies in the structure, not the material
Tell me a practical case. A roaster flue expansion joint leaked after less than half a year of use. After removing it, the dust accumulated in the trough of the bellows was dead, the guide tube was worn out, and the medium directly washed the bellows.
Did you say it was a material problem? The 316L material would have been enough at this temperature. The real reason is that the gap design of the guide tube is unreasonable, and the dusty airflow drills in from the gap, forming a vortex in the trough, and the dust is deposited, which is grinded while blocking. If the direction is not considered right, no matter how advanced the structure is, it will be useless.
Another common failure is chloride ion stress corrosion cracking. The direction of the crack is perpendicular to the direction of the ripple and can be recognized by the naked eye at a glance. Such cracks tend to occur at the crests or valleys of the bellows, where the residual stress is greatest. In case of this failure, it is useless to change the material by light, so it is necessary to reduce the stress concentration from the structure, or change the flow diversion mode, so that chloride ions have no chance to concentrate.
The pits of installation and operation and maintenance, hide if you can
Many accidents are not designed, they are faked. The small tie rod on the expansion joint, for transportation and installation, must be loosened after installation. Forgot Pine? As soon as the pipeline heats up, the expansion joint can't move, and the stress is all held on the bellows, so it is over when it is broken.
The same is true during the pressure test. Do you want to install a restraint device? Gotta pretend. If it is not installed, the blind plate force during pressing may directly break the expansion joint. There is also the amount of pre-deformation, which must be adjusted according to the actual ambient temperature before installation. If you follow this checklist, you can avoid 80% of human failures.
During the inspection, don't just look at the leakage, and see if the guide tube is abnormally worn and whether there is dust accumulation in the trough. If it is possible, scan it with an infrared thermometer gun. The area with abnormal temperature often has problems with the internal structure.
Selection Process: Five Steps, One Step
- The first step is the collection of working condition parameters.Temperature range, pressure, media composition (especially sulfur and chlorine content), dust concentration and particle size, amount of displacement in three directions, number of cycles, pipe diameter and wall thickness. Ask the technology major for these data, or measure them if they don't get them.
- The second step is the primary selection of materials.According to the temperature and medium, the material of bellows and guide tube is preliminarily determined. Don't just look at temperature resistance, but also at high temperature durability and corrosion resistance. Do a media corrosion test when you are unsure.
- The third step is structural design.Determine whether single layer or multi-layer, waveform, wavenumber, guide tube structure and clearance, and whether there is wear bushing. Here, consider the direction of dusty airflow, so that dust has no chance of entering the bellows.
- The fourth step is fatigue life check.According to the actual number of displacement cycles and temperature, the fatigue life of bellows is checked by EJMA standard or equivalent method. If the life is not enough, add the wavenumber, change the waveform and change the material until the design life is met.
- The fifth step is factory inspection.Including hydraulic test, air tightness test, stiffness test and displacement performance test. Conditionally, do a fatigue test. Although it is a little more expensive, it is better than an accident on the spot.
What data you want at each step and who to ask for it is clearly written above. If you follow this process, the expansion joint can be used for at least the designed life, and it will not leak every three days.
There is no room for copying the metal expansion joint of the roaster. Working conditions vary widely, and copying other people's models is a bet on luck. Make the working condition data accurate, make the structural design right, and leave the rest to time to verify.