A few days ago, a friend who is a steelmaker complained to me, saying that an expansion joint in their factory leaked after less than three months. When I removed it, I saw that good guy, the weld was cracked and the bellows was bulging. Only when I asked did I know that when I installed it, I didn't even know the working condition data and went up-isn't this digging a hole for myself! Today, I will dismantle six steps for you. Each step is actual combat experience. If you follow it, you can basically avoid 90% of the rework pits. Without further ado, let's go straight.
Step 1: Finding out the working condition data first and then selecting the type
Don't come up and ask the manufacturer, "Do you have converter vaporization cooling flue expansion joints for sale?", which is as unreliable as going to the pharmacy and saying, "Give me some medicine". You have to pull out your working condition data first: How high is the flue gas temperature? Are there alternate high temperature sections and low temperature sections? What is the pressure fluctuation range? Is there a big angle turn in the direction of the flue? Also, don't ignore the flow direction and flow rate of cooling water, which directly affects the calculation of compensation amount of expansion joint.
A steel mill only gave the design temperature when selecting the model, but in actual production, the converter occasionally blows over, and the temperature soars to 1200℃ instantly, and the expansion joint burns through directly. Why? Because there is no margin in the selection of materials. So, you have to take out the worst condition data-not the average, but the limit. Then take these data to find the manufacturer and ask them to produce a calculation book. Remember: the expansion joint is not the bigger the better, nor the thicker the better, the right one is the right one. If you are not sure for yourself, just ask, "How many cycles can the fatigue life of this model hold under our intermittent high temperature conditions?"-As soon as this sentence is thrown out, the manufacturer will know that you are not a layman.
Wind and earthquake loads also have to be taken into account, especially for elevated flues installed outdoors. Don't blow a six-level wind and twist the expansion joint into a twist.
Step 2: Arrival acceptance should be personally hands-on quantity-size, material
When the goods arrive, don't sign just by looking at the packaging intact. You have to take a tape measure or caliper to measure it yourself. A lot of rework is because of "a little bit of a difference in size" – 2mm difference? Sorry, if the flange doesn't match, you will have to re-machine the lathe on site, and the construction period will be delayed.
What exactly? First, the total length. Is the spacing of the interface flange consistent with the drawing? Second, the center distance and diagonal of the flange bolt hole. A difference of 1mm may not even be bolted through. Third, the wave pitch and wave height of the bellows. I usually take a vernier caliper to check the wall thickness of the bellows-some manufacturers will steal materials in this place, and the nominal 2.5mm is only 2.2mm.
How to check the material? Don't believe the certificate. Bring your own spectrum detector (just hold it in your hand) and hit the bellows and the receiver. Especially 304/316L and the like, you can know if it's impersonation after typing it. Two days ago, I met a customer who said that the stainless steel expansion joint they bought was found to contain not enough nickel-what should I do? Return the goods. After a few months of hard installation, it will corrode and perforate. That's called blood loss.
Step 3: Clean the site before installation and align the hoisting, completely completed after the mistake of 1mm
Have you cleaned up the installation site? Is there any welding slag, oil, or rubble on the ground? If these things are not clear, once the expansion joint bumps when hoisting, and a cut is made in the bellows, it will be wasted. More importantly: you have to make sure that the fixed and guide supports at both ends of the flue are welded and meet the strength requirements-the expansion joint cannot bear the flue weight, it is only responsible for compensating for displacement.
When hoisting, never use wire rope to directly strangle the bellows! I've seen people try to save trouble, and the rope is hanged in a pocket. As a result, the bellows is dented-the stress is concentrated in the later stage and it will crack. Use a nylon sling or cloth strap and tie it to the takeover section. Hoisting alignment is also critical: the expansion joint should be as concentric as possible with the flue. Can you imagine an expansion joint with a crooked suit that is stretched on one side and compressed on the other when working?
How to align it? Measure the perpendicularity of the flange surface with a horizontal ruler and a laser collimator. Deviation more than 1 mm? Then readjust the spacer or lug position. Don't think 1mm doesn't matter-when the flue heat expands, the amount of deflection will be amplified dozens of times, and the bellows will become laterally unstable for minutes.
Are you willing to spend two hours lifting and leveling, or would you rather spend two weeks replacing a scrapped expansion joint? Weigh it for yourself.
Step 4: Practical operation of welding and fastening-temperature, sequence and torque must be kept in mind
For the weld between the bellows and the joint, the interlayer temperature should be strictly controlled not to exceed 150℃. How to control it? The infrared temperature measuring gun measures while welding, and when the temperature is high, it stops and waits for the temperature to cool down. If you let the welder dry all the time, the heat input is too large, the gold phase of the bellows substrate changes, and the corrosion resistance is directly discounted.
The welding sequence is also particular. First weld the circumferential welds on both sides of the expansion joint, and then weld the longitudinal welds on the flue-this order cannot be reversed. Why? Because if the longitudinal seam is welded first, the flue has been deformed, and then the annular seam will not match, so it can only be deadpulled, and the stress is all held on the expansion joint. Is that the truth?
What about flange fastening? Don't screw the wrench to the bottom. With a torque wrench, gradually tighten to the design torque value in three times in diagonal order. The criteria I usually give are: up to 50% torque the first time, 80% the second time, and 100% the third time. If you are fast and tight at one time, the flange gasket will be pressed and biased, and the air will leak directly in the later period.
Bolts should be coated with anti-bite agent, especially under high temperature conditions, stainless steel bolts are easy to bite to death. If you don't paint it, you can't dismantle it next time you overhaul it, so you can only cut it by gas-tsk, it's another big pit.
Step 5: Water pressure test is not a formality, cold inspection should be listened, seen and touched
Many people think that pressure test is to press, as long as it doesn't explode. That's a naive idea. Converter vaporization cooling flue expansion joint pressure test you have to do two things: one is water passage leak test, and the other is cold displacement check.
It is qualified to hold 1.5 times the design pressure (usually 0.6-0.8MPa water pressure) for 30 minutes with a pressure drop of no more than 5%. But this is just going through the process, and the real checks come later. You have a person standing next to the expansion joint and listening with your ears – is there a subtle air leak sound of "fizzing"? Touch with your hands-are there any abnormal temperature points on the bellows surface? See with your eyes-are there any water droplets seeping out at the flange interface? These three tricks work better than any instrument.
In the state of no water flow, artificially push and pull the flue (with a jack or hand-pulled hoist), and observe whether the expansion joint can expand and contract freely, and whether it is stuck or skewed. If it can't be pushed, it means that there is a problem with the installation alignment, or the guide support is stuck, and it has to be dealt with immediately. Don't wait until it's in operation, it's already an accident by then.
And guess what? I encountered a case where everything was fine during the pressure test, but the cold displacement inspection found that the expansion joint had been pushed dead on one side by the flue support-it couldn't move at all. When disassembled, the bracket was welded in the wrong position, and the expansion joint had no compensation ability. If this inspection is not done, it will explode half an hour after it is put into operation.
Step 6: What do you inspect every week after it is put into operation? Abnormal noise, displacement and air leakage
After it is put into operation, everything will not be fine, and the expansion joint should also be "raised". Inspect at least once a week, focusing on three things:
- abnormal noise: When you get down to the expansion joint and listen, is there a "click" or "crunch" sound? If so, there is a high probability that the bellows is unstable or the ripples rub against each other. Stop the furnace and check it quickly.
- displacement: Draw a marking line on the flue next to the expansion joint, and measure the distance between the marking lines every week. If the displacement suddenly increases or decreases, it indicates that the thermal expansion of the flue is abnormal, which may be the loosening of the support or the internal slag.
- air leakage: Brush with soapy water on the weld and flange, blister is leak. Don't bother, it's better to brush for a minute than wait for the alarm system to go off.
In addition, check the bellows every quarter for pitting, cracking and bulging. If you find a bulge the size of a pigeon's egg-it means that there has been lamellar corrosion inside, and it must be replaced immediately, without negotiation. Don't think about "holding up again", it won't survive the next overhaul.
All right, six steps. From selection to inspection, there are pits in every step, but there are also countermeasures. As long as you follow these six steps, the converter vaporization cooling flue expansion joint can basically use the design life. If you find it troublesome, then it's okay-prepare the money and construction period for rework. Anyway, I don't feel distressed, hehe.