Find out first: the assembly logic of non-metallic expansion joints and metal expansion joints is fundamentally different
Many people come into contact with the assembly of non-metallic expansion joints for the first time, and their minds are still all about the set of metal bellows: flanges, welding rings, screwing bolts, and it's done. As a result, I was dumbfounded when I got the drawing of the rectangular non-metallic expansion joint-why are there no ripples? How is it a layer of cloth?
Non-metallic expansion joints (also called fabric fiber expansion joints) rely on flexible skin (loop belt) to absorb thermal displacement and vibration. The main body is a structure of metal frame + multi-layer fiber fabric pressing. The metal expansion joint is deformed by corrugation, and it will be fine if it is welded to death; The non-metallic expansion joint presses the skin against the flange by the pressure plate, and if it is not well pressed, it will leak. Do you think this can be the same?
Once I went to the site, the workers came to do non-metal work with the installation specifications of metal expansion joints, and the bolts were all tightened according to metal standards. As a result, on the first day of smoke ventilation, the edge of the skin was directly torn. What's the reason? The fibers were crushed. Non-metallic skin is a flexible body, and pressing too hard is equivalent to cutting cloth with a knife.
Don't start work if you don't check the three things before you start
First, working condition parameters.Temperature, pressure, media corrosiveness, these three numbers determine whether you are holding the right skin in your hand. Don't think that the expansion joint of the same model can be used universally. It is also non-metal, and the layup structure with a temperature resistance of 120 degrees and a temperature resistance of 600 degrees is completely different. Ceramic fiber layer, polytetrafluoroethylene membrane, fluororubber lining, one layer difference in the number of layers, double the price difference.
Second, the interface size.It's not just measuring the outer diameter of the flange. The diameter of the center circle of the bolt hole, the number of holes, the diameter of the hole and the thickness of the flange, if one number does not match, the angle grinder has to be used to expand the hole on the spot. The most afraid thing is that the diagonal hole distance is 2mm different, and the flange is unevenly stressed when installed, so it will be crooked if it doesn't run.
Third, the laying order of the fabric fiber layers.This one is the easiest to overlook, but it is precisely the core. From the inside to the outside, they are: stainless steel wire mesh (wear-resistant), glass fiber cloth, ceramic fiber felt, polytetrafluoroethylene film, and outer sheath (silicone rubber glass fiber cloth or fluororubber cloth). The layup is messy, the thermal insulation effect plummets, and the outer surface is directly hot. If you don't have a layup order label on the skin you get, don't guess, call the manufacturer and ask for a process list.
Core Assembly Steps: Pits in Each Step
Frame welding
The frame of rectangular non-metallic expansion joint is generally welded with angle steel or channel steel. Fillet welds must be continuously fully welded and cannot be spot welded. And guess what? Some masters try to save trouble, leave a few spot welding joints and install skin on them. After three months of operation, the weld cracked and the whole expansion joint was scrapped. To be smoothed after welding, the welding nodules must be cleared-the skin presses against the raised welding nodules, and nine out of ten are worn.
Skin laying and pressing
Before the skin is laid on, check the direction of the guide tube. The guide tube should flow towards the medium. If it is installed backwards, the airflow will directly wash the inner layer of the skin, and the life will be cut by more than half. The skin should be laid naturally flat, not tight or wrinkled. Wrinkles are like creases on clothes. When thermal expansion and contraction occur, the stress is concentrated in that position, and it is there that cracks first.
When the pressure plate is pressed tightly, the bolt pressesTwist diagonally in three passes: The first pass is pre-tightened to 30%, the second pass is 70%, and the third pass is in place. Don't use the air trigger to hit the end in one breath, the skin is unevenly stressed and waiting for leakage.
Bolt torque
M12 bolts, recommended torque 45-55N·m; M16, 90-110 N m. Don't think you can twist a few more turns if you are strong, use the torque wrench when you need to. One more twist, the skin crushes; Less twist half a turn, the air leak didn't run.
Five of the most common assembly mistakes in the field
- The bolt is tightened too tightly and leaks.As mentioned earlier, the fiber loses its elasticity after crushing, and the gap is wider.
- The folds of the skin were uneven.After installing it, it looked almost the same, and the folds of one hot and one cold cracked first.
- Install the guide tube in reverse direction.It is easy to install but difficult to find, and the high-temperature smoke is finished as soon as it is washed.
- Missing insulation fill layer.Some designs fill thermal insulation cotton between the inner and outer skin, forget to stuff it in, and the shell temperature directly exceeds the design value.
- The bolts are not tightened in diagonal order.One side is tight and the other is loose, the skin is torn askew, and the stress is all concentrated in the weak point.
How many of these five mistakes have you made? I interviewed several power plant maintenance monitors, and they said that the biggest headache is that the deflector tube is installed backwards-because it can't be seen without disassembling it.
How to check and accept after assembly? Teach you two tricks
Don't put on high-pressure gas as soon as it comes up. The test pressure of non-metallic expansion joints generally does not exceed 0.05MPa (1.5 times the design pressure, but the upper limit is 0.05MPa). If the pressure was too high, the skin would explode directly. That scene would not be good to watch.
First move: tightness check.Brush with soapy water on the pressed edge of the skin and flange, and see if there are bubbles after ventilation. Focus on checking around the bolt holes and at the corners of the platen – these are the two locations that are most overlooked. If there are bubbles, mark them, and tighten them again after pressure relief. Don't bring the screw bolts.
The second trick: visually inspect the compensation amount.Draw a reference line on the expansion joint, simulate the displacement of the pipe with a manual jack or guide chain, and see if the corrugation (fabric layer) can expand and contract freely. If it is stuck, it is mostly because the skin is too tightly installed or the guide tube interferes. At this time, rework is much less troublesome than problems after operation.
Post-maintenance: which can be replaced on site and which must be returned to the factory
The non-metallic expansion joint has an advantage – the skin can be replaced on site. As long as the frame is not deformed and the bolts are not rusty, loosen the pressure plate, tear off the old skin, lay the new skin on it, and then tighten it according to the above torque requirements, and it's done. A set of process journeymen can do it in half a day, and the cost is only a fraction of the change of the whole expansion joint.
Frame weld cracking(On-site repair welding will lead to local deformation, and the sealing surface will be wasted),Corrosion perforation of guide tube(It needs to be disassembled and dry replaced, and the on-site process conditions cannot be met),Platen corrosion deformation(This thing looks inconspicuous, and it all relies on it to compress it when assembled, and the deformed pressure plate leaks no matter how it is adjusted).
Don't let the old master carry the cutting machine to the scene and shout "I'll weld it" to the frame-the frame and pipeline system of non-metallic expansion joints are rigidly connected, and the thermal stress is all on the weld. If the weld is not good, it will lay mines for safety. When it is time to return to the factory, it is time to return to the factory, and when it is time to scrap it, it is no shame.