How to join flue soft expansion joint? Hardcore how-to guide from selection to installation
The newly installed non-metallic expansion joint had not been running for three days, and the edge of the flange began to hiss and leak. When the maintenance team removed it, it saw that the fabric layer and flange were detached, and several bolts were loose. He asked me: How can the joint of flue soft expansion joint be reliable? This matter is big or small. If the joint is not good, the compensation function of the whole pipeline will be in vain. Today, we'll break it up and break it up.
1. Find out what the flue soft expansion joint is
Don't look at the long name, this thing is a non-metal expansion joint (also called fabric fiber expansion joint) to put it bluntly. The main body is laminated with flexible materials-silicone gel cloth, fluororubber cloth and ceramic fiber blanket laminated one by one, with metal flanges or clamp interfaces at both ends. Different from metal bellows, it is specially used in low-pressure and large-displacement scenes such as flue and air duct. Its temperature resistance can reach 600℃ or even higher, and its corrosion resistance is stronger than that of stainless steel. But it has a weakness: the installation accuracy requires high, and the joint is a little sloppy, leaking, delaminating and burning through in turn.
Therefore, before asking "How to join the flue soft expansion joint", first weigh which joint form you have in your hand-flange type, welding type, clamp type, each technique is quite different.
2. Hard threshold before joint: flange matching and bolt selection
The vast majority of non-metallic expansion joints are flanged connections, but many people turn screws when they come up, so it is strange that there are no problems.
First thing – check flange size and bolt hole spacing.The flange of soft expansion joint is divided into flat plate flange and angle steel flange, and the number of bolt holes must be tightly fitted with the pipe flange. One hole less? Don't think about reaming, that's damaging the structure. Rather let the manufacturer redo it. In addition, if the sealing surface of the pipe flange is a protruding surface (RF), the flange of the soft expansion joint usually has its own sealing surface, without additional gaskets; However, if the flange surface is corroded and welding slag accumulates on site, flexible graphite pad or ceramic fiber paper must be added.
The second thing – the material of the bolts.Flue gas contains sulfur? Choose 316L or duplex steel. Flue gas temperature exceeds 300℃? Ordinary carbon steel will rust and break in one or two months, so it must be high-temperature resistant alloy steel. The length of the bolt is also particular: after tightening, the screw exposes 2~3 wires of the nut. If it is too long, it will easily hang dust, and if it is too short, it will not stand the strength.
Sealing gaskets are not pictured to save trouble. There is a PTFE compensator in our station, which is used by customers for wet desulfurization export-PTFE lining inside and ceramic fiber paper outside flange, which has not leaked once after two years of operation. The spacer is selected correctly, and the joint is half successful.
3. Four practical points of joint construction
①Flange face cleaning.Welding slag, rust and oil must be shoveled clean. In a power plant project, the workers were lazy and didn't clear the welding slag. As soon as the machine was turned on, the smoke was sprayed out from the position where the welding slag pierced the sealing gasket, like a fire water gun. It is recommended to use an angle grinder and a hundred impeller for cleaning. Don't use a wire brush-there will be metal burrs after brushing.
②Bolt tightening sequence.Don't twist in a circle in one direction. To pre-tighten diagonally, step-by-step force in three passes: the first pass to 30% torque, the second pass to 60%, and the third pass to 100%. What is the value of torque? Depending on the torque range given by the manufacturer, usually M16 bolts are between 80~120 N·m.
③Pay attention to the direction of the guide tube.Some non-metallic expansion joints have guide tubes inside, and the arrows point to the direction of media flow when they leave the factory. Install backwards, the smoke directly washes the fabric layer, and it will wear out in three months. We encountered a cement factory that installed the guide tube of rectangular non-metallic expansion joint backwards, and as a result, the soft joint was changed three times in a month. Later, the rectangular expansion joint of this station was changed, and it was installed according to the arrow. It has been fine for three years now.
④Pre-stretched or pre-compressed.The axial length of the expansion joint may need to be adjusted depending on the site installation temperature and the design compensation amount. Pre-compressed for hot installation and pre-stretched for cold installation. How to calculate the specific compression amount? Go directly to the manufacturer to install the compression data, don't adjust it yourself. Our non-metallic expansion joints will be marked on the transport limit screw when they leave the factory, just adjust them.
4. Treatment of special joints
In addition to standard flanges, there are two kinds of joints, welded and clamped, in practice.
welding typeIt is suitable for scenarios where high temperature and high pressure or must be fully sealed, such as the outlet section of desulfurization flue gas baffle door. Continuous fillet welds should be used when welding, and the height of welding feet should not be less than 6mm to avoid edge biting. Do penetration test (PT) after welding to confirm that there are no cracks. Note: Welding heat may damage nearby fabric layers, isolate with asbestos cloth before welding.
If the pipe is rectangular, use rectangular non-metallic expansion joints. Reinforcing ribs should be set at the joints, and the spacing between bolts should be as dense as 100mm, otherwise the flange will deform and leak when the wind pressure is high. In this case, we generally recommend the rectangular non-metallic expansion joint of this site, with thickened angle steel flange and double row bolt holes.
5. Several pits where the joint is most prone to rollover (with solutions)
Pit 1: Bolts are loose.Temperature changes cause thermal expansion and contraction, and the nut must be tightened twice after one week of operation. Experienced teams will re-tighten 24 hours, 72 hours, and 168 hours each after the first drive. If conditions permit, add spring washers or loosening nuts.
Pit 2: The fabric layer is disengaged from the flange.Usually, the vulcanization process is not up to standard or the rivet spacing is too large. When installing, check whether the product leaves the factory with anti-detachment reinforcement bead-the non-metallic expansion joint of our station comes standard with stainless steel bead and self-tapping screws. If not, add stainless steel strips to drill and fix them yourself, with a spacing of 100mm in a row.
Pit 3: Infiltration of media into fabric interlayer.The flue gas carries water or acid, which can corrode the fiber and cause delaminated bulging. The solution is to install polytetrafluoroethylene (PTFE) anti-corrosion liners inside the non-metallic expansion joints. This station's PTFE-lined hose and PTFE compensator do this job, specially to deal with acid flue gas after wet desulfurization.
6. Checklist that must be done after joint completion
- Visual inspection —The flange clearance is uniform, the bolt exposes 2~3 filaments, and the fabric surface is free of bulge and wrinkle.
- Sound method —Pass low-pressure air (0.1~0.2MPa), and use a stethoscope or ear to stick a pipe to hear if there is any hissing air leakage sound.
- Soap water leak test or smoke test- -Most sensitive to tiny leaks, bubbles are leaks.
After confirming that there is no leak, the most critical step:Remove the transport limit screw of the expansion joint。 Many people forget this, and as a result, the expansion joint is welded to death by the screw, and it can't move at all, losing its compensation function. After removing the screw, mark the installation date and first tightening time on the edge of the flange.
Finally, if you are really unsure how to join the soft expansion joint of the flue, go directly to the technical support of our station with the flange drawing, and don't try it blindly on the spot. A joint overturned, and the whole construction period was delayed for at least three days. The accessories and labor costs were enough to eat several meals.