Non-metal expansion joint sewing, what exactly is sewn?
Non-metal expansion joint sewing, sewing is not cloth, but life. The life of a loop belt, the sealing performance of a compensator, and the safe operation of the whole flue or air duct all depend on whether these stitches can carry it. This is not an exaggeration. After doing this for a long time, you will know that eight out of ten problems on the scene are sewn on.
Find out the difference between non-metal expansion joints and metal expansion joints first. Metal expansion joint absorbs displacement by corrugation deformation, while stainless steel corrugation has fatigue limit no matter how good it is. Non-metallic expansion joints (fabric fiber expansion joints) are not the same as rectangular non-metallic expansion joints-they are compensated by the flexibility of the fabric loop band. The loop belt is not formed at one time, and the whole loop is stitched out by multiple layers of fabric composite. This sewing process directly determines whether the compensator can hold up in high-temperature flue gas and corrosive medium. If the sewing is not good, the medium will directly penetrate the needle eye and leak out, and no matter how good the base cloth is, it will be in vain.
Material selection: You can't sew it just by finding a piece of cloth
Select materials before sewing. Some people think that non-metallic expansion joints are "cloth work", just take a piece of fiberglass cloth and sew it in a circle-that's layman's language. The actual materials are glass fiber cloth, ceramic fiber cloth, polytetrafluoroethylene coated cloth and silica gel coated cloth, which are pressed together according to the structure of a fireproof layer, a heat insulation layer and a sealing layer, and then sewn again. The core basis of material selection is the medium temperature and corrosivity. For example, the non-metallic expansion joint used behind the desulfurization flue gas baffle door contains sulfuric acid dew point corrosive medium and chloride ions in the flue gas, so the base cloth must be acid and chlorine resistant, and the PTFE coating layer must be thick enough. This link is wrong, and it won't take long to leak. At that time, the money spent on shutting down the furnace for maintenance will be enough to buy dozens of expansion joints.
The selection of materials also depends on the temperature gradient. When the temperature does not exceed 200℃, silica gel coated cloth is sufficient; Above 400℃, ceramic fiber cloth is the main force; Further up, you have to rely on multi-layer insulation structure to pile up the thermal resistance. Don't expect one layer of cloth to take all conditions. The design of non-metallic expansion joints is a "sandwich project" in itself.
Sewing process: Stitch length, tension, corner, all details
The sewing process is not as simple as imagined. First cut the piece according to the drawing, and the corner seams of the rectangular circle belt must be staggered-the four seams are stacked in the same position, which is the stress concentration point, and it will crack after several vibrations. Then there is stitching, where layers of fabric are stitched with high-temperature sewing thread (usually aramid thread or fiberglass thread), and the needle length, presser foot pressure, and sewing tension have to be adjusted. Stitch length is too thin and not strong enough; Too dense, too many needle eyes will leak air; The tension is too high, the fabric is wrinkled, the surface of the loop band is uneven, and the local wear is aggravated after installation. These things, the machine can't adjust well, all depends on the master's feel and experience.
After sewing, you have to do sealing treatment. The common practice is to cover a layer of fluorine rubber or silicone tape and sew another stitch along the original stitch to cover the needle eye. Needle eye leakage is a problem that many customers ignore-the material of the loop itself is fine, but the place where each needle passes is a tiny leakage channel. It is not obvious under negative pressure conditions. If you run in the positive pressure flue for a few days, dust and leakage will accumulate at the needle eye, and serious direct spraying will occur. Don't underestimate this covering seam, it is sealed bottoms.
How to check the quality of sewing? Two hard indicators
Strength and airtightness. The strength depends on tensile test, and the sampled tensile breaking force should meet the requirements of sewing strength of loop belt in JB/T 12235-2015 "Non-metallic Expansion Joint". Air tightness depends on applying soapy water or ammonia to detect leaks, and see if there are bubbles popping up at the stitches. Apply soapy water this trick is soil but effective, sewing whether there is any leakage, one application is accurate. In addition, check whether the seam edge is straight, whether there is a jump needle broken thread, and whether the raw edge has been treated cleanly. Our workshop has an inspection table, and each product keeps a sample record, just to avoid bringing sewing defects to the site-no matter how good the drawings are, the stitches will still be wasted if they are crooked.
On-site failure often lies in the seam
I've seen quite a few cases of on-site failure, the three most common.
The first is stitch wear. Under the vibration condition, the loop band keeps buckling, and the stitching thread rubs with the fabric repeatedly. After a long time, the stitching thread is worn off first, and then the whole layer is peeled off. This situation is especially common in the air duct at the outlet of the fan.
The second is that the temperature exceeds the limit and causes the suture to carbonize. During the selection, the medium temperature was calculated to be 450℃, and when the actual operation reached 550℃, the aramid wire became brittle and powdered directly, and the loop belt cracked in layers. When you look at the failed parts, the stitch is the first to break, because the temperature resistance rating of the stitch is often half a gear lower than that of the base fabric, which is the dead spot that is easiest to overlook when designing.
The third is chemical corrosion that rotts the base cloth. The concentration of chloride ions in the desulfurization system is high, the strength of glass fiber cloth drops sharply after erosion, and the stress at the suture is concentrated, which takes the lead in tearing. Therefore, when selecting the type, we should not only look at the displacement, but also check the medium temperature and corrosion composition. If necessary, thicken the heat insulation layer or use PTFE coating cloth instead. Don't wait for it to explode before replacing it. Once the corrosive medium in the flue gas leaks to the outside, the whole surrounding equipment and pipelines will be burned out.
Then how to pick a reliable sewing supplier? Ask three questions
What material is used for stitching? What is the stitch length? Is there a record of airtightness testing? Those who can't answer it are basically unprofessional. The suture material determines the upper limit of temperature resistance, the needle length determines the balance between strength and seal, and the airtightness record directly reflects the factory inspection ability-no record indicates that the product has not been inspected piece by piece with a high probability.
It depends on whether the product has been subjected to pressure or negative pressure test, especially the rectangular non-metallic expansion joint used for flue and air duct. Under the negative pressure condition, the ring belt is recessed inward, and the stress direction of the suture is completely different from that of positive pressure. Products that have not been verified by negative pressure are gambling.
Don't just try to be cheap. Sewing loops is a consumable, but it is also a wear part. The cost of reworking it once far exceeds the difference saved. Downtime loss, labor replacement, peripheral equipment damage-just calculate the general ledger and know that the extra money spent by reliable suppliers is to buy insurance, not to pay tuition.