1. Find out first: What exactly are non-metallic expansion joints?
To put it bluntly, a non-metallic expansion joint (also called a non-metallic compensator, or fabric fiber expansion joint) is a "soft joint" in a pipeline system. Its main body is not a metal bellows, but a layer of flexible composite material-usually made of fluororubber, silicone, polytetrafluoroethylene, glass fiber cloth, ceramic fiber felt, and wrapped with a layer of metal mesh or sheath. Internally, most of them have a guide tube, and the function of the guide tube is to guide the airflow to the center to avoid the high-speed medium directly washing the flexible ring belt.
Compared with metal expansion joints, what is the essential difference? The metal expansion joint absorbs the displacement by the deformation of the bellows, has high pressure resistance but small compensation amount, and is afraid of corrosion and stress concentration. The non-metallic expansion joint is just the opposite: it can absorb large displacements in three dimensions (axial, transverse and angular), and it is corrosion-resistant and high temperature-resistant (some can reach above 1000℃), and most importantly-cheap, lightweight and simple to install. Therefore, the typical scenes are the flue duct of power plant, desulfurization system, flue gas pipeline at the tail of cement plant and blast furnace gas pipeline of iron and steel plant. You flip through this siteRectangular non-metallic expansion jointAndNon-metallic expansion joint (fabric fiber expansion joint)Product pages are basically used in these places.
2. Don't look at these before installation, wait for rework later
How to get a seat in the model selection?
Media temperature, media composition, pipe displacement direction. The temperature determines the material of the ring belt-fluororubber is enough for ordinary flue gas, and acid-resistant polytetrafluoroethylene is needed for desulfurization wet flue gas; The displacement direction determines the structure form-pure axial displacement with single wave, large transverse displacement to obtain the upper complex structure. It may be more intuitive to press a table, but I won't go into details here. Remember one thing: if the selection parameters deviate by 10%, the life may be discounted by 50%.
Which items to check on arrival?
Don't rush to the scene after unpacking. First, check the goods item by item according to JB/T 12235-2015 (National Standard for Non-metallic Expansion Joints): Is there any bulging, delamination and scratches in the appearance? The flatness error of flange surface should not exceed 2mm/m. Pay special attention to the correct direction of the guide tube-the guide tube is generally marked with arrows, and the direction of the arrows must be consistent with the flow direction of the medium. This arrow is not a decoration. It is installed backwards and the high-speed airflow directly blows the inside of the ring belt, and it will wear out in half a year. In addition, the tie rod nut pre-compressed from the factory, don't cheap to loosen it. Many masters feel that "the factory press is too tight, so I can loosen it and install it". As a result, as soon as it is loosened, the ring belt is reset elastically. After installing the flange, it is found that the length is wrong, and it is hard pulled, and finally it leaks. Remember: Factory pre-compression is calculated by the manufacturer according to the displacement, keep it as it is before installation, and adjust the tie rod nut according to the instructions after installing the system (Q&A on this siteHow to adjust the tie rod nut of expansion jointWith detailed instructions).
Which clauses are red lines in JB/T 12235-2015?
First, the flange connecting bolts must be tightened with a torque wrench, not by hand feel. The standard specifies the tightening torque value of different specifications of bolts (for example, M20 bolts, the torque is about 200-250N·m). Article 2, direct welding or cutting on the loop belt is not allowed during installation, and must be isolated with asbestos cloth first. Article 3. During the hydraulic test of the system, the non-metallic expansion joint must be provided with temporary support to prevent the hydraulic pressure from exploding the ring belt.
3. Upper flange, screw bolt, adjust pull rod-the installation steps are disassembled and discussed
Step 1: Flange butting
First align the flange of the non-metallic expansion joint with the pipe flange. Sealing gaskets must be installed between the flange surfaces, usually with asbestos rubber sheets or PTFE pads. The bolt holes should be aligned, don't knock hard. If the misalignment exceeds 3mm, it is more cost-effective to change the flange interface than to plug the air leakage in the later stage.
Step 2: Sequence of screwing bolts
Cross-crossing method, tighten in two to three times. Screw to 50% torque the first time, 80% the second time and 100% the third time. Don't screw it in one direction, it will cause the flange to warp and deform. What if it's deformed? The local clearance of the flange is large, and the gasket is not compressed firmly, which will inevitably leak after being put into operation.
Step 3: Adjustment of tie rod nut
This is the most confusing part. The role of the tie rod is to limit the expansion joint from excessive stretching or compression during transportation and installation, while bearing the system blind plate force. After installation, the tie rod nut should be loosened to the length that meets the displacement requirements (refer to the instructions for details). However, there is a situation: if the pipeline system itself has a fixed bracket, and the expansion joint only compensates for the thermal displacement, then the tie rod nut should be loosened to the position corresponding to the maximum displacement; If the expansion joint also has to bear axial thrust (for example, without a fixed bracket), the tie rod cannot be completely loosened, and it needs to be adjusted to the pre-tension force specified by the design. How to tune it? Q&A on this siteFunction of expansion joint tie rodIt is very clear: adjusting the pull rod is to adjust the pre-deformation amount. When leaving the factory, the manufacturer has marked the cold and hot positions, so just adjust it according to the marks.
Let's talk about the "how many people have been cheated by factory pre-compression"-many customers saw that the goods were flat when they received them, thinking that they were broken, so they loosened them first. As a result, it was much looser. After installing it, it was found that the loop belt was elongated and the flange was not aligned. In fact, when non-metallic expansion joints leave the factory, they are compressed to the minimum transportation size. After arriving at the site, the packaging fixtures are removed first, and then the tie rod is adjusted to the installation length. This length is "L0" marked by the manufacturer, so don't make your own decision.
4. It leaked after half a year of use? Don't hurry to scold the manufacturer, first check yourself against these common faults
Two days ago, I met a customer, saying that the non-metallic expansion joint I bought began to smoke after half a year, and I was so angry that I wanted to return it. I asked him to take a picture and send it, and it was clear at a glance: circle with bulge. There are only three reasons for bulging-either the medium temperature exceeds the design value (for example, the design is 200℃, but the actual run is 350℃), or the direction of the guide tube is reversed, which causes the airflow to directly scour the inner wall of the ring belt, or the bolt preloading force is insufficient, which causes the flange surface to leak and then string into the ring belt layers. As a result, the manufacturer sent someone to measure the temperature on the spot, and found that the instantaneous temperature of the boiler rushed to 400℃ when it started and stopped, and the silica gel layer of the ring belt was directly carbonized. This pot manufacturer does not carry it.
Flange deformation? Check for violent screwing of the bolts during installation. Loose bolts? It is mostly due to the creep of the bolts in the hot state that leads to the decrease of the preloading force, and the preloading force is forgotten to be re-tightened after being 80% of these faults are buried by the installation details. So don't curse when something goes wrong, check it first by this list:
- Is there any bulge, crack or delamination on the surface of the ring belt?
- Are there any signs of leakage at the flange joint?
- Are the bolts loose? Use a torque wrench to check if the torque has dropped.
- Is the tie rod nut shifted? Are there any abnormal friction marks?
If nothing is wrong, then see if the pipe support has settled or deformed-in many cases, the expansion joint itself is not broken, but the displacement of the pipe exceeds the design value and it is torn.
5. Maintenance really doesn't have to be watched every day, but don't forget the two time nodes
Re-tighten bolts on day 7 after commissioning
Why Day 7? Because after one week of hot operation, both flange gaskets and bolts will creep (especially in high temperature conditions), the preload force will decrease by 30%-50%. At this time, tighten again, and the torque returns to the original value, which can avoid subsequent loosening and air leakage. If you forget, by the time you find a leak, the band may have been partially worn out.
Quarterly Appearance Tours
To put it bluntly, it is to walk around and see if there is anything abnormal. Focus on whether the ring belt is discolored or cracked, whether the flange bolts are corroded, and whether the guide tube falls off and sounds abnormal. Simple right? However, many factories don't look at it for a year, and only when they shut down for maintenance do they find that the ring belt has been worn out.
Replacement cycle? There is a lot of difference between the flue gas duct and the air duct, so don't one size fits all
The life of non-metallic expansion joints depends on the working conditions. Clean hot air ducts, no problem for five or six years. However, the desulfurization flue gas pipeline contains acid, humidity and particulate matter, and the aging rate of the ring belt is much faster-it has to be replaced in two or three years. If your home is a flue gas pipeline at the tail of the kiln of a cement plant, it is normal for the temperature to fluctuate greatly, the dust content to be high, and even to change it once a year. There is only one criterion to judge: when there are visible cracks or bulges on the surface of the ring belt, it is time to prepare for replacement. Don't hold it hard, hold it until it leaks and then change it, which is more costly.
Okay, that's all for this non-metallic expansion joint instruction manual. The core is: don't be sloppy in type selection, install according to the steps, tighten again after operation, and inspect daily. By doing these four things, the non-metallic expansion joint will at least serve safely until the moment you forget it exists.