1. Where exactly does the arrow on the expansion joint point? — — First understand the definition of "positive"
Many people got the expansion joint at the scene, and their first reaction was to find the arrow. The direction of the arrow is indeed the key to the installation, but many people take it for granted that the arrow points in the direction of the media flow. Actually, it's not that simple. Take our common general-purpose corrugated expansion joint and high-temperature axial expansion joint, for example, the arrow points to the flow direction of the medium, but more importantly-it is also the opening direction of the guide tube. The function of the guide tube is to protect the bellows from the erosion of high-speed fluid. Once installed backwards, the guide tube becomes a "baffle", which generates vortex noise at least and directly rushes through the bellows at worst. Two days ago, I met a power plant customer. They installed a high-temperature axial expansion joint backwards, and it leaked after less than three months of operation. When I opened it, the bellows was flushed out of a hole, and a pile of welding slag accumulated at the tail of the guide tube. You say it was wrong or not?
2. What moths will happen when "dressing backwards"?
Installing it backwards is not just a problem with the deflector. There are also clear directional requirements for the adjustment of tie rod nuts for tie rod expansion joints (such as double hinge transverse expansion joints or straight tube pressure balance expansion joints). The tie rod is used to restrain the displacement of the bellows in a certain direction. If it is installed backwards, the force beared by the tie rod will not be on the designed path, which will cause the bellows to twist at least and the tie rod bolts to break at worst. Another point: some expansion joints (such as external pressure single axial expansion joints) have protective sleeves on the outside of the bellows. If they are installed backwards, they will cause friction between the protective sleeve and the pipe, which will wear out over time. Alas, this kind of problem is particularly common in the cement industry and power station industry. On-site workers save trouble and install it directly without looking at the drawings. As a result, a bunch of hidden dangers are found during the annual inspection.
3. Don't be misled about those expansion joints that don't distinguish between pros and cons
Are all expansion joints divided into positive and negative? Not. For example, the directly buried (fully buried) expansion joint is designed with bi-directional compensation in mind, and the guide tube is symmetrical, so it has no impact if it is installed backwards. There are also some non-metallic expansion joints (such as fabric fiber expansion joints and rectangular non-metallic expansion joints), which are made of flexible materials in the middle and have no guide tube, so there is no positive or negative. However, note that although the non-metallic expansion joint does not divide the flow direction, it has the difference between the upper and lower mounting surfaces-there is usually a nameplate mark on it and a drainage hole on the bottom. If you install the drainage hole upward, wait for the water to come in on rainy days. So don't pretend as soon as you hear "no difference between pros and cons". Every product has its own temper.
4. How to quickly judge the direction during on-site installation?
Here are some practical tricks for you. First, look at the position of the guide tube. Stand up the expansion joint and look from one end to the other. You can see the side of the guide tube shrunk inside, which is the direction of the medium flowing in. Second, look at the adjustment nut of the tie rod. Generally, the adjustment nut on the pull rod is facing outward, which is convenient for later adjustment of displacement. If you find that the nut is snug against the flange surface, it is mostly installed backwards. Third, look at the nameplate – all of our products leave the factory with flow direction arrows placed prominently and with installation instructions. It's simple to say, but many installation teams don't read the instructions at all and do it as soon as they come up. This is why we have always advised customers to confirm the flow direction when specifying "expansion joint with guide tube" when purchasing, and don't wait for it to be installed before regretting it.
5. An old-fashioned but always overlooked problem: pre-stretching vs. pre-compression
The "anyway" of the expansion joint is not only the direction, but also the pre-deformation before installation. For example, the expansion joint on the steam pipe should be pre-stretched or pre-compressed according to the ambient temperature when installed. You install it backwards, and the amount of compensation is wasted. For example: northern winter installation, pipe temperature is low, expansion joints should be pre-stretched; Summer outfits are the opposite. If workers install it directly regardless of the season, after the pipeline heats up, the bellows may be crushed to death, and the life span will be greatly reduced. This is particularly obvious in the corrugated expansion joints used in the power station industry and the metal corrugated expansion joints in the cement industry. When you have time, flip through our FAQ. There is a "Does the screw of the expansion joint need to be disassembled?" It talks about the adjustment method of installing the tie rod, which is directly linked to "anyway".
6. Summary: Do these three things before pretending to save 90% of the trouble
First, after unpacking, read the nameplate and arrows clearly, and don't rush to hoist them. Second, compare the pipeline drawings to confirm the flow direction of the medium, especially in high temperature and high pressure situations. Third, check whether the guide tube is centered and whether the tie rod nut is loose. If site conditions permit, it is best to do an air pressure test or at least a visual inspection. In the final analysis, the expansion joint can last for ten years if installed correctly, but it will be scrapped in three months if installed backwards. Which do you choose?