What exactly is a metal full height expansion joint?
Don't rush to make it too complicated yet. Metal full-height expansion joint, the core is one word: high. The "height" here does not refer to the overall height of the expansion joint, but to the wave height of the bellows. A larger wave height means a larger amount of displacement each ripple can absorb. For example, ordinary corrugated expansion joints are like running in small steps, moving a little at a time; The metal full-height expansion joint is a big step forward, specially to clean up those hard bones with large displacements.
This expansion joint is essentially a subdivision type of bellows compensator, but it has a clear division of labor with ordinary corrugated expansion joints: the ordinary model is suitable for medium-low pressure and small-to-medium displacement scenarios; The metal full height expansion joint is specially designed for the working conditions of low pressure, large displacement, high temperature and large thermal displacement. It can be regarded as a "flexible connector" in the pipeline system. In places such as boiler flue duct, cement kiln tail and desulfurization system, the thermal expansion of the pipeline is often tens or even hundreds of millimeters. If the ordinary compensator is hard to carry, the bellows will be fatigued and cracked after several heat cycles.
Which occasions must it be used?
The flue duct of power station, the flue gas system at the tail of cement kiln, and the flue at the inlet and outlet of desulfurization tower-these places have in common: high temperature, large pipe diameter and considerable thermal displacement.
The temperature of the smoke air duct of the power station boiler is often above 300℃ during operation. Coupled with the vibration of the fan and the displacement caused by the self-weight of the pipeline, the ordinary expansion joint often leaks in less than one maintenance cycle. This is especially true for the flue gas pipeline at the tail of cement kiln, which has high temperature and contains dust particles, which requires extremely high wear resistance and high temperature resistance of corrugated pipe. There is also a desulfurization system. Although the flue gas temperature is relatively low, the medium is corrosive and the flue section is large, so the amount of compensation required is not small.
Under these working conditions, the metal full-height expansion joint can absorb the same total displacement with less wavenumber by virtue of larger single-wave compensation, which not only reduces the length of the expansion joint itself, but also reduces the overall stiffness, and the pipeline is less stressed. The corrugated expansion joints for power station industry, metal corrugated expansion joints for cement industry and desulfurization flue gas baffle doors of this station are facing these practical engineering problems.
Which parameters should I keep an eye on when selecting?
Choose metal full-height expansion joints, don't just look at wave height. Nominal diameter, design pressure, design temperature, compensation amount and fatigue life, none of these five items can be leaked.
Let's start with the path. The diameter determines whether the expansion joint can match the pipe diameter, which is the most basic. Don't worry too much about the design pressure-the metal full-height expansion joint itself is more suitable for low-pressure scenarios, but "low pressure" does not mean "no pressure". The design pressure must be greater than the actual working pressure, leaving a safety margin.
The design temperature and compensation amount are a pair of combinations. The higher the temperature, the lower the permissible stress of the material, and the shorter the fatigue life under the same displacement. If the actual displacement exceeds the rated compensation amount of the expansion joint, the bellows will soon generate stress concentration at the trough and form cracks. Here are two more details that are easily overlooked: the stiffness and the deflector. The stiffness determines how much thrust the expansion joint exerts on the pipeline support. If the stiffness is too large during type selection, the pipeline support has to be additionally reinforced; The guide tube is responsible for reducing the erosion and wear of the bellows caused by high-speed airflow. When selecting the model, be sure to confirm whether it is in place.
All the parameters are checked before you dare to install the expansion joint on the pipe. Otherwise, when the equipment runs and the displacement is not enough, it will not just be a problem of removing and replacing it, but the whole pipe system will have to be reworked.
The easiest mistakes to make when installing
When the equipment arrives at the site, the installation link is the real test. I've seen too many cases of expansion joints being scrapped early because of installation errors.
The most common mistake is improper handling of tie rods and screws. Metal full-height expansion joints usually leave the factory with tie rods or screws, which are used to protect the bellows from excessive deformation due to its own weight or external force during transportation and hoisting. After installation in place, the tie rod nut needs to be adjusted according to the design requirements-what should be loosened must be loosened, and the screw that should be removed must be removed. Some construction teams try to save trouble and leave the tie rod and screw on them as they are. As a result, the bellows are locked tightly, and they can't expand and contract freely at all, and the ability to compensate for displacement is completely offset. More seriously, when the pipeline is running, the thermal displacement is too large, and a huge local stress is generated between the tie rod nut and the pipe wall, which twists the bellows into a "twist".
Another high frequency error is the pointing arrow loaded backwards. The expansion joint cylinder is usually marked with the media flow direction arrow, which determines the correct installation direction of the guide cylinder. When the arrow is installed backwards, the deflector loses its role in protecting the bellows from airflow impact and dust wear. Especially the smoke air duct with high dust content, the guide tube is useless after reverse installation, and the bellows is directly exposed to high-speed dust air flow, which may wear out within a few months. How to adjust the tie rod nut, whether the screw is removed or not, the adjustment of the tie rod nut of the expansion joint and the screw of the expansion joint in this site need to be removed are described in detail in these two questions and answers. It is best to turn it out before installation.
What if there is a leak after a few years of use?
If the expansion joint leaks after a long time, don't rush to the conclusion that the product quality is not good. The troubleshooting ideas should be in order.
The first step is to see if the bellows has corrosion or intergranular cracks. If the medium contains chloride ion or sulfide, stainless steel bellows is prone to stress corrosion cracking. This kind of crack is often very fine and may not be clearly seen by the naked eye, so it needs to be detected by permeation testing. The second step is to check whether the pre-deformation during installation is correct. The metal full-height expansion joint usually needs to be pre-stretched or pre-compressed according to the actual temperature of the pipeline during installation. If the pre-deformation amount is not calculated correctly, the actual displacement of the bellows after operation will deviate from the design value, and the fatigue life will be greatly shortened.
Instead of waiting until the leak is repaired, it is better to avoid it in advance during the selection and manufacturing stages. In terms of material selection, choose the right stainless steel grade according to the corrosiveness of the medium, and even consider corrosion-resistant alloys; In the manufacturing process, the forming mode of bellows, the fillet radius of wave trough and the welding process will affect the fatigue life. It's like buying shoes and trying them on feels tight. It's not the shoes, it's your mismeasured foot size. Taking these factors into account in advance is much more reliable than plugging leaks afterwards.
After careful calculation, the metal full-height expansion joint is not mysterious, it is born for large displacement and low pressure conditions. The key is to understand the selection parameters thoroughly, put the installation details in place, and pay attention to monitoring when reusing. If you are worried about the pipeline compensation scheme for power stations, cement or desulfurization projects, you might as well compare the above ideas to see if you have stepped into some pits.