Starting from the pain points of working conditions: Why do ordinary metal expansion joints not last long in wear-resistant pipelines
Pneumatic conveying pipeline for cement industry, air-delivered fly ash, wind speed 25m/s, temperature around 120℃. When ordinary stainless steel corrugated expansion joints are installed, the bottom of the corrugated valley will be worn out in three months or half a year. Why is the position of wear always at the bottom of the ripple valley? Because the medium forms a vortex at the corrugation, the granular material repeatedly hits the trough at a certain angle, and the wall thickness becomes thinner after the corrugation itself is formed. Under the double blow, the metal wall can't bear it at all.
Many field engineers have encountered this situation-they have obviously chosen materials with high enough temperature resistance, and 316L or even 310S have been used. The temperature is fine, but abrasion has become the biggest killer. Increasing the wall thickness to 3mm and 4mm only drags the failure time from three months to eight months. The failure logic of abrasion condition is not temperature, but repeated scour of particle kinetic energy superposition. At this time, just relying on the thickened metal wall, the direction is wrong.
What about that? Add "armor" to the flow surface of the bellows. This is where the lined ceramic metal expansion joint comes in.
What exactly is lined with ceramics: Structural disassembly and material boundaries
Lined with ceramic metal expansion joints, from the structural point of view, the outer layer is still metal bellows, which bears flexible and pressure loads, and the inner layer has an extra circle of ceramic lined pipes. This liner is not glued on, but is inlaid or masonry on the inside of the bellows in a specific way to separate the high-temperature dusty medium from the metal wall of the bellows. The thickness of the ceramic layer is generally 10-25mm. Alumina ceramics or silicon carbide ceramics are commonly used, which have high hardness and good wear resistance. The temperature resistance is generally above 700℃, and it can even be carried to the beginning of 1000℃ instantly.
Structurally, attention should be paid to leave a reasonable void or flexible filling layer between the ceramic liner and the bellows. Why? The bellows should be expanded and contracted, and the ceramic sheet is rigid, so the two can't be hard connected, otherwise the ceramic will collapse as soon as the bellows moves. To put it bluntly, bellows are responsible for "moving", and ceramics are responsible for "carrying grinding", each doing their own work.
Where are the material boundaries? The Achilles heel of ceramics is poor impact resistance. It's okay for you to make it wear-resistant, but if there are large foreign objects in the line or a violent water hammer occurs in the system, the ceramic may chip and fall off. Therefore, the lined ceramic metal expansion joint is suitable for the occasions where abrasion is mainly and the impact can be controlled, but it is not suitable for the pipeline with serious mechanical impact.
What are the applicable scenarios for lining ceramics with the ratio of PTFE and non-metal expansion joints
In the product information of the station, PTFE-lined hoses and PTFE compensators are another technical route. The core advantage of PTFE materials is that they are resistant to strong acid and alkali corrosion, but the upper limit of temperature resistance is there-the long-term service temperature is usually-20℃ to 180℃. When you put the PTFE liner in the fly ash pipeline at 120℃, the temperature is barely within the range, but PTFE is not wear-resistant, and the particles will fluff and crack as soon as they are flushed, so they won't last long.
Non-metallic expansion joints (fabric fiber expansion joints) are widely used in large smoke ducts. They are high temperature resistant, corrosion resistant and have large compensation, but they have low structural strength and rely on skin to bear pressure. They are suitable for low pressure or micro positive pressure systems. Pneumatic conveying this positive pressure of 0.1-0.3MPa, the medium with hard particles, the non-metallic skin can't hold it, and it is a matter of time before it wears out.
-Strong corrosion + medium to low temperature → lined with PTFE/PTFE compensator
-Low pressure high temperature flue gas → non-metallic expansion joint
-High temperature + particle abrasion + certain pressure → lined with ceramic metal expansion joint
-Pure high temperature, no abrasion → ordinary high temperature axial expansion joint is enough
The cost of choosing the wrong type is far more than the money of replacing a spare part. The production loss of a single downtime is often orders of magnitude more expensive than the expansion joint itself.
Three details that are easiest to overlook when selecting a model: temperature, media, and installation direction
The first detail is temperature, but don't just look at the design temperature of the pipe. During the start-up and shutdown stage of the pneumatic conveying system, short-term high temperature peak may occur, such as clearing plugs and heating pipes when the system is just started, and the instantaneous temperature may rush above the design value. At this time, the high temperature limit and thermal shock stability of ceramic materials should be confirmed. Alumina ceramics have low thermal expansion coefficient and acceptable thermal shock resistance, but cracks may still appear when quickly cooling and heating. When selecting the model, ask the peak temperature and peak frequency clearly, don't just look at the average value.
The second detail is the medium. Ceramics are wear-resistant, but "wear-resistant" has a prerequisite-particle hardness. If the medium is hard particles with Mohs hardness of 7 or more such as quartz sand or corundum powder, silicon carbide ceramics are more suitable than alumina ceramics. If the medium contains chloride ions or fluoride, alkaline environment or acidic atmosphere can also corrode ceramics, don't think for granted that ceramics are "immune to all poisons". Provide the composition, concentration, particle size, particle shape and flow rate of the medium to the manufacturer, so that the manufacturer can calculate the wear rate.
The third detail is the installation direction. The expansion joint lined with ceramic metal has strict requirements on the flow direction of the medium. The direction of the guide tube is installed in the reverse direction, and the medium directly washes the joint between the bellows and the ceramic, which is equivalent to leaving an inlet for abrasion. Before installation, look at the flow direction identification arrow on the housing to verify that the medium enters and exits from the small port and the large port of the guide tube. After installing it backwards, it is not easy to find the problem immediately with the naked eye, but the life span will plummet to less than half of the original.
Things in operation and maintenance that no one tells you in advance
How to judge after the ceramic liner falls off? The early signs are increased vibration and obvious abnormal noise in the expansion joint area, and local hot spots appear on the outer surface of the pipe wall in severe cases. After the ceramic sheet falls off, the high-temperature dusty medium directly washes the metal wall of the bellows, and the penetration speed is very fast, and it may be perforated in a few days. Therefore, it is safe to regularly check the temperature distribution of the external surface of the expansion joint. A large temperature difference indicates that the lining has fallen off.
Installation precautions are also easily overlooked. The ceramic liner is afraid of bumps, and it is strictly forbidden to impact during hoisting and transportation. Some construction teams throw the expansion joint to the ground and then drag it over. There may have been hidden cracks in the lining, which will break for you after two or three months of installation-it is not a product quality problem, but a construction damage.
When shutting down for maintenance, open the expansion joint to take a glance at the wear of the liner, focusing on the media inlet side. The severe wear area is often directly opposite the end of the guide tube, and this area is most prone to wear out. Manufacturers generally thicken the ceramic layer here when designing, but when it is grinded to a certain extent, it should be replaced.
To be honest, the price of lined ceramic metal expansion joints is several times that of ordinary metal expansion joints, but you calculate the overall cost of ownership: ordinary expansion joints are replaced once every six months, and each shutdown loses hundreds of thousands; Ceramic-lined expansion joints last three to five years, but the total cost is much lower.
Conclusion: Lined ceramics are not all-purpose, but it is really worry-free to use them correctly
High temperature, dusty, abrasive media pipeline with certain pressure. In this field, its wear resistance far exceeds that of ordinary metal corrugated expansion joints, and its application range is wider than that of PTFE-lined and non-metal compensators.
Before selection, find out the details of temperature curve, medium characteristics and flow direction installation. After selection, install according to the specifications, and regularly check the surface temperature and vibration during operation. This thing can really help you save a lot of maintenance time and downtime costs. On the other hand, if the working conditions are not thoroughly understood, the installation drawing will save trouble, and no matter how good the lining ceramics are, it will be useless-this truth is true on any kind of expansion joint.