What does an inner cannula do? Find out its three duties first
Many peers who do pipeline design, when they mention the inner casing of the metal expansion joint, their first reaction is-oh, the guide tube. Yeah, not entirely. The formal name of the inner casing in the industry is the diversion tube, but it does more than "diversion".
To put it in detail, it has three duties: the first is to divert the flow, so that the medium flows smoothly through the bellows along the axial direction, and prevents the fluid from spinning in the trough and forming a vortex; The second is wear resistance, especially when conveying the medium of particles, the inner sleeve blocks the front of the corrugation, and carries high-speed scour for the corrugation; The third is heat insulation. Under high-temperature working conditions, the inner casing can block part of the radiant heat directly to the root of the bellows, thus reducing the wall temperature of the bellows.
The corrugated expansion joint without the inner sleeve is like a rider on the field without protective gear-it works, but at your own risk.
Which working conditions are inseparable from the inner casing? Self-examination against these scenarios
Does it mean that all expansion joints have to be fitted with inner sleeves? Not. In the on-site cases I have come into contact with in recent years, the following three types of working conditions must be installed:
- High velocity airflow + particulate mediaFor example, the wind speed of the air duct at the head and tail of the kiln in the cement industry and the powder feeding pipe of the power station is often 20m/s, and the particle hardness is high. The corrugated bottom without the inner casing will be worn out in one year. The metal corrugated expansion joints in the cement industry and the corrugated expansion joints in the power station industry are standard in the factory configuration.
- High temperature flue gas: In the pipeline behind the desulfurization flue gas baffle door, although the flue gas temperature is not extreme, the sulfur-containing components are corrosive. The inner casing can reduce the direct contact of high-temperature flue gas with the corrugated inner wall. With the appropriate material, the life is completely different.
- Scenarios requiring low flow resistance: The inner wall of the inner casing is smooth, which can reduce the fluid resistance. For example, the double hinge expansion joint used in air-cooled island vacuum pipeline is very sensitive to flow resistance, and the design of inner casing directly affects the vacuum degree of the system.
On the other hand, if the medium is clean steam or water, and the flow rate is not high, there is really no need to worry about whether to install it or not-instead, there will be an extra friction pair.
How to determine the material and size of the inner casing? Doorway in Design Selection
The inner casing is not just a piece of iron, rolled and welded. There are several key points in its selection.
Let's talk about the material first. Q235B or 304 is enough for ordinary working conditions; If the flue gas has high sulfur content, it has to go up, 316L or more corrosion-resistant alloy; If it is an occasion with particularly severe wear, the upward surface of the inner casing should be surfaced with cemented carbide or replaceable wear-resistant bushings should be installed. If the material is chosen wrong, the inner casing will be scrapped first, and the bellows will be worn out.
Let's talk about the size. There should be a gap between the outer diameter of the inner casing and the inner diameter of the bellows. This gap is particularly small-it is too small. As soon as the expansion section is compressed, the inner casing directly pushes to the bottom of the corrugated valley, which is equivalent to the compression of the corrugated white; It's too big, and the airflow carries particles through the gaps and rushes straight to the root of the ripples, which greatly reduces the protection effect. The gap is generally controlled between 10mm and 30mm on one side, depending on the size of the caliber. There is also the overlap length, the length of the inner sleeve extending into the corrugated section, which usually covers the highest position of the wave crest, otherwise the particulate medium forms an impact dead angle at the wave crest.
Reverse orientation installation, cracked welds-the most common pits for field installation
Two months ago, a customer called and said that a general-purpose corrugated expansion joint on their site had leaked less than a month after it was put into operation. When disassembled, the inner sleeve was installed in the reverse direction. The medium in his pipe is directional. After being installed backwards, the air flow directly flows against the gap between the inner casing and the corrugation. The root of the corrugation is like sandpaper, and it is full of spots.
The opening direction must be toward the direction of the flow of the medium。 That is, the medium first passes through the smooth inner wall of the inner casing and then flows out from the outlet of the inner casing. Why? If it is installed in reverse, the medium flows in from the annulus between the outer wall of the inner casing and the inner wall of the corrugation, and the corrugated surface is unprotected, and the particles are all hit in the trough.
Another high-frequency problem is that the transport fixtures are not disassembled. When the expansion joint leaves the factory, there is usually a spot welding fixing or transportation tie rod between the inner casing and the bellows to prevent movement during transportation. Some installation teams try to save trouble, and the main fixing brackets on the pipeline are installed. As a result, the tie rod is not loose, and the compression displacement of the expansion joint directly cracks the weld of the inner casing. This problem should be especially careful in directly buried (fully buried) expansion joints. Once buried, it is the cost of civil excavation to deal with it.
How much does the inner casing affect the life of the entire expansion joint?
Let's put it straight, whether the inner casing is designed well or not directly determines the service life of the inner casing of the whole set of metal expansion joints-note that it is the service life of the whole set of equipment, not the life of the inner casing itself.
The design life of a set of corrugated expansion joints for power station operating conditions is calculated as 15 years. When the wall thickness of the bellows is 1.5mm and there is no inner casing, the wear rate of high-speed dust-containing flue gas to the corrugation is about 0.5mm/year. That is to say, the bellows will be too thin to be used in about three years. After adding the inner casing, all the wear is borne by the inner casing, and the bellows has almost zero wear, and the overall life can be more than 10 years. This is why when the design institute lays out flue gas pipes, the expansion joint model should be clearly marked with an inner casing.
On the other hand, the welding quality of the inner casing is poor, the welding seam is not tightly sealed, and the medium penetrates into the interlayer between the casing and the corrugation, which is more likely to cause gap corrosion than without installation. Many non-professional manufacturers don't care much about this detail, but a lot of things happened in the actual operation. Therefore, when choosing the expansion joint, asking more about the material, thickness and welding process of the inner casing is much more realistic than just looking at the price.
The inner casing of the metal expansion joint is not an optional accessory, it is the core functional part that determines that the whole set of expansion energy saving can not hold the working condition. Whether to install or not depends on the working conditions; How to install it depends on experience. These two things didn't know in mind, and it was only a matter of time before things went wrong later.