How fast does an expansion joint break without a deflector?
In high-speed airflow or dusty media pipelines, the life of a corrugated expansion joint without a guide tube may be only one-tenth of the design life, or even shorter. And guess what? What the bellows fears most is not tension and compression deformation, but direct erosion of the medium.
Why is the bellows made into a wavy shape? Because of absorbing displacement. But this wave shape just becomes a "trap"-the airflow at the trough will form a vortex, and dust particles repeatedly hit the bottom of the trough under the action of centrifugal force. Over time, the wall thickness of the corrugated valley is grinded from 1.2mm to 0.3mm, and the pressure fluctuates slightly, and the cracks come out. This failure mode has nothing to do with fatigue, it is pure wear and tear.
The function of the guide tube, to put it bluntly, is to add a layer of "armor" inside the bellows. The medium first passes through the guide tube to form a stable flow field, and the dust and vortex are blocked outside the bellows. Without this layer of armor, the bellows would be directly exposed to the medium, and it would be calculated on a daily basis.
The design of the guide tube is more specific than you think
The deflector is not just welding a steel pipe. Wall thickness, gap, overlap direction, each item is competing.
How to determine the wall thickness? Look at media abrasiveness. The flue gas pipeline is generally 3-4mm, and the dust concentration is high to 6mm. But you can try adding it to 8mm? The thermal expansion of the guide tube itself becomes larger, which will withstand the end and pull off, but pull the bellows out. Therefore, the thicker the wall thickness is, the better, and the thermal expansion and stiffness must be matched.
The gap is even more picking with millimeters. The annular clearance between the outer diameter of the guide tube and the inner diameter of the bellows, usually between 10-30 mm. If the gap is small, the guide tube will get stuck when the bellows is compressed; When the gap is large, the air flow bypasses the edge of the guide tube and directly pours into the trough, and the guide tube is useless. One millimeter away, and the end is either worn out or stuck, it's that extreme.
The overlap direction is also particular. The overlap seam of the guide tube must follow the medium flow direction, and the medium flow direction side presses the next piece to prevent the airflow from "lifting" the overlap. When installed backwards, the high-speed airflow will drill into the overlap joint, forming high-frequency vibration, and the bellows will soon crack.
Which operating conditions cannot be diverted?
Flue gas pipeline of power station, air duct of cement industry and desulfurization system are the main battlefields of diversion metal expansion joint.
What goes on in the flue gas pipeline of the power station? Sulfur-containing flue gas and fly ash, the flow rate is up 15m/s, and the temperature fluctuates greatly. When the ordinary corrugated expansion joint is installed, dust accumulates in the trough, and the deformation of the corrugated pipe is limited after the ash scale hardens. Then the stress is concentrated, and the crack quietly spreads from the trough. The factory has to stop every few months to change it-how much money will be lost in power generation, you can calculate it yourself.
The cement industry is even more ruthless. The air duct is filled with cement raw powder, which is an order of magnitude stronger than fly ash. We have a customer who didn't use the diversion type, and the expansion joint took less than half a year. The bellows wore out and leaked, the thermal parameters of the kiln system were completely chaotic, and the clinker quality was problematic. Later, it was replaced with a metal corrugated expansion joint (with guide tube) in the cement industry, which has not been touched for three years.
The slurry pipeline of desulfurization system, the medium is limestone slurry, the content of solid particles is high, and contains chloride ions, corrosion and wear double blow. At this time, let alone not installing the guide tube, even the material of the guide tube has to be upgraded to corrosion-resistant alloy. Ordinary bellows are basically consumables under this working condition.
What to weigh when choosing a model?
It is a fact that deflected metal expansion joints are more expensive than general-purpose corrugated expansion joints of the same specification. What's so expensive? There is an extra section of guide tube material, more design and manufacturing hours, and more requirements for installation direction.
But that can't be the case. An expansion joint with a guide tube can last for five years under severe working conditions; If you don't bring it, it will be scrapped in half a year. Converted into the average annual cost, it is more than half cheaper with a guide tube. Plus downtime loss, labor replacement cost, spare parts warehousing cost-hey, compared to this, it is really expensive not to install a deflector.
Of course, if the medium is clean low-pressure steam, ordinary gas, and the flow rate is low, the general-purpose corrugated expansion joint is completely enough. The principle of selection is very simple: see whether the medium contains solid particles, see whether the flow rate exceeds 10m/s, and see whether dust may accumulate in the trough. If you get any of these three items, don't feel bad about the money of the diverter.
Don't reverse the arrow when installing
When the diversion type metal expansion joint leaves the factory, the medium flow direction arrow must be marked on the cylinder. Many on-site installers don't pay attention to this detail, thinking that they are all round pipes anyway, and they are the same in front and reverse clothes-if you think so, just wait for rework.
The direction of the arrow determines the windward side of the guide tube overlap seam. When installed backwards, the airflow impacts the overlap seam to produce abnormal noise, and the whole pipe is buzzing; In serious cases, the overlap is opened, the guide tube falls off and gets stuck in the bellows, the expansion joint loses its compensation function, the thermal stress of the pipeline has nowhere to be released, and the connecting flange will be pulled and deformed.
The clearance between the bellows and the guide tube has to be re-measured during installation. If the pipe is forcibly pulled, the gap will become smaller, and the displacement of the bellows will be discounted. On site, it is best to measure four points in the circumferential direction with a gauge to ensure uniformity. Alas, these details seem cumbersome, but each is the lesson of predecessors in exchange for.
The expansion joint is an obscure thing on the pipe, and it can be really bad if it breaks. As an internal component, the guide tube is invisible and intangible, but it really determines how long the equipment can run. When selecting the model, ask "with or without a guide tube", and take a look at the direction of the arrow when installing, which will save real money.