What exactly is a deflector? What does it have to do with the expansion joint body?
Don't rush to look at those lofty definitions, let's start with the internal structure. When you take apart a corrugated expansion joint, you will see a thin-walled cylinder inside, welded to the inner wall between the bellows and the end tube. This tube is a metal expansion joint guide tube, which is in direct contact with the medium in the pipe. Its relationship with the bellows is a bit like a "shield" and a "warrior"-the bellows is responsible for absorbing displacement, and the guide pipe helps it block hard damage such as erosion, wear and corrosion.
To put it bluntly, the deflector is the "personal guard" of the expansion joint. Without it, high-temperature flue gas, particulate media or high-speed fluid will hit the bellows directly, and it may be scrapped in a few months. Two days ago, I met a customer who was a power plant. The expansion joint at the outlet of one of their induced draft fans was not equipped with a guide pipe. As a result, the bellows was worn out by fly ash, and it cost hundreds of thousands to shut down and repair.
None of the four core functions can be missed
What kind of work does the deflector do? Let's make it clear one by one.
- Protect bellows from flushing:This is fundamental. High-speed flowing media (such as steam and flue gas) can quickly cause fatigue and wear if they impact the bellows directly. The guide pipe directs the medium into a smooth flow path, and the bellows can expand and contract with peace of mind.
- Reduce media flow resistance:The bellows itself is uneven and resistant. The inner wall of the guide tube is smooth, which can reduce pressure loss. Especially for long-distance pipelines, the accumulation of less makes more, and the energy-saving effect is considerable.
- Prevent debris accumulation:There are inevitably welding slag, rust and scaling debris in the pipeline. These debris, if stuck in the ripple trough, can hinder compensation or even lead to instability. The guide pipe "straightens" the flow channel, and the debris is not easy to remain.
- Absorbed partial thermal displacement:Some guide tubes are designed as bellows structures (called built-in guide tubes), which can also absorb a small amount of axial displacement and share the burden of bellows. However, this function is not standard, it depends on the specific design.
Does it mean that all expansion joints have to be equipped with a guide tube? The answer is no. For example, the sleeve-type pipe expansion joint is compensated by the sliding of the inner and outer pipes. There is no bellows inside, so naturally there is no need for diversion. What about the rubber compensator? It uses rubber flexible body to directly bear pressure, and does not involve flow diversion. However, when the bellows is the main body, the medium flow rate is high or contains particles, the guide tube is just needed.
Material selection and design: "dish" according to working conditions
The material and thickness of the guide tube directly determines how long it can last at the site. Here are a few typical scenarios:
- High temperature flue gas (such as power plant flue):With stainless steel 321 or 316L, it is resistant to high temperature and oxidation. If the temperature exceeds 800℃, you must also consider lining ceramic fiber or choosing alloy.
- Particulate media (such as pneumatic conveying pipelines in the cement industry):The deflectors must be thickened or wear-resistant layers surfaced on the inner surfaces. Some working conditions simply use ceramic patch guide tubes. Many of the products of metal corrugated expansion joints in cement industry are equipped with thickened guide tubes as standard.
- Strong corrosive environment (such as desulfurization system):At this time, stainless steel can't bear it, so you have to use PTFE or Hastelloy (C276) as the guide tube. The station's PTFE-lined hoses and PTFE compensators are typical of this type of application.
Look at the difference of product models. General-purpose corrugated expansion joints are generally equipped with standard thickness stainless steel guide tubes, which meet most occasions. However, if it is a high-temperature axial expansion joint, the guide tube should take into account axial compensation, and sometimes it will be made into a corrugated guide tube. And what about corrugated expansion joints used in power station industry? Because of the large diameter and high medium temperature, the guide tube is often welded in sections and leaves sufficient thermal expansion gap.
Which models must be equipped with a deflector? Which ones are not used?
Straight pipe pressure balanced expansion joint, it has working bellows and balanced bellows inside, the medium flows between the two, if no guide pipe is added, the high-speed fluid directly flushes the balanced bellows and fails quickly. Therefore, this type of guide tube is standard, and it must be ensured that the medium flow direction is consistent with the arrow.
Compound hinge transverse expansion jointSame thing. It absorbs lateral displacement by hinge, the curvature of flow channel changes greatly, and the guide tube plays the dual role of rectifying flow and preventing scouring. You look at the cross-sectional views of these models, the deflectors are all welded to the end tubes.
Conversely,Sleeve type pipe expansion jointBy the sealing packing and the inner and outer tubes slide, the medium flows through the inner tube, and there is no problem of flushing the bellows, so the guide tube is naturally saved.rubber compensatorWith rubber elastomer, it is also not required. AndNon-metallic expansion jointThe inside is made of fiber fabric, which is soft, and the guide tube may puncture it, so it is usually not added.
Installation Direction and Common Myth: Arrows Are Not Decorations
The arrow printed on the expansion joint housing represents the flow direction of the medium. The deflector must be installed in this direction, otherwise the medium will "backflow" through the gap between the deflector and the bellows, flushing the bellows root-equivalent to being installed for nothing. A lot of on-site accidents are due to this detail.
Leave sufficient clearance between the guide pipe and the inner wall of the pipe. The standard is 3~5mm. If it is small, it will get stuck and swell to death, and if it is large, it will surge easily. In addition,How to adjust the expansion joint tie rod nut?Note: The guide tube usually does not participate in axial force transmission, so just adjust the tie rod nut to the installation length, do not screw it to death. The role of the tie rod is to limit the excessive stretching of the bellows, and it is not used to press the deflector.
Does the deflector have any effect on the stiffness of the expansion joint? Yes, but small. According to the calculation formula of bellows stiffness, the stiffness is mainly determined by bellows wall thickness, wave pitch and wave height. The deflector as an inner liner adds only a tiny amount of overall axial stiffness (generally less than 5%), which is negligible. However, in super large diameter (such as above DN3000), the dead weight of the deflector will significantly affect the mode, and stiffening ribs are required.
Failure cases and "local methods" to prolong life
- Scour thinning:Long-term impact and local wear of particle-containing media. Solution: Increase the wall thickness of the deflector (from 3mm to 6mm), weld the wear resistant layer or install a replaceable liner.
- Weld cracking:High frequency vibration or thermal stress causes weld fatigue. It is recommended to use full penetration welding, stress relief annealing after welding, and fillet transition at the weld.
- High temperature oxidation shedding:When the temperature exceeds the material limit, the guide tube deforms and falls, blocking the pipeline and causing pipe burst. When selecting materials, be sure to select according to the actual maximum temperature +20℃ margin.
How to prevent it? Regular checkups are king. Every time the machine is shut down for maintenance, measure the wall thickness of the guide tube with ultrasonic waves, especially the onward surface at the entrance. If the thinning exceeds 30%, change it quickly. In addition, for long pipelines, the length of the guide tube can be appropriately increased, so that the medium can form a stable flow field earlier and reduce the turbulent impact.
After all, the deflector is an inconspicuous but "beam-carrying" component. Choose the right and install it, and expand and save energy safely and steadily for five or eight years; Ignore it and it malfunctions every three years. Next time you buy an expansion joint, remember to ask more questions: "Is the guide tube equipped? What material?"