It's also compensation. What is the difference between sleeve type and corrugated type?
In these years of pipeline compensation, the most common problem encountered is: "Which one should I choose between sleeve expansion joint and bellows expansion joint?" In fact, the compensation principles of the two are completely different. Bellows rely on the elastic deformation of thin-walled corrugations. To put it bluntly, it means "bending and carrying hard". When the displacement is large, the corrugated shell is prone to fatigue cracking. And what about the metal sleeve expansion joint? It relies on the relative sliding between the inner and outer casings, which is equivalent to a "telescopic slide". The compensation amount can reach 300mm or even greater at a time, and the bellows can't even think about it.
However, don't think the sleeve style is invincible. Its sealing is achieved by packing compression, unlike all-metal seals like bellows. Therefore, as soon as the medium pressure is high (for example, it exceeds 2.5MPa), or there are toxic and harmful gases in the pipeline, the risk of sleeve leakage comes up. This is why sleeve type is a frequent customer in many steam pipelines, but high-pressure chemical pipelines still have to use bellows or metal hoses-each with its own site.
Two days ago, there was a power plant customer. The steam pipeline temperature was 540℃ and the expansion amount was close to 400mm. He asked if he could use bellows? I said it was OK, but three or four bellows had to be connected in series, and a guide frame had to be added. Doubling the cost didn't count as maintenance. In the end, he honestly went on the sleeve style, and it was done in one. Isn't this just "prescribing the right medicine"?
The structural code of a sleeve expansion joint – what does it slide on?
Disassemble a metal sleeve expansion joint, the core is three things: inner tube, outer tube, and sealing packing. The inner tube is fixed to the outer sleeve at one end and slides in the cylinder like a piston at the other end. There are seal grooves on the inside of the casing, stuffed with flexible packing (graphite packing, PTFE pad, etc.), and then pressed with gland bolts. When the pipe is thermally expanded, the inner pipe is pulled outward; When cold shrinking, shrink inward. The whole movement does not involve any elastic elements, pure mechanical sliding.
Deflector tube. The guide tube is a section of the liner welded to the inlet end of the inner tube. Its function is not only to "divert the flow"-more importantly, it prevents the high-speed medium from directly washing the sealing packing, and it can also reduce the local resistance. If the medium carries particles (such as flue gas, slag) and there is no guide tube, the packing will be washed out in a few months. Therefore, the No.7 Q&A specifically mentioned "the specific function of the guide tube", which is by no means an optional accessory.
In addition, the compensation direction of the sleeve expansion joint is unidirectional. If you want it to move both axially and transversely, you have to choose a duplex structure or add hinges. Pure sleeve style with only one degree of freedom-axial telescoping. This also determines its application scenario: straight pipe, with either bellows or rotary compensator at the turn (such as the rotary compensator products of this site).
Which Occasions Must Be Sleeved? Don't carry the bellows hard
Many people in design institutes, when they mention that the expansion joint is a bellows, never think about the sleeve at all. But in some cases, using bellows is to dig a hole for yourself.
- High temperature and large displacement scenarioFor example, the temperature of the main steam pipeline of the power station and the outlet of the industrial boiler exceeds 500℃ and the displacement exceeds 200mm. The rigidity of bellows decreases obviously at high temperature, and the fatigue life decreases sharply. However, the sleeve type has a simple structure and little influence of temperature on sliding friction. As long as the filler can hold it (for example, with high-temperature graphite packing), it can be used all the time.
- Low pressure large diameter pipeline: Like flue gas desulfurization system and circulating water pipeline, the pressure is less than 0.1MPa, but the diameter is always one or two meters. When the bellows is so big, the wall thickness should be thickened, and the cost should be doubled several times; Sleeve type is coil welded with ordinary steel plate, which is cheap and solid.
- buried pipeline: Directly buried (fully buried) expansion joint is actually a variation of sleeve type. The outer pipe is directly buried in the soil, and the inner pipe expands and contracts with the pipe. Bellows buried? You can't find a crack when it leaks. Therefore, many heating pipelines are directly buried, and sleeve type is standard.
Do not use sleeve type for highly toxic, flammable and explosive media. No matter how good the sealing packing is, there will be slight leakage over time. At this time, honestly choose corrugated expansion joint or metal hose, safety first.
Pits that are easy to step on during selection and installation, especially guide tubes and seals
Install the guide tube backwards. The opening direction of the guide tube must face the flow direction of the medium. If it is installed backwards, the medium directly rushes into the gap between the inner tube and the outer casing, and instantly blows the packing away. Be sure to look at the direction of the arrow on the housing before installation (mentioned in Q&A No. 10), no arrow? Take your own flashlight to light the position of the guide tube, and you can clearly see where the gap is facing.
Excessive compression of the sealing packing. Some people feel that the tighter they press, the less they leak. What is the result? The sliding resistance of the inner pipe increased sharply, the pipe could not be pushed, the expansion joint became a fixed fulcrum, and the whole pipe was holding the stress. Correct approach: Pre-tighten the gland bolt first, and then pull the inner tube manually (or do a cold draw), as long as you feel that there is slight damping. After running, fine-tune according to the leakage situation, and don't crush it to death at one time.
Ignore the fixing bracket. The sleeve expansion joint only compensates for displacement and does not absorb the blind plate force. If the sleeve type is installed at a single point on the pipeline, reliable fixing brackets must be set at both ends of the expansion joint, otherwise the pipeline will destroy the valves and equipment like a bulldozer. This is the same as bellows, and many peers have suffered losses on it.
Just use graphite packing for water and steam; Polytetrafluoroethylene compensator (lined with tetrafluorine) or rubber compensator is obtained for corrosive acid-base medium. Don't be pretentious.
Comprehensive Comparison and Maintenance Suggestions: When to Choose Sleeve Type Pipe Expansion Joint?
Looking at the whole compensator family, the specialties of metal sleeve expansion joints are obvious: simple structure, large displacement, low cost and high temperature resistance. The shortcomings are also obvious: it is not resistant to high pressure, there is a possibility of micro-leakage, and it cannot absorb lateral displacement. So the choice logic is simple- -
- Axial straight pipe segment with pressure ≤2.5MPa, temperature ≥300℃ and displacement ≥150mm: Sleeve type is preferred. For example, the four major pipelines of the power station (main steam, reheating section, etc.), heating trunk line and industrial waste gas pipeline.
- Pressure> 4.0MPa, or medium toxic/flammable/absolute zero leakage required: Please use corrugated expansion joints (such as corrugated expansion joints and high-temperature axial expansion joints used in the power station industry of this station), or welded metal hoses.
- Small angular displacement requirements (e.g. pipe turns): Don't worry about the sleeve, upper double hinge transverse expansion joint or curved tube pressure balance type.
In maintenance, sleeve type is less worry than bellows. There is no problem of fatigue failure, the main thing is to check the sealing packing. Open once a year to see if the packing has aged and hardened and if the gland bolts are loose. If there is a leak, just tighten the gland or add a new ring of packing, without stopping to replace the whole expansion joint. In contrast, once the bellows is cracked, it has to be cut and replaced, and the construction period and cost are high.
So, stop thinking that there is only one kind of expansion joint, bellows. Metal sleeve expansion joint, this old man can still play in many new projects. With the right model, the right orientation, and the right brackets, it will last a lot more than you think.