People who do pipeline design probably have encountered the owner holding the drawing and asking, "Is your metal expansion joint with sleeve just adding a guide tube?" Every time I hear this, I have to explain it from the beginning. Simply write it clearly today-the metal expansion joint with sleeve is not a standalone model, it is a structural form that adds a layer of protective sleeve to the inner or outer wall of the bellows. This layer of casing is completely different from the guide tube: the guide tube is the flow direction of the medium, so that the fluid goes along the center, not directly on the ripples; The casing tube is wear-resistant and anti-corrosion, which blocks particles and corrosive media outside the bellows.
Two days ago, I met a customer, the raw powder pipeline of cement plant, the original universal corrugated expansion joint without casing, but the trough was worn out and leaked in 8 months. Later, it was replaced with a casing, and it ran in the same working condition for almost three years. It was still good to see that the bellows was cut open, but half of the casing was worn off. Do you think this gap is big?
What the hell is the sleeve carrying
Let's start with protecting this layer. What the bellows fears most is that the particulate matter directly washes the trough. That position has the largest deformation and the thinnest wall thickness, and it is a matter of time for it to wear out. The first function of the casing is to block a wall between the bellows and the medium, so that the particulate matter hits the casing first.
Reduces fluid-induced vibrations. In high-temperature steam pipeline and air powder pipeline, when the flow rate is combined, the bellows is prone to high-frequency vibration. The rigidity of the casing is much greater than that of the bellows, which can change the excitation frequency of the fluid and suppress the vibration. To put it bluntly, it is to add a "Dinghai Shen Needle" to the bellows.
The third effect is to improve the pressure distribution. When the bellows is subjected to internal pressure, the stress is most concentrated at the trough. The existence of casing can block a part of the pressure load, so that the bellows is stressed more evenly. This is particularly evident on high-pressure large-diameter pipelines.
So, stop using the sleeve as a stand-in for the deflector. The guide tube is tube "flow", and the sleeve is tube "protection". Both can exist simultaneously, but functional boundaries must be distinguished.
What operating conditions must be cased
Here are a few judgment parameters. Don't just pat your head based on your feeling.
- Flow Rate:If the medium flow rate is greater than 15m/s, it is recommended to add casing. More than 25m/s, without casing is pure life gambling.
- Dust content:Casing must be considered when the solid particle concentration exceeds 50 mg/Nm³. Cement raw powder, desulfurization flue gas, don't hesitate, go directly.
- Temperature:When the medium temperature exceeds 400℃, ordinary carbon steel casing will be oxidized and peeled, so it has to be upgraded to stainless steel or heat-resistant alloy material. Don't expect a layer of carbon steel to conquer the world.
There is another situation-high-frequency vibration. For example, the compressor outlet pipeline has a high vibration frequency. Even if the medium is clean, it is recommended to add a casing, which can significantly reduce the dynamic stress of the bellows.
How to choose with sleeve? The key is the clearance
Choose a metal expansion joint with a sleeve, and it is not enough to report a "with a sleeve". The four parameters of caliber, pressure, temperature and displacement are indispensable. They determine the thickness, material, and structural form of the casing.
The most prone to problems is the gap between the casing and the bellows. This gap must be strictly controlled-it is too small, and the casing will be stuck as soon as the bellows expands, and the compensation amount will be wasted; It's too big, and the particles drill into the gap, and still wear the bellows, which is equivalent to not taking it. Under the conventional design, the clearance is controlled at about 3~8mm, and the specific value depends on the caliber and displacement. For example, the gap of a large-diameter thick-walled expansion joint should be larger, otherwise it will be stuck as soon as the thermal displacement comes.
The general-purpose corrugated expansion joint, high-temperature axial expansion joint and sleeve-type pipe expansion joint of this station all have corresponding schemes with casing. However, note that no standard drawing can be applied to all working conditions. The metal corrugated expansion joint used in cement industry is completely different from the corrugated expansion joint used in power station industry. Take the structure of the power station to the cement factory, and sooner or later something will happen.
Installation and maintenance: Three pits don't step on
In the first pit, the casing is installed in reverse direction. The opening direction of the casing must be consistent with the flow direction of the medium, and the direction of the arrow is the flow direction. Install backwards, the medium is directly poured into the gap between the bellows and the casing, and the bellows is used as a target. Look at the mark before installation, and check it again after installation. Don't be too troublesome.
The second pit, forced alignment. The overall stiffness of the expansion joint with casing is higher than that without casing. If the expansion joint is used to forcibly absorb the misalignment of the pipeline during installation, the casing will be deformed by force, the gap will disappear, and the bellows will suffer. Adjust the pipe, don't let the expansion joint be a jack.
In the third pit, the inspection only looks at the bellows. Many people check the expansion joint, open the insulation and see that the bellows is not cracked, so they think it's okay. You can't see the wear of the casing. The correct way is to regularly check the end of the casing for wear marks, or measure the residual thickness of the casing with a thickness gauge. Especially in abrasive media conditions, casing is the real consumable.
What about that? When making the equipment ledger, list the casing wear record as a separate item, and compare the data every time the machine is shut down for maintenance, so as to estimate the remaining life.
The sleeve is not once and for all. Only when the type is selected and installed correctly can the life be pulled full. If you don't know the specific working conditions, ask the manufacturer for previous case data, and don't guess it yourself.