Two faces of failure: What is the difference between plane instability and columnar instability?
The metal expansion joint is unstable. To put it bluntly, the bellows can't bear the pressure and its shape has changed. The two most common:Planar instabilityAndcolumnar instability。 Plane instability looks like a ripple bulge, a local bulge, usually occurs at the crest or trough of a wave, a bit like a tire bulging a bulge. The columnar instability was hard-the whole bellows was twisted like a twist, and the axis was crooked.
Why is columnar instability more dangerous? The plane instability can last for a while, and the local stress is concentrated but the whole can barely work. Once columnar instability occurs, the bellows directly loses its load-bearing capacity, the pipeline displacement compensation completely fails, and the medium leakage and pipeline pull-off are all matters of minutes. On the spot, there were cases of columnar instability of flue gas pipelines in power stations. The whole expansion joint was like a bent straw, and millions were lost when the furnace was shut down for emergency repair. Did you say dangerous or not?
Where does instability come from? Pressure, temperature, installation deviation – who is the real pusher?
When the bellows is subjected to internal pressure, axial force and circumferential stress will be generated. The higher the internal pressure, the easier it is to flatten the bellows. Here is a key parameter calledCritical pressureIt and the bellows'stiffnessDirect hook. The stiffness formula includes wave height, wave pitch, wall thickness, and number of layers-if you change any one, the instability limit will change.
What about the temperature? High temperature will make the yield strength of the material decrease, and under the same pressure, the bellows is more prone to plastic deformation. For example, the yield strength of 304 stainless steel at 400℃ is only about 60% of that at room temperature, and the temperature reduction is not considered in design, which doubles the risk of instability.
Installation deviation is often a tough role that is overlooked. Two days ago, I met a customer in a cement factory. During on-site installation, the direction of the guide tube was reversed, and the medium directly washed the side wall of the bellows, causing local wear and vibration, and it became unstable in half a year. There are also tie rod nuts that are not adjusted properly, the pre-stretching amount is given wrong, what should be pulled is not pulled, the allowance that should be left is not enough, and problems occur as soon as you drive.
Pits stepped on by design selection: What is the poor ability of different structures to resist instability?
When it comes to type selection, it is not enough to look at the amount of compensation on the sample. Let me sort you out:
- Universal corrugated expansion joint: Single-layer bellows, hard to carry by its own stiffness, suitable for low pressure and small displacement. However, when the internal pressure is high, the risk of columnar instability is the greatest.
- External pressure single axial expansion joint: The bellows is subjected to external pressure, which is equivalent to being clamped rather than stretched open, and its ability to resist instability is obviously improved. It is safer to use on steam pipes.
- Straight pipe pressure balanced expansion joint: With balance ring and tie rod, the axial force generated by internal pressure is digested by itself and will not be transmitted to the fixed bracket. It is the most resistant to columnar instability, but it is also the most expensive.
High temperature axial expansion jointIf used on flue gas pipes, the stiffness reduction at high temperatures must be checked. Many designers only set the normal temperature formula, and as a result, they become unstable as soon as they are put into production. AndCompound hinge transverse expansion jointThe lateral displacement is large but the ability to resist internal pressure is limited, so don't use it as an axial compensator.
The most overlooked details of on-site installation
In the installation process, three details are the most terrible:
Direction of guide tube: The direction of the guide tube arrow must be consistent with the flow direction of the medium. On the contrary, the medium directly washes the bellows, wear and vibrate together, and instability is only a matter of time. It is clear in the Q&A on our website about the role of the expansion joint guide tube.
tie rod nut adjustment: The tie rod nut is locked before installation to prevent transport deformation. Once installed in place, the nut must be loosened as required by design and adjusted to the specified length. Many people forget to loosen the nut, or tighten it too tightly, and the bellows can't expand and contract freely, which is equivalent to holding it hard. The pressure all acts on the bellows, and the risk of instability soars. How exactly to adjust? We are inHow to adjust the tie rod nut of expansion jointThere are steps in the question and answer, just follow them.
Amount of pre-stretch: When installing in cold state, the bellows should be pre-stretched (or pre-compressed) in reverse direction according to the thermal expansion of the pipe. Giving it wrong is equivalent to leaving the bellows overloaded in the first place. On-site workers install it directly to save trouble. As soon as the pipeline heats up, the bellows is directly pushed to the limit, and the column is unstable and doesn't run.
What should I do if I'm unstable? Practical advice for engineers
Don't panic when you find instability. Go in three steps:
Step 1: Emergency shutdown.Once the bellows appears obvious deformation, especially columnar instability, it will be stopped immediately to reduce pressure. Continued operation can cause bellows to tear and media to eject, which is an extremely high safety risk. Steam pipe instability may also burn people, don't hesitate.
Step 2: Local replacement.If only a single expansion joint is unstable, and the pipe itself is fine, only this one can be replaced. However, note that when replacing, check whether the peripheral bracket is displaced and the support is settled. Otherwise, the newly replaced expansion joint may be pulled out again quickly.
Step 3: Overall upgrade.If the same pipe section is unstable repeatedly, it means that the design selection is defective. It is recommended to upgrade from generic toExternal pressure single type axial typeOrStraight pipe pressure balanced expansion jointAt the same time, check the thrust of the fixed bracket. There is a classic case in the cement industry: the common metal corrugated expansion joint was originally used, which became unstable three times in half a year. Later, it was changed to a structure with a balance ring, and a guide bracket was installed. So far, it has been running without fault for five years.
Attached Typical Case: Power Station Flue Gas Pipeline and Cement Industry
Case 1: Instability analysis of high-temperature axial expansion joint of flue gas pipeline in power station
The flue at the outlet of the desulfurization tower of a power plant, the medium temperature is 350℃ ~400℃, and the useHigh temperature axial expansion joint。 After one year of operation, the bellows developed columnar instability. The disassembly found that: ① the elastic modulus reduction of the material at high temperature was not considered in the design, and the critical pressure value was too high; ② The pre-stretching amount is reversed during installation (cold compression). Rectification plan: Upgrade the material of the expansion joint to Incoloy 825, increase the wave height and wall thickness, and calculate the pre-stretching amount strictly according to the thermal displacement. No abnormalities were observed after three years of operation after replacement.
Case 2: Improvement scheme of metal corrugated expansion joint in cement industry
The exhaust gas pipeline at the kiln end of a cement plant has high dust content in the medium, and the original use is ordinaryUniversal corrugated expansion joint, plane instability twice in three months. On-site investigation found that the guide tube was seriously worn, and the inner wall of the bellows was washed and thinned by dust. Improvement scheme: Switch toExternal pressure single axial expansion jointWear-resistant bushing is installed on the outside of the bellows, and the guide tube extends to more than 50mm downstream of the bellows. At the same time, a tie rod limiting device is added to prevent excessive compression of the bellows. After improvement, the service life is extended from three months to more than two years.
In the final analysis, prevention is much more important than remedy for the instability of metal expansion joints. It is much better to check the stiffness twice when designing and look at the instructions twice when installing than to sigh after staring at the twisted bellows.