Hey, friends who are engaged in power stations and chemical industry should all have the experience-the thicker the pipeline and the fiercer the parameters, the more difficult the expansion joint will be. Especially on the main steam pipeline of supercritical unit, the medium temperature rushes to 600℃, and the pressure reaches above 25MPa. At this time, if the expansion joint drops the chain, parking is a trivial matter, the steam turbine enters foreign objects or the pipeline is unstable, and the accident level will go up.
What exactly is the expansion joint carrying under high pressure conditions?
Many people think that the expansion joint is to absorb heat expansion, and the structure is like an accordion. Just let it expand and contract. That's only half true. In addition to digesting thermal displacement, bellows under high-pressure conditions have to carry three things at the same time: pressure thrust, high-frequency vibration and medium corrosion. Don't underestimate the pressure thrust-it is called blind plate force in the industry. The internal pressure acting on the end face of the bellows will form a huge axial thrust. The larger the diameter and the higher the pressure, the more scary this force will be. DN500 tube, 5MPa pressure, blind plate force easily exceeds 100 tons. You just rely on the rigidity of the bellows itself to top it? I can't stand it.
Let's talk about the failure mode. I have seen many cases of accidents-the bellows was not broken, but cracked by stress corrosion at the wave root, or the whole was unstable. What is Bogan? Where the waveform is the most concave, the stress concentration is the most intense. Once there are chloride ions or sulfides in the medium, and alternating stresses are superimposed, cracks quietly grow out of the wave root. By the time you inspect and find a leak, it has been cracked in many cases.
Technical route of new high-pressure metal expansion joint
What's new about the new high-pressure metal expansion joint? Let's go through them one by one.
The first is the multi-layer bellows. Note that high pressure is not equal to pressing the corrugation with a thick plate. With the same pressure resistance, it is softer and more resistant to fatigue with multi-layer thin sheet stacking than single-layer thick sheet. For example, many corrugated expansion joints used in power station industry are two-layer, three-layer or even four-layer structures, and the layers restrain each other while maintaining elasticity. The high-temperature air ducts in the cement industry are similar, with high temperature and large dust. The direction of material selection is not exactly the same as that of power stations, and wear resistance and temperature resistance are emphasized.
The second is the guide tube. Don't underestimate this lining cylinder. First, it reduces the direct erosion of the medium to the inner wall of the bellows, and second, it reduces the flow resistance. The key is the gap design of the guide tube-if the gap is large, the high-pressure medium will drill in, and the root of the bellows directly contacts the high-speed fluid. No matter how strong your bellows is, it can't withstand erosion. If the gap is small, it may get stuck after thermal expansion, and the bellows loses its compensation ability. The matching tolerance here is the technical threshold of each family.
Let's talk about the end structure. The universal corrugated expansion joint is completed by welding at both ends, but not in high-pressure situations. The new design adds overall flanging reinforcement at the end, thick wall short joint transition, and even directly makes an integral structure with the pipe to move the weld out of the high stress zone. This is like human joints. Ligaments alone are not enough, but bones have to be stuck.
Stability does not depend on bellows, but on surrounding structural parts
When some engineers select models, they stare at the wave number, layer number and material of the bellows, thinking that as long as the bellows is strong enough, it will be fine. However, more than half of the credit for the stability of high-pressure expansion joints lies in the tie rods, hinges and pressure balance structures.
Straight pipe pressure balance type expansion joint, through the combination of working bellows and balancing bellows, the blind plate force is digested inside the expansion joint, and the pipe and fixed bracket hardly bear pressure thrust. The pressure balance expansion joint of curved pipe is aimed at the curved pipe section, and solves the double problems of transverse displacement and pressure thrust. What about external pressure single axial expansion joint? Bellows are subjected to external pressure, which avoids column instability under internal pressure, and are particularly friendly for large diameter thin-walled bellows.
These structural innovations are the core of the new design. No matter how strong the bellows itself is, without a reasonable balance structure, the pipe system will still be pushed around by the blind plate force.
Field data: How much has the stress dropped and how is the life calculated?
Tell me a practical case. On the main steam pipeline of a supercritical unit, the original common axial expansion joint appeared wave root cracks in less than one maintenance cycle. Later, it was replaced with a new high-pressure metal expansion joint, and the number of bellows layers and waveform parameters were adjusted in design, and an external pressure balance structure was added. After the transformation, the measured stress amplitude of bellows dropped from the original 280MPa to about 150MPa-how to interpret this number? The fatigue life is calculated according to EJMA standard. Every time the stress amplitude decreases by half, the life can be multiplied several times. Conservatively estimated, the design fatigue life has been increased from the original 2,000 times to more than 8,000 times. For peak shaving units with frequent start-and-stop, this gap is the difference between shutting down more than once a year and shutting down more than once two years.
But I'm gonna throw cold water. The data looks good, and the installation has to keep up. This link of pre-stretching is the easiest to be overlooked-the amount of pre-stretching is calculated according to the cold installation temperature, and the on-site workers don't care about this, and they just pull half of it against the drawings. The pre-tension is not in place, the bellows deflects to one side in the hot state, and the actual stress amplitude is not the design value at all. There is also the direction of the arrow of the guide tube, which must be consistent with the flow direction of the medium. If it is reversed, the airflow will go straight to the bellows, and it will be worn out for you in less than three months.
Selection to avoid pits: Don't treat thickening as a panacea
High pressure is not equal to simply thickening the wall thickness.
The large-diameter thick-walled expansion joint looks strong, but the larger the wall thickness, the greater the stiffness, and the greater the reaction force generated when compensating for displacement, instead transmitting huge loads to the fixed bracket and pipe system. You calculate the thrust the bracket takes-sometimes more than the blind plate force.
How do you choose different working conditions? If there is a large transverse displacement in the pipeline direction, the transverse expansion joint of compound hinge is given priority. The hinge structure can constrain the axial displacement, so that the compensation is concentrated in the transverse direction, and the structure of multi-wave group has stronger pressure resistance. For buried pipelines, directly buried (fully buried) expansion joint is the correct solution. It has outer sheath and end seal, so it is not afraid of soil corrosion and groundwater intrusion. In addition, in some ultra-high temperature, corrosive medium or large-section flue situations, metal bellows may really be unable to handle it, so don't be hard-non-metallic expansion joints (fabric fiber expansion joints) are combined with rectangular expansion joints, which are temperature resistant, corrosion resistant and absorb three-dimensional displacement, which is a systematic solution.
Future Trends: From Single Components to Intelligent Pipeline Systems
Finally, let's talk about directions.
New material coatings have developed rapidly in recent years. Taking the power station industry as an example, nano-ceramic coating or high-temperature-resistant alloy spraying on the surface of corrugated pipes can improve its erosion and corrosion resistance. The digital monitoring of bellows has also landed-the fiber Bragg grating sensor is attached to the wave peak, and the strain data is returned in real time. As soon as the pipeline stress is abnormal, the background will immediately alarm. With the use of desulfurization flue gas baffle door and electric plug-in insulation door, the expansion joint has changed from a simple compensation element to a perception and execution unit in the whole intelligent pipeline system.
Do you ask if this thing is expensive? Definitely more expensive than a regular expansion joint, but it saves the loss of an unplanned downtime-that's not a matter of hundreds of thousands. Again, in the face of a tough battle, the equipment has to keep up.