Structural disassembly: What does the external pressure single axial expansion joint look like? What's the difference between it and the general-purpose type?
Look at the names first —External pressure type metal bellows expansion jointAlso called external pressure single type axial type expansion joint. The biggest difference between it and our common general-purpose corrugated expansion joint (that is, the kind you usually see on low-pressure water pipes) is the position of the corrugated pipe.
The general type bellows is directly exposed outside the pipeline, and the medium pressure directly acts on the inner wall of the bellows, and the bellows itself has to bear the internal pressure. The external pressure type is just the opposite-the bellows is installed inside the cylinder, the medium flows through the outer wall of the bellows, the pressure is borne by the outer cylinder, and the bellows is only responsible for absorbing axial displacement. To put it bluntly, the pressure shell is the outside steel pipe, and the bellows is like a protected spring.
Inlet end pipe, outlet end pipe, working cylinder (external pressure vessel), bellows assembly, guide cylinder, tie rod and nut. Compared with the general-purpose model, it has a heavier outer cylinder, which reduces the risk of direct exposure of the bellows. When you look at the real thing, the external pressure type is obviously thicker than the general-purpose type of the same caliber, because it is covered with a layer of pressure-bearing shell.
Core advantages: Why do everyone choose it in high pressure, high temperature and large displacement scenarios?
Bellows are not subjected to internal pressure and have excellent stability。
What are you most afraid of in high-pressure pipelines? Bellows instability. Under high pressure, the waveform of ordinary general-purpose type is easily squashed or bulged, and once it is unstable, it will leak or even burst. The external pressure type unloads the pressure to the outer cylinder, and the bellows hardly bears pressure, but is only responsible for expansion and contraction, so it can carry a very high pressure-usually the design pressure can be 2.5MPa, 4.0MPa or even higher.
Let's talk about the heat. The external pressure type structure has its own guide tube, and the high-temperature medium is separated by the guide tube, so that the dead corner in the valley of the bellows is not directly washed, so as to avoid local overheating and stress concentration. With high temperature resistant alloys (such as Incoloy 800H), it can still be stable in steam pipes above 600℃.
What about large displacements? The bellows of external pressure type can be made longer, because there is no need to fear the columnar instability caused by internal pressure, and the single wave compensation amount can be more than 30% larger than that of the general type. Two days ago, I met a customer who was doing power station renovation. Originally, I used a general-purpose type, but the displacement was not enough, so I added two expansion joints; Replace with external pressure single axial type, one can be done, saving flange and installation space.
Practical application: power station, cement, chemical industry-which working conditions must be it?
- Main steam/reheat steam pipeline of power station: Temperature above 540℃, pressure above 10MPa, displacement tens of millimeters. This place external pressure type expansion joint is standard, matching this station'sCorrugated expansion joint for power station industrySeries, which has been used for more than ten years without any problems.
- Kiln Head/Kiln End Waste Heat Pipeline in Cement Industry: High temperature flue gas with dust, the pressure is not high but the temperature changes drastically. The external pressure guide tube can reduce dust accumulation and avoid bellows from being worn out.
- Chemical high pressure heat exchanger import and export: For example, in ammonia and methanol plants, the medium in the tube is flammable and explosive, and the pressure level is high. External pressure type plus double seal structure, safety redundancy pull full.
To put it bluntly, as long as the medium pressure exceeds 1.6MPa, the temperature exceeds 300℃, and the axial displacement exceeds 20mm, your first reaction should be to look at the external pressure type. Of course, there are exceptions to special circumstances, which will be said later.
Selection and pit avoidance: caliber, pressure, guide tube, tie rod-none of them can be wrong
What are you most afraid of when choosing external pressure type metal bellows expansion joint? Slap the head.
Caliber and Pressure: First, look at the design pressure and temperature of the pipeline to determine the material and wall thickness of the outer cylinder. Note that the external pressure cylinder itself is designed according to the pressure vessel, so you can't just look at the bellows parameters. For example, DN300 pipeline, the design pressure is 2.5MPa, the wall thickness of the outer cylinder is at least 8mm, and the material is 20G or Q345R.
guide tube: Not optional. The action of the guide tube isQ&A on this siteThere are detailed diagrams in it-it allows the medium to flow smoothly, avoids vortex scour, and protects the bellows at the same time. When selecting the type, pay attention to the thickness of the guide tube should be the same grade as the pipeline, and the diameter should be one gear smaller than the inner diameter of the bellows, otherwise the flow rate is too high to cause vibration.
tie rod: The external pressure type usually does not come with a tie rod? Wrong! Many designs will be equipped with tie rods to limit the ultimate displacement and prevent the bellows from over-stretching. But how to adjust the tie rod nut? according toHow to adjust the tie rod nut of expansion jointAfter installation, screw the nut to just touch the ear plate, do not lock it, and leave the design compensation amount. If it is completely locked, the expansion joint is crippled.
If the pipeline has large lateral displacement, the external pressure single axial type is not applicable, so it is necessary to replace the double hinge or straight pipe pressure balance type. Don't push it hard, there's an accident.
Installation and maintenance: Screw disassembly or not? How is the direction of the arrow correct? How much do tie rod nuts adjust?
Can the screw be removed or not?Which screw? If it is a transport screw (used to hold the bellows against transport deformation), it must be removed after installation. refer toDo you need to remove the screw of the expansion jointIf it is not dismantled, the bellows cannot be expanded and contracted, and it will fail directly. However, if it is the tie rod screw used to adjust the compensation amount, don't disassemble it, but keep it.
How is the direction of the arrow correct?The arrow indicates the flow direction of the medium, generally pointing to the outlet end. However, the external pressure expansion joint has a characteristic-the medium flows through the outside of the bellows, and the flow direction has little influence on the force of the bellows, but the arrow indicates the installation direction of the guide tube. If it is installed backwards, the guide tube will hinder the flow of the medium and increase the pressure loss, which will lead to uneven scour in severe cases. Just press the arrow, don't be smart.
How much do tie rod nuts adjust?Conventional practice: Before installation, loosen the nut to the designed length (the manufacturer will mark it), and tighten it symmetrically after installation in place until the nut just touches the ear plate, at which time the bellows is at free length. Then twist it back half a turn? Never! Just keep the contact state directly, too much gap will cause abnormal noise and vibration. If the pipe has pre-tension/pre-compression requirements, adjust the nut position as per the drawing.
Common Myth Clarification: External pressure expansion joint ≠ universal, these cases are not as good as straight pipe pressure balance type
To be honest, the external pressure metal bellows expansion joint is really powerful, but don't be superstitious. There are a few situations where it doesn't work:
- The pipeline has large lateral displacement or angular displacement: External pressure single axial type can only absorb axial displacement, and forcibly restricting lateral displacement with tie rod will cause bellows to twist. In this case, you should choose the double hinge transverse type orStraight pipe pressure balanced expansion joint。
- Constrained space to fit long cylinders: The external pressure type has greater axial rigidity, and its length is one section longer than the general type. If the pipes are tightly spaced and can't fit, it might as well be compensated with elbows or sleeves.
- Low pressure and large diameter: For example, the flue above DN2000 has a pressure of only a few kPa and a low temperature. The external pressure type is expensive and heavy, so it is not as cost-effective as the rectangular non-metallic expansion joint or fabric fiber expansion joint.
Therefore, the more expensive the selection is, the better, and the better it can resist pressure, the better, but the matching of working conditions. The external pressure type is the king in the scenario of high pressure, high temperature and large displacement, but outside this range, it is a heavy iron lump. Did you pick the right one?