1. What does the external pressure metal corrugated expansion joint look like? What is the essential difference between it and the common internal pressure type?
Two days ago, I met a customer who came with a drawing and asked, "Does your external pressure expansion joint just turn the internal pressure shell over and install it?"-that's really not the case.
The most intuitive difference in appearance of external pressure metal corrugated expansion joint is:The bellows are encased in a pressure-bearing housing。 The medium flows through the outside of the bellows, the pressure acts on the housing, and the bellows itself only bears external pressure. In our common internal pressure type (that is, general corrugated expansion joint), the medium flows through the inside of the bellows, and the pressure directly acts on the inner wall of the bellows.
Structurally, the external pressure type is usually composed ofCorrugated pipe, pressure-bearing housing, end pipe, tie rod assemblyConstitution. The bellows is "upside down" on the pipe, and the outer shell covers the bellows entirely, and both ends are connected to the pipe through end pipes. If you take it apart, you will find that the peaks and troughs of the bellows are oriented opposite to the internal pressure type-this is also where many people get confused.
To put it bluntly, the internal pressure bellows is stretched open like a "spring", and the external pressure bellows is "compressed" in a shell. These two structures determine that their mechanical characteristics and application scenarios are completely different.
Second, why can it withstand high pressure and large displacement? Core Working Principle and Structural Advantage Disassembly
Well, here's the question: why can the external pressure type carry high pressure and large displacement?
The core lies inPressure action position。 Internal pressure type bellows, internal pressure puts the bellows at risk of "columnar instability" – the more pressure, the easier the bellows can be pressed and bent like a spring. The solution is to either thicken the wall thickness or increase the number of layers, but the cost skyrockets.
The external pressure type is different. The high-pressure medium is all borne by the shell, which is cylindrical, and its compression resistance is inherently much stronger than that of bellows. The bellows only bears external pressure, and when the bellows is subjected to external pressure, the trough will be pressed tighter because of the pressure, and the structure will be more stable. Think of it as a "taut spring", and external pressure instead helps it fix its shape.
In terms of displacement capacity, external pressure type is usually adoptedSingle axial typeDesign (corresponding to the "External Pressure Single Axial Type Expansion Joint" on our website), the bellows can be made longer and with more wavenumbers because there is no problem of columnar instability under internal pressure. Therefore, the amount of axial displacement that it can absorb is often 2~3 times that of the same caliber internal pressure type.
guide tube。 The external pressure type usually does not have a guide tube inside (because the medium travels from the outside), so the medium flow resistance is less, which is particularly suitable for systems sensitive to pressure drop. Of course, if the medium has particulate matter, you can also install an inner liner to prevent wear, which depends on the specific working conditions.
Third, power station, cement, desulfurization... What occasions must it be? Analysis of typical application scenarios
External pressure metal corrugated expansion joint is not universal, but in some cases, it is the optimal solution, or even the only solution.
Power station industry:The parameters of the main steam pipeline and the reheating steam pipeline are always the pressure above 10MPa and the temperature 540℃ +. In this kind of high pressure, high temperature and large displacement condition, the internal pressure type either has too thick wall thickness, which leads to poor flexibility, or has short service life. The external pressure type has become the standard configuration of power station pipeline compensation by virtue of the shell bearing pressure and the long bellows absorbing displacement. Many of the "corrugated expansion joints for power station industry" on our website are external pressure structures.
Cement industry:The hot air pipeline and the outlet pipeline of the decomposition furnace on the cement production line have high temperature (800℃ ~1000℃), large diameter and large displacement, and the medium contains dust. The external pressure type smooth shell is not easy to accumulate dust, and the maintenance is convenient. With the design of high-temperature axial expansion joint, it can run stably for many years. Like our "Metal Corrugated Expansion Joint in Cement Industry" series, the external pressure type accounts for a high proportion.
Desulfurization system:The medium in the desulfurization flue is corrosive flue gas, with low temperature but high humidity and small pressure fluctuation. The advantage of external pressure type is that if the bellows is damaged, the shell can also play a certain sealing role, so it will not stop immediately. Moreover, the external pressure structure is convenient for lining with anticorrosive materials (such as lining with PTFE). The "Desulfurization Flue Gas Baffle Door" on our website is often used in conjunction with this type of expansion joint.
In addition,Heating pipe network, air cooling island, vacuum pipe(For example, "double hinge expansion joint of air-cooled island vacuum pipeline" and the like) will also use external pressure type, and the core logic is the same: it requires large displacement and the pressure is not low.
4. Keep an eye on these 4 parameters when selecting the model: pressure, temperature, displacement and interface size
Many procurement rollovers are turned over in the parameters without figuring out. Remember, external pressure selection stares at four numbers:
- Design Pressure:Don't just look at nominal pressure. The pressure resistance grade of external pressure type shell must match the design pressure of pipeline, and the external pressure stability calculation of bellows should be done separately. Under the same nominal pressure, the bellows of external pressure type can be thinner, but the wall thickness of the outer shell cannot be saved.
- Design temperature:Temperature determines the material selection and fatigue life of corrugated pipe. The carbon steel shell has limited temperature resistance, so it should be replaced with stainless steel or alloy steel at high temperatures. Never apply room temperature selection directly to high temperature-the allowable stress of materials will drop greatly at high temperature.
- Axial displacement:This is the strength of external pressure type, but it should be noted that the larger the displacement, the longer the bellows, and is there enough installation space? At the same time, the adjustment range of the tie rod should be considered. The function of the tie rod is to limit excessive displacement and protect the bellows (see the question and answer of "Function of Expansion Joint Tie Rod" on our website for details).
- Interface size/connection method:Includes nominal diameter, wall thickness, flange standards (HG/T, GB, ASME, etc.). Flanged or welded? Welding is generally recommended for external pressure type to reduce leakage points.
Cold tight。 For high-temperature pipelines, pre-stretching or pre-compression is usually required during installation, and the cold tightening amount should be specified when selecting the type. This is directly related to the working life of the expansion joint.
V. During installation and routine maintenance, several pits that are easiest to overturn (with key points of tie rod adjustment)
To be honest, no matter how good the quality of external pressure expansion joint is, it will be waste if it is not installed correctly. I've seen too many cases.
Pit 1: Drive without loosening the tie rod nut.For the convenience of transportation when the expansion joint leaves the factory, the adjustment nuts on the pull rod are tightened, which locks the bellows. After installation and before the pipe is pressurized, the nut must be loosened to the installation length specified in the design. Many people forget this step. As soon as the pipe heats up, the bellows has no room to expand and contract at all, and it directly cracks.
Pit 2: Install the tie rod in reverse direction.External pressure tie rods usually only absorb the thrust (blind plate force) generated by internal pressure, and are not used to limit displacement. Adjusting too tightly restricts the bellows from working properly. Correct practice: screw the nut until it just touches the washer, and then return half a turn to ensure it can slide freely.
Pit three: neglected drainage/steam exhaust.There will be a cavity between the external pressure housing and the bellows. If steam or condensed water is transported, a drainage port must be set in this place, otherwise the accumulated water will corrode the bellows, and it will freeze and crack at low temperature. Remember to leave the position of the drain valve when installing.
Pit Four: A pony pulls a big cart.Choose the external pressure type and think everything is okay? Not right. If the design of the pipe support is not reasonable, the expansion joint is subjected to additional bending or torsional moment, and it will still be scrapped early. Although the external pressure type has strong compression resistance, its bending resistance is not much stronger than that of the internal pressure type. Pipe guide brackets and fixed brackets must be arranged in accordance with design specifications.
What do you see for daily inspections?See whether the shell is bulging, whether the weld is cracked, and whether the tie rod nut is loose. It doesn't matter if the bellows can't be seen. If the shell is not broken, it means that the bellows is probably fine.
After understanding these, you can look back at the question "What is the external pressure metal corrugated expansion joint?"-it is not just a product, but also a set of engineering solutions for high pressure and large displacement conditions. Stop mixing up internal pressure and external pressure.