First, let's make clear a misunderstanding: it's not a bellows, it's a sleeve
Many friends who do pipeline design in power plants and cement plants default to bellows when it comes to expansion joints. The name metal sleeve expansion joint sounds a little twisted, but it is completely different from the corrugated expansion joint. It is also called sleeve-type pipe expansion joint. Its core structure is a thick pipe sleeved with a thin pipe, and the middle is sealed by packing, so that the inner pipe can expand and contract freely in the sleeve. Such a simple thing is the protagonist of absorbing axial displacement in the pipelines of power plants and cement plants.
Why not use bellows? In high-temperature and high-pressure steam pipelines, the corrugated pipe is affected by thermal fatigue and stress corrosion, and its life often can't last the overhaul cycle. However, the metal sleeve expansion joint absorbs displacement through sliding friction, has no corrugation fatigue problem, is resistant to high temperature and wear, and has a simple structure-think about it, how can an expansion joint without corrugation have fatigue?
Structural disassembly: five components, each doing its own job
The metal sleeve type expansion joint consists of a sleeve, an inner tube, a sealing packing, a guide ring and a gland. The sleeve is an outer shell, and the inner tube slides in the sleeve with the thermal expansion and cold contraction of the pipeline; The guide ring ensures that the inner tube is not eccentric and stuck when sliding; The packing material is filled in the annular space between the sleeve and the inner tube to act as a seal; The gland is used to compress the packing and prevent the media from leaking.
Metal sleeve type seals rely on packing, not bellows. The material of the filler, the filling density and the tightness of the gland directly determine that the whole expansion energy saving can not be used. Bellows seal is the flexible metal wall absorbs displacement by elastic deformation, and once the corrugation cracks, it is catastrophic leakage. Even if there is a slight leakage in the packing seal, the seal can be restored by tightening the gland on the spot-this difference is a world of difference in the high-temperature steam pipe network.
Don't confuse the metal sleeve type with the general-purpose corrugated expansion joint. The universal corrugated expansion joint realizes three-dimensional displacement compensation by corrugation deformation, while the metal casing type only does axial expansion and contraction. It has no lateral stiffness and no angular deflection ability, and the selection is used in the wrong place, which is to dig a hole for itself.
Three parameters of model selection: compensation amount, pressure level and temperature
When choosing a metal sleeve expansion joint, three parameters must be taken: compensation amount, pressure grade and medium temperature.
How to calculate the amount of compensation? Use the thermal expansion formula of the pipe Δ L = L × α × Δ T, L is the length of the pipe between the two fixed brackets, α is the linear expansion coefficient, carbon steel is about 0.012 mm/ (m·°C), and Δ T is the operating temperature minus the installation temperature. For example, a 100-meter steam pipe with an operating temperature of 300°C and an installation temperature of 20°C has a thermal elongation of 100×0.012×280=336mm. This is the total amount of axial displacement that the metal sleeve expansion joint needs to absorb.
The pressure level is easy to understand. What is the design pressure of the pipeline? The expansion joint has to be determined according to this pressure. The medium temperature determines two things: one is how to choose the material of the inner tube and sleeve, and the other is what type of packing is used. Ordinary carbon steel and asbestos fillers can still be used below 400°C, but heat-resistant steel and flexible graphite fillers have to be replaced above 400°C.
Guide bracket. The metal sleeve expansion joint can only absorb the axial displacement, and the transverse displacement of the pipeline must be restrained by the guide bracket. The spacing of the brackets is arranged according to the pipeline design specification, and the distance between the guide bracket and the expansion joint shall not exceed 4 times the nominal diameter, so as to ensure the smooth sliding of the inner pipe and no stiffness.
The easiest pits in installation and operation and maintenance
Many on-site construction teams don't know about the operation of cold drawing pre-deformation at all. The original intention of cold drawing is to pre-compress the expansion joint by half of the compensation amount during installation, so that the expansion joint can work near the middle position after the pipeline runs, with margin at both ends. However, it should be noted that the cold drawing amount cannot exceed half of the compensation amount, and it must be operated before the pipeline is installed and the pressure test is completed. A few days ago, I met a customer. After the pipeline heated up, the expansion joint died. After checking the reason, the direction of cold drawing was reversed-alas, this is a typical construction disclosure that was not done in place.
The fastening of the packing gland is also particular. It is not the tighter the more dead the better. If the packing is pressed too tightly, the sliding resistance of the inner tube will become greater, which will accelerate the wear of the sealing surface. The correct method is: the gland bolt is uniformly stressed during installation to ensure that the packing is not loose and leaky; If leakage is found after operation, tighten it evenly at a small angle. Do not tighten on one side, so that the gland is tightly crooked.
Do not use metal sleeve expansion joints as hinges. It has no angular compensation ability. If the pipeline has lateral displacement or angular deviation, it can be pulled hard by flange bolts, and the consequence will be eccentric wear of the sealing surface and fracture of the guide ring. If you really want to absorb the lateral displacement, you should honestly choose the lateral expansion joint of compound hinge, or add the corrugated expansion joint.
Collaborative use: Practical case of steam pipe network and flue gas pipe
The metal sleeve expansion joint is routinely used alone, but it is often paired with other compensators. For example, in the long-distance steam pipe network, a metal sleeve expansion joint is used in the middle of the straight pipe section to absorb the axial displacement, and a rotary compensator is arranged near the elbow to solve the directional displacement. The combination of the two is much lower in cost and durable than all corrugated pipes.
Contains dust, sulfur and frequent temperature fluctuations. Metal sleeve type expansion joint is used with external pressure single type axial type expansion joint. One can solve the axial expansion and contraction of large displacement, and the other can absorb the small displacement of high frequency. And guess what? This is how many cement plant kiln head flue gas pipes are dry, and it is much worry-free to use them than to use all corrugated pipes.
But on the other hand, if the pipe medium is a highly corrosive liquid, or absolutely zero leakage is required, then the metal sleeve type has little advantage. In this working condition, it is the right medicine to choose a PTFE metal hose or a rubber PTFE compensator.
How exactly to choose? Give me a way of thinking without detours
First, whether the pipeline has large displacement axial expansion and contraction, yes, consider the metal sleeve type;
Second, whether the medium temperature and pressure are high or not, exceeding 450°C or 2.5MPa, the structure and material cost of metal sleeve type should be recalculated;
Third, there is no lateral displacement of the pipe system. Yes, don't use the sleeve type;
Fourth, the leakage requirements are strict or not, strict, priority is given to bellows, lax, metal casing type is more economical.
In the final analysis, metal sleeve expansion joints are not universal glues, nor are they obsolete goods. It has the advantage that the corrugated expansion joint cannot replace in the environment of high temperature, large displacement and large dust, such as the main steam pipeline of power plant and the flue gas pipeline at the tail of kiln of cement plant. Understanding it and making good use of it is much more reliable than blindly pursuing "tall" bellows.