The most problematic areas in the pipeline system are often not pumps or valves, but those seemingly unremarkable flexible connectors. Usually, no one cares about the soft connection expansion joint of metal. Once it leaks, cracks or breaks through fatigue, the production line stops, and the loss is tens of thousands or even hundreds of thousands. Today's matter has to be broken into pieces and explained clearly.
First distinguish the concept: are metal hoses, metal soft connections and expansion joints the same thing
The purchase list says "metal soft connection expansion joint", but when you get to the shelf, you will find metal hoses, PTFE-lined metal hoses, universal corrugated expansion joints, external pressure single axial expansion joints... The first reaction is confusion. These three names are often mixed, but they are really not the same thing.
Metal hose emphasizes "pipe", and its body is corrugated pipe with mesh sleeve, which is mainly used to absorb vibration and compensate for installation deviation. Generally, it is short in length and small in diameter, such as gas corrugated pipe and vacuum special hose.
Expansion joint emphasizes "joint", which is a complete pipe element, including bellows, end pipe, guide tube, tie rod and other structures. General-purpose corrugated expansion joints, directly buried (fully buried) expansion joints and high-temperature axial expansion joints all belong to this category. The term metal soft connection is more biased towards functional description-products that function as flexible connections.
So, are you buying an expansion joint for thermal displacement compensation in pipes, or a metal hose for vibration damping at equipment interfaces? Figure out this problem first, so that the direction will not go wrong.
First look at the medium, second look at the temperature, and third look at the pressure: three parameters that must be clarified before selecting the type
"How much is the expansion joint of DN200?"-This is like asking "How much is a house of 200 square meters?" I can't answer it. The first three parameters of selection must be clarified: medium, temperature and pressure.
The media determines the material.Is steam, hot water, flue gas or corrosive media transported? No problem with 304 stainless steel bellows for ordinary steam pipes; However, if it is the wet flue gas in the desulfurization system, which contains high concentration of chloride ions, which is easy to cause stress corrosion cracking, it is necessary to consider 316L or even higher grade corrosion-resistant alloys. If the medium is strong acid and alkali, the conventional metal bellows can't bear it at all. At this time, it is necessary to line PTFE hose or PTFE compensator.
Temperature determines the allowable stress of the material.The upper temperature resistance limit of metal bellows depends on the corrugated material. The performance of 304 stainless steel is very good at room temperature, but the creep accelerates after exceeding 400℃, and the permissible stress decreases obviously. The temperature of the main steam pipeline of the power plant and the waste heat pipeline of the kiln head of the cement industry is always five or six hundred degrees or even higher. High-temperature axial expansion joints must be selected, and heat-resistant alloy should be used as the corrugated pipe material. If the temperature is calculated incorrectly, the stress relaxation of the bellows occurs at high temperature, and the compensation amount will be greatly reduced.
Pressure determines wall thickness and wave number.The higher the pressure, the thicker the wall thickness of the bellows, but the increase of wall thickness will lead to greater stiffness and decreased compensation ability. Therefore, under high-pressure working conditions, the compensation amount is often compensated by increasing the wave number. Here is an engineering experience: when the nominal pressure exceeds 2.5MPa, the structural form of the conventional bellows has to be re-checked, and it may have to be replaced with a single axial expansion joint with external pressure-the external pressure structure makes the bellows bear external pressure, which has better stability and is less prone to column instability.
These three parameters do not exist in isolation, they are coupled to each other. For example, when high temperature + high pressure + corrosive media appear at the same time, the difficulty of selection is directly doubled. At this time, you can't just look at the product sample, but you have to let the manufacturer do stress analysis and fatigue life check with the working condition parameters.
From general-purpose type to directly buried type: Structural differences of metal soft-connected expansion joints under different working conditions
The same pipeline displacement compensation requirements, different working conditions, the expansion joint structure is completely different.
Universal corrugated expansion jointIs the most common, single-type structure, mainly absorbs axial displacement, suitable for straight pipe sections. It consists of a corrugated pipe and two end connecting pipes, which has simple structure and low cost. However, its internal pressure thrust needs to be withstood by the pipe bracket, so the main fixing bracket must be installed during installation. Don't underestimate this. Just follow the bracket drawings issued by the design institute, and don't change them casually.
Direct buried (fully buried) type expansion jointIt is used for directly buried heating pipelines, directly buried in the soil, without setting up an inspection well. It has a protective sheath on the outer layer, and thermal insulation material is filled between the inner bellows and the outer guard. This expansion joint has extremely high requirements for waterproofing. Once the groundwater seeps in, the failure of the insulation layer is small, and the corrosion of the outer wall of the bellows is fatal. Therefore, the end sealing structure of the directly buried expansion joint is very critical, so special attention should be paid to the sealing process of the manufacturer when purchasing.
Compound hinge transverse expansion jointAndCompound straight pipe bypass pressure balanced expansion jointIt solves a different problem. The former uses two bellows and hinge structure to absorb lateral displacement and do not produce internal pressure thrust; The latter balances internal pressure thrust through a bypass pipe and is suitable for situations where thrust is not allowed to be transmitted to a fixed bracket, such as turbine inlet and outlet pipes. In another case, there is a turn in the direction of the pipe, and at this timeCurved tube pressure balance expansion jointIt comes in handy-it comes with its own balanced bellows, which absorbs the combined displacement of curved sections.
Therefore, selection is really not as simple as flipping through the sample and picking a size. Is the installation location overhead or directly buried? Does the pipe go in a straight line or a turn? Can the bracket withstand internal pressure thrust? Don't hurry to inquire before you figure out these questions.
Installation and tie rod adjustment are not standardized, no matter how good the expansion joint is, it will be useless
If you choose the right model, there is a problem in the installation link, and all your previous efforts are wasted. A typical example: the expansion joint leaves the factory with the tie rod nut in a locked state to prevent the bellows from being stretched or compressed during transportation and hoisting. However, at the scene, some installation teams directly welded the expansion joint with the tie rod to the pipeline, forgetting to loosen the tie rod nut.
And the result? After the pipeline heats up, the thermal expansion cannot be absorbed by the bellows, and all the thrust acts on the tie rod. In light case, the tie rod is bent and deformed, and in worse case, the bellows is forcibly cracked. Many steam pipelines leaked shortly after they were put into operation, and it was found that this was the reason. What about that? After the expansion joint is installed, the tie rod nut must be adjusted to the designated position to allow the bellows to be free, or to the specified value according to the pre-tension/pre-compression requirements. This tie rod is for transportation protection, not for you to be a permanent stress member.
Another common mistake is that the deflector is installed in the reverse direction. The function of the guide tube is to reduce the direct scouring of the inner wall of the bellows by high-speed fluid, and at the same time reduce the flow resistance. The direction of its opening must be directed towards the direction of the medium flow. If it is installed backwards, the medium directly washes the root of the bellows, and it will not take long to wear out. The direction of the arrow on the pipe is clearly marked, just follow it.
The pipes on both sides of the expansion joint must be provided with guide brackets to ensure that the displacement occurs along the design direction. If the pipe twists, the bellows are subjected to additional torque, and this failure mode is very insidious – the surface looks like the bellows are intact, but there are already micro-cracks at the trough.
Common Failure Modes and Service Life Judgment of Metal Soft Joint Expansion Joints
Bellows failure never happens suddenly, it has precursors.
The most common failure modes areFatigue crack。 Every time the bellows expands or contracts, the corrugated root bears an alternating stress. When the number of cycles is up, microscopic cracks will appear, and then gradually expand through the tube wall. What is the basis for judgment? Look at the cumulative displacement cycles. For example, the design life is 10,000 full displacement cycles, but if the actual working conditions start and stop frequently and the temperature fluctuates greatly, it may reach the life in two or three years.
Secondly,Stress corrosion cracking。 This is the most troublesome because there is no obvious thinning from the appearance, and the crack propagates along the grain boundary, is dendritic, and generally appears at the trough. Austenitic stainless steel is sensitive to chloride ions, and the risk of stress corrosion increases significantly in the environment above 60℃ and the chloride ion concentration exceeds 100ppm. Judgment method: If there are filamentous cracks on the surface of the bellows, and there is no obvious plastic deformation around the cracks, it is probably stress corrosion.
The third isInstability。 The column instability or plane instability of the bellows occurs under high pressure, which is manifested as lateral bending of the bellows or single corrugated depression. This situation is usually because the pressure exceeds the design value, or the bellows is twisted during installation. Once instability occurs, the bellows is basically scrapped and has no repair value.
Then how to judge whether the expansion joint should be replaced? Give a few hard indicators: penetrating cracks appear on the surface, leak, change; If the bellows has indentation, bulge or obvious deformation, change it; If the compensation amount fails to meet the design requirements, the pipeline vibrates abnormally, so change; If the service life exceeds the design fatigue life, it is recommended to arrange maintenance and replacement. Under the working condition of corrosive medium, the wall thickness of corrugated pipe should be tested every time it is overhauled. If the wall thickness is reduced by more than 20% of the design value, it should be replaced with a new one directly.
Finally, it is the same sentence: the metal soft connection expansion joint is a mature industrial product, and the selection idea is nothing more than the matching of four variables: medium, temperature, pressure and displacement. But it is the details that really open the gap. If you spend an extra half hour on model selection, you may save a few days of discontinued production and hundreds of thousands of repair costs. Calculate this account yourself.