The name of shock-absorbing metal expansion joint has been shouted a little widely in recent years. Many users come to inquire with the word "shock absorption". When they ask about the working conditions, they can't explain why. As soon as they come up, they want the most expensive one. Alas, that's really not how selection works.
What vibration does this equipment "reduce", how much pipe displacement, how much rigidity the system allows, and whether the installation space is enough-these are not clear, and it will be scrap iron when bought back. Today, I will break these five things apart and explain them clearly.
1. What does the shock-absorbing metal expansion joint "reduce"?
Mechanical vibration (direct transmission from pump, compressor, fan), fluid pulsation (fluctuation of medium pressure, water hammer, air column resonance), and secondary stress release caused by thermal expansion and contraction. Many people think that installing a corrugated expansion joint can solve all three things, which is a big mistake.
Bellows are essentially displacement compensating elements whose core task is to absorb thermal displacement-axial expansion and contraction, lateral deflection, angular rotation. Shock-absorbing metal expansion joints do have vibration isolation capabilities, but that is only a "by-product". To really isolate vibration, both stiffness and damping have to be looked at. The bellows has low stiffness and small damping, which can isolate some high-frequency vibrations, but it has almost no choice for low-frequency mechanical vibrations.
The expansion joint is a "flexible connection", not a "shock absorber"。 It really can't do the work of using the bellows as a rubber shock-absorbing pad and expecting it to eat all the vibration of the pump unit. If the pipeline vibration problem is serious, it should be a metal hose with a spring shock-absorbing bracket, or a rubber compensator for secondary vibration isolation. Don't let the bellows take the blame.
Second, structural disassembly: every part is not given for free
The shock-absorbing metal expansion joint looks simple, but when it is disassembled, there are many doorways.
BellowsIs the core, it determines flexibility, stiffness and fatigue life. Multi-layer corrugation has higher pressure resistance than single-layer corrugation, but it is also more stiff, which is not good for shock absorption-here is a trade-off.
guide tubeNot an ornament. It reduces the flow resistance of the medium, protects the inner wall of the bellows from being washed by high-speed fluids, and — many people don't know — it can suppress fluid-induced vibrations. The gap between the guide tube and the bellows is not well designed, which will instead induce vortex-induced vibration, which belongs to the design detail.
tie rodThe function is to constrain. The tie rod restricts the bellows within the set displacement range to prevent instability caused by excessive stretching. The expansion energy saving with the tie rod bears the internal pressure thrust, but the tie rod nut is not adjusted properly, which will change the actual force state of the bellows.
External pressure sleeveIs another idea: put the bellows outside the medium, and pass the medium inside the sleeve. The advantage is that the bellows is not limited by the instability of the internal pressure column, the single wave compensation is larger, and the sleeve itself is a layer of sound insulation. Therefore, the external pressure single axial expansion joint is especially used in steam pipelines.
Under the vibration condition,Universal corrugated expansion jointAndCompound hinge transverse expansion jointAre the two models that appear most frequently. The former has simple structure and low stiffness, which is suitable for absorbing axial displacement; The latter uses two bellows and hinge structure to convert the displacement into angular displacement, and has excellent lateral compensation ability. It is especially suitable for pipelines with limited space and large lateral displacement.
Third, the first step of type selection: first calculate the thermal displacement of the pipeline, and then talk about shock absorption
I answered a phone call that day, and the customer asked, "How much is your shock-absorbing metal expansion joint, DN800?" I said how big is the thermal displacement of your pipe? He froze.
Displacement doesn't even count, what type to choose?
Large axial displacement, take priorityUniversal corrugated expansion joint、External pressure single axial expansion joint, orStraight pipe pressure balanced expansion joint— — The latter is mostly used in situations where internal pressure thrust needs to be completely eliminated, such as feed water pump outlet and steam turbine interface.
Large lateral displacement, withCompound hinge transverse expansion jointIt has small lateral stiffness and can also absorb part of the angular displacement. If the horizontal pipe section is to be displaced both laterally and axially, it is necessary toCompound straight pipe bypass pressure balanced expansion jointThe price is indeed expensive, but the working conditions don't allow you to make do.
Predominantly angular displacement?Curved tube pressure balance expansion jointIs the standard answer, used on L-shaped or Z-shaped pipe sections, by the combination of angular displacements to absorb thermal expansion.
First determine the type and magnitude of displacement, then select the structure type, and then check the stiffness and fatigue life。 Pick the most expensive one as soon as you come up. That's not a selection, but a show of wealth.
4. What determines the shock absorption effect: stiffness is a hard index
The vibration transmittance is related to the frequency ratio and the damping ratio. The excitation frequency is determined by the equipment speed, and you can't change it. The only thing you can change is the natural frequency of the system-keep the natural frequency away from the excitation frequency, and the vibration isolation effect will come out.
While the natural frequency is proportional to the square root of the stiffness. The lower the rigidity of the bellows, the lower the natural frequency and the wider the vibration isolation band. Therefore, when the shock absorbing metal expansion joint is selected,Stiffness must be written into the technical protocol as a critical parameterNot just by looking at the amount of compensation.
In actual engineering, many design institutes will string another section next to the corrugated expansion jointMetal hoseOrrubber compensatorDo secondary vibration isolation. The metal hose has good flexibility and can absorb installation deviation and high-frequency vibration; The rubber compensator has great damping and has wonderful effects on medium and low frequency vibrations. Two matching, each tube a section, the effect is much stronger than a single installation of a shock-absorbing metal expansion joint.
However, the rubber compensator cannot be used on high-temperature media, and the temperature resistance generally does not exceed 120℃. If this number is exceeded, or the medium is corrosive, only useNon-metallic expansion joints (fabric fiber expansion joints)。 The stiffness of non-metallic expansion joint is one or two orders of magnitude lower than that of metal corrugated pipe, and the vibration isolation effect is naturally good, and it is resistant to high temperature and corrosion. It is basically the flue gas pipeline of power plant and the kiln head waste heat pipeline of cement plant. The cost is that the pressure bearing capacity is weak, so it can only be used in low pressure or even slightly negative pressure applications.
5. Pits in installation and maintenance: one step is wrong and the whole disk is lost
Choose the right model, install the wrong still leaks.
Let's startPre-stretching。 Many shock-absorbing metal expansion joints have been pre-stretched when they leave the factory, so they can't be pulled again casually during installation. If you think "it's safer to pull it", if you pull it too far, the bellows will directly deform plastically, and the life will drop from a cliff.
Againtie rod nut。 The function of the tie rod nut is to adjust the initial compression amount of the bellows. If it is not correctly adjusted, the bellows will be unevenly stressed, and one side will be stretched and the other side will be compressed, which will soon cause fatigue cracking. Others asked, "Does the screw of the expansion joint need to be disassembled?"-if it is a transportation protection screw, it must be disassembled after installation, otherwise the expansion joint is equal to a display, and no displacement can be compensated, and the pipeline stress is fully pressed on the equipment at both ends.
The easiest thing to overlook isguide bracket。 The expansion joint can only absorb axial displacement. If the pipeline is not provided with a guide bracket near the expansion joint, the pipeline will bend laterally as soon as it expands thermally, and the bellows will bear the shear force and bending moment that should not be beared. Can it not crack?
The shock-absorbing metal expansion joint of steam pipeline leaked after two months, and the owner scolded the product as garbage. At a glance at the scene, there is a guide bracket missing 30 cm away from the expansion joint, and the pipeline is laterally deviated by more than 40 mm in the hot state, which is purely an installation problem.
The other is the kiln head pipe of cement plant, the non-metallic expansion joint is old and bad, and it has been replaced three times. What's the reason? Installation did not leave enough thermal displacement margin, and the expansion joint was forcibly torn. Designers take it for granted to cut the material according to the installation size at room temperature, and drag the expansion joint as soon as the pipeline heats up.
These cases are not isolated. Expansion joints are often wrongly accused of being "leakage points". In fact, more than 70% are problems of type selection or installation.
Selecting shock-absorbing metal expansion joint is essentially to find a balance between three dimensions: compensating displacement, reducing stiffness and controlling cost. Calculate the displacement clearly first, then talk about the model, stiffness and installation scheme-the order should not be chaotic. The money that should be spent is spent wisely, and the wrongful money that should not be spent is not paid for a penny. This is the serious selection logic.