For more than ten years in the dry pipeline industry, every once in a while, someone asks with drawings: Should this place install metal hoses or expansion joints? Can the regular ripples work? When I saw the medium temperature of 350℃, the pressure of 2.5MPa, and the corrosive medium, I knew that this was another example of taking ordinary compensators hard. Today, I will break up the armored metal expansion joint and explain it clearly, so as to save you from taking detours.
What is the difference between armored metal expansion joint and ordinary corrugated expansion joint? Let's look at the structure first
Ordinary corrugated expansion joints (that is, what we often call general corrugated expansion joints) absorb heat displacement by a layer of thin-walled corrugated pipes, which are directly exposed to the outside. What about armored metal expansion joints? Outside the bellows is a protective layer, usually a metal braided mesh or an annular sleeve. This layer of "armor" is not a decoration, it is a serious load-bearing structure.
The bellows is responsible for flexible expansion and contraction, and the outer armor layer is responsible for sharing the internal pressure thrust, resisting external mechanical damage, and blocking some fluid erosion. To put it bluntly, the ordinary bellows is a solo fight, while the armor is the cooperation of two people. From the stress model, after adding armor, the stress level of the bellows itself can be reduced a lot, and it is not blowing to double the fatigue life.
What working conditions must be armored? High temperature, high pressure, corrosive media don't wait for leakage before regretting
When do you have to use armor, you ask? Let me tell you, in these situations, don't hesitate:
- High temperature and high pressure steam pipeline— — When the temperature exceeds 400℃ and the pressure exceeds 1.6MPa, the pressure bearing capacity of the bellows itself is not enough, and the armor layer must rely on to share the pressure. This is basically the way for corrugated expansion joints used in power stations.
- Corrosive media— — In the acid, alkali and salt spray environment, the wall thickness of the bellows is so thin, and it is a big accident to corrode a little. The armor layer can play the role of sacrificing protection, and an additional line of defense.
- Environments where mechanical damage may occur— — Buried laying, dense pipeline areas, and passages where people walk around. If the bellows is smashed and rubbed, it is an internal injury, and the appearance can't be seen. The armored ones are much more leathery.
Two days ago, I encountered a case in a chemical plant. The ordinary corrugated expansion joint used in ammonia pipeline had intergranular corrosion cracks after less than eight months of operation, and hundreds of thousands of losses were caused by pipe replacement and shutdown. Later, it was replaced with an armored structure, and it was still running safely after running in the same working conditions for more than two years. How to calculate this account is up to you.
Which parameters do you focus on in the selection? The diameter, pressure, compensation amount and connection mode are all indispensable
The selection of armored metal expansion joints is similar to that of ordinary expansion joints, but there are several parameters that should be paid special attention to.
Diameter:Needless to say, the DN of the pipeline is as much as it is. However, it should be noted that the expansion joint with armor layer should have a circle larger than the ordinary one when the diameter is the same, so the layout space should be reserved in advance. Otherwise, it would be really embarrassing if the goods arrived and couldn't fit in.
Design Pressure:Look at the rated pressure, but also the test pressure. The advantage of armored expansion joints is that they can withstand higher pressure levels at the same size. For example, the armored structure of DN200 can achieve the pressure level that only the ordinary structure DN300 has, and the advantages are obvious when the pipeline layout is compact.
Compensation amount:Axial compensation, lateral compensation, angular displacement, which one is needed? The armor layer has little effect on the axial compensation, but if the lateral displacement compensation is to be done, the armor layer will limit the bending deformation, so a greater margin should be left in the design. Therefore, it is a basic skill to calculate the displacement of the pipe system before selecting the type.
Connection method:Flanged connection, welded or threaded? The flange connection is convenient for maintenance, but the flange itself is also the leakage point; The welded connection seal is reliable, suitable for high-pressure applications, but it is troublesome to replace. Armored metal expansion joints with large diameter (above DN400) are basically welded or flanged, while small diameter joints can be threaded. According to the on-site installation conditions.
What are the pits for installation and maintenance? Deflector, tie rod nut, fixing bracket, these details determine life
Installing expansion joints is a delicate job, and many people plant them in small details.
Direction of the guide tube. If the guide tube is installed inside the armored metal expansion joint, don't install it backwards. The function of the guide tube is to allow the medium to flow smoothly and reduce the vortex currents to scour the bellows. The direction of the arrow must be consistent with the flow direction of the medium. Reverse installation is equivalent to no installation, and the bellows will still be washed and worn.
How to adjust the tie rod nut? When leaving the factory, the tie rod nut is adjusted according to the installation length. Be careful when disassembling and assembling during transportation and installation. The adjustment amount of pre-stretching and pre-compression has factory marks, and you can adjust it according to the marks. But remember one thing: when testing the pressure of the pipeline, the locking nut on the tie rod must be loosened, so that the expansion energy saving can expand and contract freely. Otherwise, if you install an expansion joint, it is equivalent to installing a rigid short joint, and the thermal expansion and contraction of the pipe will all be held in the system, and the consequences will be thought of yourself. For the specific operation method, please refer to the adjustment specification of expansion joint tie rod.
I have to talk a little bit about fixing the bracket. Fixed brackets must be set on both sides of the expansion joint, and the brackets must be able to withstand the internal pressure thrust of the pipeline, which is considerable under high pressure conditions. If the support is not strong enough, the pipe will be displaced as soon as it runs, and the expansion joint bellows will quickly be broken. The year before last, there was a customer, DN600 steam pipeline, the fixed bracket was casually welded with channel steel, and the expansion joint was cracked on the second day after it was put into operation. Therefore, the position and strength of the fixed bracket (main fixed bracket and guide bracket) must be calculated by design, and must not be patted on the head by experience.
How to distinguish between armored metal expansion joints and metal hoses? Something can happen if you get mixed up
Armored metal expansion joints and metal hoses look a bit alike, both corrugated pipes plus braided mesh, but have completely different uses. Metal hoses are flexible connectors, which are mainly used to absorb installation deviations, vibrations and a small amount of displacement, and the length is generally not long; Armored metal expansion joints are pipe compensators that absorb thermal expansion displacement and are subject to medium pressure and temperature, usually with flanges or welded end pipes at both ends.
Some people use metal hoses as expansion joints on large-diameter steam pipelines. As a result, after less than half a year of operation, the bellows broke in fatigue and high-temperature steam spewed out. Fortunately, no one was hurt. This is not alarmism, it is a painful lesson that has actually happened.
Start by figuring out what features your plumbing system needs. Need to absorb heat displacement? Select the expansion joint. Need a flexible connection? Choose a metal hose. Wear-resistant and pressure-resistant and afraid of corrosion? Then don't worry about it, armored metal expansion joints are a serious choice. Understand these, and you will have a bottom in your heart if you talk to the supplier again.