Thermal expansion and cold contraction of the pipe, hard pulling and hard pulling is definitely not OK
When the factory pipeline is running, as soon as the temperature rises, the pipe extends; Once the temperature dropped, it retracted. Coupled with the vibration caused by pumps and fans, the foundation of the pipeline itself settles-if there is no place to release these displacements, the stress will all be held on the pipe wall and welding joint, and problems will occur sooner or later. And guess what? Metal expansion joints are specifically for this. It relies on the elastic deformation of the bellows to absorb all the thermal displacement, mechanical vibration and pipeline deviation, so that the pipeline system should move and the pipeline system should not move.
To put it bluntly, metal expansion joints are the "joints" of the pipeline system. Without it, the steam pipe heats up by tens of degrees, and a few meters long pipe can break the bracket and crack the flange. So you see, in power stations, cement, chemical industry, metallurgy and other industries, almost all pipelines with a little temperature and pressure can see its shadow. The common types of general-purpose corrugated expansion joints and axial expansion joints can cover most working conditions because they have done a solid job of "absorbing displacement" at the core. This is also the first reason why metal expansion joints are popular-they are useful enough and have a wide range of applications.
Type selection is not a pat on the head, different working conditions are far different
But popular is popular, and the selection can't pass, and no matter how good the expansion joint is, it is useless. Different industries and different media, the selection logic is completely different.
In the power station industry, the main steam pipeline and reheating pipeline have a temperature of five to six hundred degrees Celsius and a pressure of more than ten MPa. Ordinary rubber parts can't bear it at all, so corrugated expansion joints or high-temperature axial expansion joints have to be used in the power station industry. This kind of product requires high temperature resistant alloy materials and wall thickness calculation, and is corrugated enough to withstand repeated thermal cycles without fatigue. What about the cement industry? The dust is large and the wear is severe. If the metal corrugated expansion joint does not add a guide tube, the particulate matter in the smoke directly washes the corrugation, and it can be worn out in a few months. At this time, the guide tube is not an optional part, but a life-saving configuration.
Besides, the desulfurization system, the flue gas is highly corrosive, and the desulfurization flue gas baffle door and rectangular expansion joint specially serve this working condition. The rectangular section is to match the flue shape and reduce local drag; The material should be resistant to chloride ion corrosion. At this time, you can take general-purpose products to set them, just wait for rework.
What about the pull rod? When must it be matched? When the pipeline pressure is high, the blind plate force is large, or the lateral displacement needs to be constrained, the tie rod comes in handy. Large tie rod expansion joints and compound hinge transverse expansion joints, these models with restrained structures are designed to prevent pressure thrust from flying the pipeline. Remember one sentence: choosing expansion joints is not buying general parts, but tailor-made according to working conditions.
Don't confuse metal expansion joints with rubber compensators
I met a customer two days ago and asked if metal expansion joints and rubber compensators were the same thing. Alas, the difference between the two is so great.
The rubber compensator is made of rubber + cord cloth, which has good elasticity and strong vibration damping effect, but it is hard in temperature and pressure resistance. Generally, the operating temperature is just over 100℃, and the pressure is about 1.0MPa. Used in low-pressure and large displacement scenarios such as HVAC circulating water and pump inlet and outlet, it is really fragrant-cheap, flexible and good vibration isolation. But if you use it on a steam line, it won't last three months. The rubber will age and crack, and the air will leak sooner or later.
Non-metallic expansion joints (fabric fiber expansion joints) are another way to go. It is resistant to corrosion and low temperature, but has weak pressure bearing capacity, and is mainly used in flue gas systems and low-pressure air ducts. The advantages of metal expansion joints are temperature resistance, pressure resistance and life-stainless steel bellows can bear hundreds of degrees of high temperature and tens of MPa pressure, and can be used for more than ten years if properly designed.
Therefore, it is not "which is better" at all, but "it is good to use the right place". Metal expansion joints are popular precisely because they have gnawed down the hardest bones (high temperature, high pressure, large displacement). It doesn't make sense if you compare the price with rubber; But when you calculate the full life cycle cost, the frequency of replacement of metal parts is low, which is more cost-effective.
There are more pits in installation than in type selection
If you choose the right product, if the installation goes wrong, you will still spend money for nothing. How to install large tie rod expansion joint? Do you want to remove the expansion joint tie rod nut? What exactly does the direction of the arrow mean? These questions I get on the phone every day.
Let's start with pre-stretching. For pipelines installed at room temperature, after operation, the temperature rises and the pipe elongates. If the expansion joint is not pre-stretched from the factory, half of its absorption will be wasted. The correct way is to pre-stretch the bellows during installation, so that it has a certain compression reserve in the cold state and just returns to the neutral position in the hot state. However, note that pre-stretching is not done in all working conditions-it depends on how the compensation amount is distributed, and don't fit all.
Let's talk about the guide bracket. The expansion joint can only absorb axial displacement, and if you let the pipe deflect laterally near it, the bellows will quickly twist and damage. Therefore, guide brackets must be set on both sides of the expansion joint. The first bracket is more than 4 times the pipe diameter from the expansion joint, and the second one is more than 14 times the pipe diameter-this is an empirical data and a rule.
And that tie rod nut, many people don't know how to loosen it after installing it. The tie rod nut is locked when it leaves the factory, which is used to fix the bellows against transportation deformation. After installation in place, the nut must be loosened to the specified position to allow the expansion energy saving to expand and contract freely. The nut is not loose, and the expansion joint is a rigid short joint. If it is installed, it is equal to not being installed. What about the direction of the arrow? That's where the medium flows, and reverse clothing will impact the ripples, causing local vortices and vibrations.
If you install it wrong, no matter how good the expansion joint is, it will be useless. This really isn't scaring people.
With more and more complex working conditions, metal expansion joints are "preferred" for a reason
In recent years, the pipeline system has become larger and larger, the temperature and pressure parameters have become higher and higher, and directly buried pipelines and large-diameter pipelines have emerged in an endless stream. The metal expansion joint is also popular because it continues to grow "new skills".
You see, the directly buried (fully buried) expansion joint is specially designed for directly buried heating pipelines, and the outer protection pipe directly carries soil corrosion and groundwater; Large-diameter thick-walled expansion joint, with a pipe diameter of several meters and a wall thickness of tens of millimeters, can still absorb displacement; What about rotary compensators, which absorb thermal elongation by rotating the angle, are used on overhead pipe networks to save brackets than traditional expansion. Instead of changing the size of these products on the original basis, the structure, materials and sealing forms are all upgraded. Coupled with the double hinge expansion joint of air-cooled island vacuum pipeline and the pressure balance expansion joint of curved pipe, which are specially designed to overcome difficult working conditions, factories have more and more choices.
But many choices also mean that judgment is difficult. Exactly Which Plumbing System Is Right For Your Home? Is a universal corrugated expansion joint enough, or is it necessary to externally pressure a single axial expansion joint to reduce the pressure thrust? Is it just regular carbon steel, or does it have to be 304L or even 316L? If you are not sure, ask the manufacturers more about the working conditions and ask them to produce calculation books and selection suggestions, which is much more reliable than checking online for half a day.
In the final analysis, metal expansion joints are popular not because of their nice name, but because they can hold up, last for a long time and save maintenance costs in real knives and guns. Did you really choose the right plumbing system for your home?