What exactly is a free metal expansion joint? Is it the same thing as ordinary corrugated expansion joints?
There are many names in the industry, some are called metal compensators, and some are called bellows expansion joints. In fact, free metal expansion joint is a device that absorbs the thermal displacement of the pipe by elastic deformation of the bellows. The core is the bellows — it has to withstand pressure, corrosion, fatigue, and it has to be flexible and flexible. You go see what we have on the stationUniversal corrugated expansion joint、High temperature axial expansion jointBasically, they all belong to this category. The difference lies in the word "freedom": it means that the bellows has no additional constraints or guiding structures, and mainly relies on its own stiffness to resist internal pressure thrust. Therefore, the requirements for fixing brackets are relatively high. This is the same as the one with the tie rodCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointNot the same, the latter relies on a tie rod or a balance ring to share the thrust. Remember: Free metal expansion joints are not a panacea. They are best suited for scenarios where axial displacement is dominated. If the lateral or angular displacement is large, you will have to switch to another type.
Which occasions must it be used?
To be honest, steam pipelines, hot oil pipelines, main steam systems of power stations, hot air pipelines in cement industry-as soon as the temperature goes up, the pipelines are hot and elongated, which is called a fierce. If you don't install the expansion joint, the weld cracks, flange leaks and bracket deformations will come to your door in minutes. I met a customer two days ago, and the hot air duct of the cement kiln head was usedMetal Corrugated Expansion Joints in Cement IndustryThe free metal expansion joint structure was selected, but the result was that the cold tightness was not calculated well, and it was directly jacked to death during test run. Therefore, it is not enough to know which type to use-working temperature, pressure, media corrosiveness, and displacement direction (axial, transverse or angular) must be clarified. For example, the power station industryCorrugated expansion joint for power station industryUsually, it should be equipped with a guide tube to avoid high-speed steam flushing the bellows; However, the expansion joint used behind the desulfurization flue gas baffle door has to be acid-and alkali-resistant, and the non-metallic or PTFE-lined scheme is more reliable. You can't use free metal expansion joints to deal with strong acid smoke, can you? That bellows pierced in three days.
How to choose a model without rolling over?
The first step is to determine the amount of displacement: is it mainly axial expansion and contraction or lateral deflection? Free metal expansion joints are good at absorbing axial displacement, but if you need to compensate for lateral displacement, it's best to useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 The second step is to calculate the pressure: the pressure bearing capacity of the bellows is limited, so multi-layer bellows or reinforcing rings should be used in high-pressure applications, such asLarge diameter thick wall expansion joint。 Step 3, look at the installation space: Not enough space?External pressure single axial expansion jointIt can make the bellows external and reduce the axial length. And direct buried pipeline, we have to use itDirect buried (fully buried) type expansion jointWith anti-corrosion and waterproof structure. When choosing a model, you can refer to the "Expansion Joint Model and Size" Q&A on our site, which lists the common specifications. Don't pat your head in the office. If you are not sure, ask the manufacturer for a calculation book.
The easiest moths in the installation process
First, cold tightening-the compensator is pre-stretched or pre-compressed when it leaves the factory. The site installation should be adjusted according to the design value, and do not weld to death directly. A user installedHigh end axial expansion jointIf the cold tightening amount is not right, the bellows will bulge after two months of operation.
Second, guide brackets-free metal expansion joints are unlike those with tie rods that can resist thrust by themselves. Pipelines must be provided with guide brackets in front and behind them to prevent instability. What is the spacing? There are formulas in the pipeline design manual, so don't save it.
Third, the direction of the guide tube-many expansion joints have arrows on them, pointing to the direction of media flow. Installing the guide tube backwards will block the bellows, resulting in local erosion and wear (refer to "The direction of the arrow of the expansion joint refers to" in the question and answer). And guess what? Once the customer installed the deflector backwards, and the bellows was flushed through in half a year, and it was finished after replacing it with a new one.
Fourth, tie rod screw-if it is a model with a tie rod (such asLarge tie rod expansion joint), remember to remove the transport screw after installation! If you don't dismantle it, you don't install it, and the expansion joint can't move at all. This question is clearly explained in the question and answer "Does the screw of the expansion joint need to be removed?", but some people always forget it.
How to guarantee longevity and reliability?
In the final analysis, bellows are thin-walled parts, and their fatigue life is directly linked to the number of displacement cycles. When designing, it is recommended to select the type according to the actual operating conditions, and don't press the safety factor to the dead-too many bellows layers will affect the flexibility. The daily inspection focuses on three points: whether there is any abnormal noise (maybe the bellows is unstable), whether there is any leakage (weld or bellows crack), and whether there is any abnormal bracket displacement (the fixed bracket is loose). Under normal use, it is not a problem for free metal expansion joints to run for ten or eight years, but note that stainless steel should be replaced in corrosive environments, and creep should be considered in high temperature situations. The Q&A on our site also has the topic "Service Life of Expansion Joints", which can be flipped through. Another point: Don't save money, the material, wave pitch and wall thickness of the bellows are a little worse, and the life is ten times worse.
In short, selection is not filling in a form, you have to understand the working conditions thoroughly. The life of the pipe, half on the expansion joint.