1. What key information is hidden in the specification sheet? Confusion of nominal diameter, wave number, pressure and other levels
Get a metal expansion joint specification sheet, the first few columns are usually nominal diameter (DN), wave number, pressure rating (PN), and connection mode. Many people glance at the past, and think it's just the size and pressure. What's so good about it? As a result, it was found that it was wrong. Either it leaked or the bellows burst directly.
Don't just look at the numbers for the nominal diameter, which corresponds to the inner diameter of the pipe, but the actual inner diameter of the expansion joint may be slightly different because of the guide tube or wall thickness. For example, the corrugated expansion joint used in the power station industry of DN500 may have an inner diameter of 500mm, but if it is a large-diameter thick-walled expansion joint, the wall thickness will increase, and the actual circulation area will be slightly smaller. The wave number directly determines the compensation amount. The more the wave number, the larger the axial compensation amount, but the corresponding axial stiffness will also decrease. Some manufacturers mark the wave number on the specification sheet as "effective wave number", excluding the straight edge segment at the end. This should be asked clearly.
Stress levels are more critical. The difference between PN0.6 and PN1.6 is almost three times. If you choose PN0.6 on the steam pipe, it is no different from a time bomb. Another point: The pressure on the metal expansion joint specification sheet is usually the design pressure, not the working pressure, so a safety margin must be allowed.
Second, different working conditions, different models: high temperature, large diameter, vacuum... Did you choose the right one?
Two days ago, I met a customer. He asked me about the specification sheet of the general corrugated expansion joint, saying that it would be used in desulfurization flue gas pipelines. I said you should see if there is any special product in the flue gas baffle door. As a result, there is corrosive medium in his working condition, and the temperature often soars to 300℃. General-purpose corrugated pipe is generally stainless steel 304, which cannot withstand chloride ion corrosion. It is necessary to use 316L or higher materials, or directly attach non-metallic expansion joints (fabric fiber expansion joints).
Under high-temperature working conditions, such as the exhaust pipe of power station boilers, high-temperature axial expansion joints must be used, usually made of Incoloy 825 or high-nickel alloy. What about large-bore pipes? Ordinary bellows may not be rigid enough, so it is necessary to use double hinge transverse expansion joint or straight pipe pressure balance expansion joint, which will have a clear "structural form" column on the specification sheet. The vacuum working condition is more special. Ordinary expansion joints may be deflated by atmospheric pressure. Special vacuum hoses or expansion joints with reinforcing rings must be used. The range of "vacuum degree" will be marked on the specification sheet.
Do you think these specification sheets can be done without looking carefully? For the same nominal diameter, the models corresponding to different working conditions are very different. If you choose the wrong one, you can stop the machine or break the pipeline.
Third, several misunderstandings that are easy to step on pits: metal expansion joint ≠ corrugated expansion joint? Is the tie rod nut removed or not?
Does anyone think that metal expansion joints and corrugated expansion joints are the same thing? Tsk, strictly speaking, metal expansion joint is a general term, and corrugated expansion joint is the most commonly used one of them, but there are other structures such as sleeve-type pipe expansion joint and rotary compensator. But in most cases on the market, these two words are mixed, so you don't need to take it too seriously. What is really easy to step on pits is the problem with the tie rod nut.
There is often a line "Is the tie rod nut adjustable" on the specification sheet, and many installers screw it to death when they see the nut. In fact, the function of the tie rod is to prevent the bellows from over-stretching, and the nut should be loosened during installation so that the bellows can expand and contract freely. After installation, adjust the nut to the designated position, leaving the pre-tension amount. If you screw to death directly, the expansion joint becomes a rigid connection, completely for nothing. There was a cement plant project before, and the metal corrugated expansion joint in the cement industry leaked within two days after it was installed. After checking, it was found that the tie rod nut was not loosened, and the corrugated pipe was cracked. So, before installing, be sure to read the "Installation Instructions" column on the specification sheet, don't take it for granted.
4. Teach you step-by-step selection with specification table: from power station, cement to desulfurization flue gas, case disassembly
Let's talk about a practical case. A power station project needs to be equipped with expansion joints for the main steam pipeline, with a temperature of 540℃, a pressure of 9.8MPa and a pipe diameter of DN300. Open the metal expansion joint specification sheet, first look at the pressure level, PN10 or above is enough, but the maximum working temperature of PN16 universal corrugated expansion joint on the specification sheet is only 350℃. Then it won't work. Look at the high-temperature axial expansion joint, the material is Cr-Mo steel, the working temperature can reach 600℃, the pressure grade is PN16, and the wave number is selected according to the compensation amount. Moreover, the steam pipeline has lateral displacement, so it can't only rely on axial expansion joints, so it must be matched with compound hinge transverse expansion joints to absorb lateral displacement. Finally, the combination of high-temperature axial type + compound hinge was selected. There is also a column of "pre-stretching amount" on the specification table, which is adjusted according to the ambient temperature during installation.
Another example is the outlet pipeline of cyclone preheater in cement industry, which has high temperature, large dust and low pressure. Select the metal corrugated expansion joint in the cement industry, and "wear-resistant guide tube" will be marked on the specification table. This must be available, otherwise the bellows will be worn out quickly by powder washing. Desulfurization flue gas pipeline is highly corrosive, and non-metallic expansion joints are more suitable. The specification table will indicate the "temperature range of acid and alkali resistance". In fact, there are three steps to select: confirm the working conditions (temperature, pressure, medium, displacement direction) → look at the specification table to find the corresponding product series → check the key parameters (material, compensation amount, connection size). Don't jump, don't guess from experience.
5. Check the last few rows on the specification sheet before installation: arrow direction, guide tube, pre-stretch amount-don't wait for installation to regret it
The last few lines of the specification sheet are often inconspicuous, but they are directly wasted when installed backwards. The first is the direction of the arrow, indicating the flow direction of the medium. Why have arrows? Because of the presence of the guide tube. The role of the guide tube is to protect the bellows from the medium washing, it only has it at one end, if it is installed backwards, the medium impacts the bellows frontally, and it doesn't take long to perforate. Second, the material and length of the guide tube are usually marked with "guide tube: SUS304" or "no guide tube" on the specification sheet. For particle-containing media, the flow guide must be carried and in the correct direction.
The third is the amount of pre-stretch (or pre-compression). This value is typically expressed in millimeters, such as "pre-stretched 15 mm". When installing, use a tool to stretch or compress the expansion joint to the specified length, and then fix the tie rod nut. This is done to keep the bellows in the most advantageous stress state for normal operation. If you ignore this line and install it at the natural length, the compensator may fail early.
The specification sheet of metal expansion joint is not a dead letter, it is the underlying map of type selection, installation, operation and maintenance. Next time, take the specification sheet and go through these five core parameters one by one, so the probability of stepping on a pit will be reduced by at least 80%.