What exactly is a single type metal expansion joint? Don't be in a hurry to choose, understand the structure
This thing is, to put it bluntly, a short section with a bellows. Corrugated pipe, end pipe, guide tube, tie rod, these are the cores. The bellows is a flexible section that absorbs the axial displacement caused by thermal expansion and contraction of the pipeline; The end pipe is used for welding or flanging to the pipe; The guide tube plays a role in protecting the inner wall of the bellows and reducing the erosion of the medium; The tie rod limits excessive stretching or compression of the bellows.
Many people follow it withExternal pressure single axial expansion jointMix up. The difference is that the external pressure type bellows is subjected to external pressure, has better stability, and is suitable for high pressure and large diameter working conditions. But the basic principle is the same: trade corrugated deformation for pipeline safety. You ask why not use a sleeve compensator? The sealing of that thing is prone to problems. The metal bellows is formed integrally, without dynamic sealing points, and the probability of leakage is much lower.
How to divide single and duplex? If you choose the wrong type, you will have to rework it on site
Single-type metal expansion joints have only one set of bellows, which are suitable for absorbing axial displacements in one direction, such as the thermal elongation of a straight section of pipe. If the pipe has lateral or angular displacement, it must be consideredCompound hinge transverse expansion joint、Compound straight pipe bypass pressure balanced expansion jointSuch a multi-bellows combination structure.
The displacement direction and constraints of the caretaker system. For straight pipe sections, the displacement is not large, and the pipe frame can guide axial expansion and contraction, so single type is enough. However, if there is displacement in two directions, or the space is limited and the middle fixing bracket can't be set, the single type can't stand it-if it is hard-fitted, the bellows will twist and stress, and it will crack for you in a few months.
What about that? Do the stress analysis of the pipe system first, don't slap your head from experience. Especially for high-temperature and high-pressure steam pipelines, the wrong choice is not a problem of rework, but a problem of safety.
You don't have to memorize a bunch of formulas when selecting a type, but you must keep an eye on the four numbers
Nominal diameter, design pressure, design temperature, displacement, plus a fatigue life. The diameter determines whether it matches the pipe, the pressure and temperature determine the material (304, 316L or higher grade alloy) and the number of layers of the bellows, and the amount of displacement determines the number of corrugations.
The number of cycles is high, and the bellows should choose multi-wave number or multi-layer structure, otherwise it will crack in a few years. Think about it, if a bellows starts and stops once a day, it will be 365 cycles a year, and the design fatigue life is only 1,000 times, and it will expire in three years. Therefore, you have to tell the manufacturer the actual working conditions clearly, and don't just give a temperature and pressure. If you want to know how to check the model and calculate the stiffness, you can directly correspondModel and size of expansion jointAndStiffness and Calculation Formula of BellowsTo check.
Eight out of ten pits at the installation site are on the tie rods and guide tubes
The nut on the tie rod is locked when the single expansion joint leaves the factory, which is in the shipping protection state. After installation, the nut must be loosened to the cold tight position marked in the drawing, otherwise the expansion joint cannot move, which is equivalent to white installation. Two days ago, I met a customer, who said that the pipe bracket was crooked when the test run was finished. In the past, I saw that the tie rod nut was not loose at all-this is not the expansion joint, but myself for not reading the instructions.
The guide tube must be directed toward the flow direction of the medium. If the medium is installed backwards, it will directly flush the inner wall of the bellows, and it will not take long to wear out and leak. Another detail is the direction of the arrow. The arrow on the expansion joint generally indicates the flow direction of the medium or the direction of installation. See it clearly before installation, and don't rely on your feeling.
Alas, these things sound low-level, but the scene really happens every day.
Good installation does not mean once and for all, and routine maintenance has to keep up
The most common failure modes are bellows fatigue cracking and corrosion cracking. The former is related to the inaccurate estimation of fatigue life during model selection, while the latter is often caused by chloride ions or irregular sewage discharge in the medium. It is recommended to check whether the tie rod nut is loose and the bellows surface is abnormally deformed once in the first month after it is put into operation, and then visually check once every six months.
Cooperate with the guide bracket setting of the pipe to ensure that the displacement is indeed absorbed by the expansion joint, rather than holding back on the bracket. If you find pits, rust, or obvious color changes on the bellows surface, you have to evaluate whether it's time to change it. As long as the selection, installation and maintenance are correct, it is common for single metal expansion joints to last ten or eight years.
In the final analysis, when it comes to pipeline compensation, what type of expansion joint to choose is only the first step. How to install and maintain it is the real doorway. Figure it out, and the safety of the pipeline is guaranteed.