Thermal expansion and contraction are the number one killer of piping systems
The linear expansion coefficient of the metal swings right there-as the temperature changes, the length changes. Take the steam pipe, for example, from normal temperature to 300℃, it can be elongated by 3-4 mm per meter. A 50m pipe with a total elongation of nearly 20cm. Think about it, a displacement of 20 centimeters can be carried by the pipeline? Without metal expansion joints, the pipe either pins the fixing bracket or pulls the flange off. Two days ago, I encountered a case: the main steam pipeline of a power plant was not equipped with a high-temperature axial expansion joint, and the elbow directly cracked after two months of operation, resulting in six-digit shutdown losses. This is not an isolated case. The expansion joint is clearly marked on the drawings of the design institute. As a result, the construction party removed it in order to save costs-who paid the bill in the end? The owners themselves.
Heating pipes in the north, when steam is delivered in winter, the pipe extends, and shrinks after stopping heating. A back and forth is a fatigue. Without the metal expansion joint to absorb this displacement, the weld cracks in a year. So the question is not "install it or not", but "what type and how to install it". The reason for the installation of metal expansion joints is to put it bluntly: let the pipeline system catch a breath when the temperature changes.
In addition to temperature, vibration and displacement have to be solved by it
The mechanical vibration caused by the start and stop of the equipment, the local displacement caused by the settlement of the foundation, and the seismic shock-these dynamic loads depend on the rigidity of the pipeline itself, and the result is weld fatigue and seal failure. Corrugated expansion joints and compound hinge transverse expansion joints used in power station industry are specialized in dealing with such problems. Someone asked: Is the pipeline itself flexible and can't hold it? Can't hold up. The elastic deformation of a pipe is related to its length and material, but the long-distance pipe can't absorb local displacement at all. For example: on the air-cooled island pipeline, some people saved the double hinge expansion joint of the air-cooled island vacuum pipeline. As a result, the vacuum system leaked and the unit efficiency dropped. Tsk, save little money and lose big money.
Don't forget about foundation settlement. The foundation settlement of chemical plants varies from a few millimeters to a few centimeters per year, and the pipes generate angular and lateral displacements as they settle. At this time, the double hinge transverse expansion joint or the large tie rod expansion joint comes in handy. Not pretending? Stress concentration at pipe joints reduces the life of flange gaskets by more than half.
Select the pair type: Matching of metal expansion joints under different working conditions
General-purpose corrugated expansion joint is suitable for conventional pressure and temperature, but when encountering large-diameter thick-walled pipelines or scenarios that need to bear internal pressure thrust, straight pipe pressure balanced expansion joint or curved pipe pressure balanced expansion joint must be used. In the high-temperature dust environment of the cement industry, the metal corrugated expansion joint with wear-resistant guide tube in the cement industry is standard-the guide tube protects the bellows from being washed by high-temperature particles. And guess what? Someone took the external pressure single axial expansion joint to install the desulfurization flue gas pipeline-the medium contained sulfur and humidity, and the material was not selected correctly, so it was perforated in half a year. So think clearly before installing: What is the media? What is the temperature pressure? Is the direction of displacement axial, transverse or angular?
Mistaken selection is more terrible than not fitted. For example, ordinary external pressure expansion joints are used for directly buried pipelines, which are not anticorrosive and insulated. The underground humid environment accelerates corrosion and leaks in two years. The correct method is to use directly buried (fully buried) expansion joint, with anti-corrosion layer and insulation layer after leaving the factory, and directly buried in the ground. There are also rubber compensators for water treatment pipelines, which are corrosion resistant but have limited pressure resistance; Non-metallic expansion joints (fabric fiber expansion joints) are used in flue gas pipelines, which have high temperature resistance but low pressure bearing capacity. There is no universal expansion joint, only a scheme that matches the working conditions.
Installation is not all at once: Common misunderstandings and correct practices
Do you want to remove the expansion joint tie rod nut when installing it? Many people screw it to death and use it as a rigid part-so why install it? The function of the tie rod is to protect the bellows during transportation, adjust the length of the tie rod according to the working conditions during installation, and maintain the amount of pre-stretching or pre-compression. For example, the steam pipeline is pre-stretched when installed to absorb thermal expansion; Cryogenic pipes are pre-compressed to counteract cold shrinkage. How exactly to adjust? Refer to the question and answer "How to adjust the tie rod nut of the expansion joint", and adjust the measured displacement again. Don't rely on your feeling.
The direction of the guide tube must be consistent with the direction of the medium flow, and the arrows are clearly marked, but someone always installs it backwards. The direction of the guide tube is wrong, and the medium directly washes the bellows, which shortens the life to one tenth of the original. There are also directly buried expansion joints, and buried pipelines must be anti-corrosion and heat preservation, otherwise the corrosion rate is far beyond your imagination. Inspect all welds and sealing surfaces after installation and perform pressure tests if necessary. Don't wait until it is put into operation to find the leak-at that time, you can calculate how much money you will lose after an hour of downtime.
The reason for metal expansion joint installation is not metaphysics, but engineering common sense. Every winter is a test for the plumbing system. Don't let the saved expansion joint become a maintenance order for next year.