Start with the thermal expansion and contraction of a pipe
When the steam pipeline was first started, the temperature soared from room temperature to 500 to 600 degrees, and the tens of meters long pipeline could be forcibly stretched by several centimeters. If you keep it from moving, it is common for flanges to collapse and welds to crack. At this time, the expansion joint comes in handy-to put it bluntly, it is the "breathing valve" of the pipeline, specifically designed to "eat" this deformation. Whether it is a metal bellows or a non-metallic fabric layer, the core principle is the same: it absorbs the displacement through its own elastic deformation, so that the pipe should expand and expand, shrink and contract, and the stress is all released on the expansion joint.
So the first question- -What does an expansion joint do?The most fundamental thing is to compensate for thermal displacement and prevent excessive stress in the pipeline system due to temperature changes. But if you think that is what expansion joints are capable of, you would be underestimating it.
In addition to displacement compensation, I also did three things
The high-frequency vibration of the gas turbine exhaust pipe makes your ears buzz when you stand next to it. Once the rubber compensator was installed in the middle, the vibration was directly isolated by more than half, and the noise was also reduced. When the flue gas baffle door and the expansion joint are used together, the air leakage rate of the desulfurization system can be reduced to the lowest-whether the sealing effect is good or not directly related to the energy consumption and emission of the whole system. To be honest, many times the candidate type is not designed to compensate for displacement at all, but is an expansion joint only because of vibration damping or sealing requirements. For example, textile fiber expansion joint is used in dust removal pipeline, mainly because of its vibration absorption capacity and air tightness. Don't underestimate these "side businesses". In actual working conditions, they are more important than the main business.
Metal, non-metal, rubber-three materials and three tempers
High-temperature steam pipeline in power station industry, the first choiceHigh temperature axial expansion jointOrStraight pipe pressure balanced expansion joint。 Why? High temperature resistance, high pressure, corrugated pipe material must be austenitic stainless steel. Cement industry dust removal pipeline, high temperature also contains dust, with metal rectangular expansion joints easy to accumulate dust and get stuck, this time has to be replacedRectangular non-metallic expansion joint-The fabric fiber layer has good flexibility and is not afraid of dust wear. If you encounter corrosive media, such as pickling tanks or chemical lines, PTFE compensators orPTFE-lined hoseThat's the right answer. Choose the wrong material and pierce it in three months. It's really not to scare you. I have seen a chemical plant that installed ordinary metal bellows on dilute sulfuric acid pipelines, and it leaked in less than two months.
Install that little trick
Two days ago, I met a client.Universal corrugated expansion jointThe screws were all disassembled, and as a result, the bellows collapsed directly during the pressure test-this kind of thing is really uncommon. The role of the tie rod is to limit the bellows from being excessively compressed or stretched during transportation and installation, not for you to disassemble and play with. When the expansion joint of the large tie rod is installed, the pre-tension is not adjusted properly, and the bellows directly exceeds the limit under the hot state, and the life is cut in half. The guide tube is installed backwards, and it is more pitty-the medium directly washes the inner wall of the bellows, and it takes less than a few months to wear out cracks. The specific function of the expansion joint guide tube is to guide the flow medium and avoid high-speed airflow or particulate matter directly hitting the bellows. Therefore, be sure to see the direction of the arrow clearly before installation, which is an indication of the flow direction of the medium, not a display.
The life span has to be calculated in detail
The national standard JB/T 12235-2015 for non-metallic expansion joints clearly states that the fabric fiber layer has a fatigue life limit, and it has to be replaced in 3-5 years. If the working condition of metal bellows is stable, it will be fine for 10 years or more. However, if you start and stop frequently or encounter water hammers, it is not unusual for you to crack in two or three years. Don't wait until the pipe bursts before replacing it-regularly check the bellows surface for corrosion points and abnormal displacement, which is stronger than anything else. Sleeve-type pipe expansion joints appear to be simple in structure, but the sealing packing ages quickly and has to be replaced regularly. Generally speaking, the service life of expansion joints is strongly related to four factors: type selection, installation, working conditions and maintenance. Don't expect it once and for all.
So back to the beginning question —What does an expansion joint do?It is more than simply "compensating for displacement". It is also a shock absorber, gasket, noise reducer, and can even save you a lot of equipment repair costs. The premise is that you have to choose the right material, install the right position, and use the right method.