Why can high-rise gas pipelines not be separated from metal expansion joints
The operating conditions of gas pipelines in high-rise buildings and ordinary multi-storey houses are completely different. As soon as the height goes up, the expansion and contraction of the pipe caused by the temperature change will be enlarged-in summer, the wall of the pipe can reach fifty or sixty degrees when exposed to the sun, and in winter, the shady side may be minus ten degrees. The temperature difference of these tens of degrees acts on the riser tens of meters high, and the axial expansion and contraction easily exceeds 30mm. Coupled with the uneven settlement of the building and the horizontal swing caused by wind vibration, the actual displacement of high-rise gas pipeline is much larger than that of ordinary buried pipeline.
A rigid connection will not last for years under this working condition. The joint is repeatedly subjected to tensile and compressive stress, and the weld cracks sooner or later. Gas leakage is no joke, so the specification explicitly requires the use of metal compensators for high-rise gas pipelines. Although the rubber compensator is cheap, the aromatic hydrocarbon medium in the gas will accelerate the aging of the rubber, and the surface will crack, become hard and lose elasticity. It will have to be replaced in three to five years, and once it breaks, it will directly leak. This is why the national standard excludes rubber compensators from high-rise gas pipelines.
Metal expansion joints use bellows to absorb displacement, which is commonUniversal corrugated expansion jointThe axial displacement of the riser is dominant. 304 or 316L stainless steel bellows are resistant to corrosion and fatigue, and the design life matches the pipe body. In the matter of high-rise gas, choosing the wrong material is laying a mine, and there is no need to discuss it.
Five Parameters Must Be Understood Before Model Selection
Pressure, temperature, medium, displacement, caliber, none can be less. Don't be too long-winded, these five parameters directly determine the number of corrugated layers, wall thickness and structural form of the expansion joint.
The pressure of high-rise gas pipeline is usually 0.1-0.4MPa, which belongs to the low pressure category. The temperature is basically normal temperature, but considering the extreme conditions of sun exposure in summer and low temperature in winter, it is recommended to choose 304 or 316L stainless steel for bellows. 316L has stronger resistance to chloride ion corrosion, and it is preferred in coastal high humidity areas.
The displacement should be calculated clearly. The axial displacement of the riser is dominated, but the lateral displacement of the horizontal pipe section should also be considered-the displacement generated by wind vibration and building swing in the horizontal direction may exceed the axial direction. At this time, you can't take ordinary axial type hard equipment, you have to chooseExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointThe former has strong anti-instability ability, and the latter can absorb bidirectional lateral displacement.
The caliber should be consistent with the pipe, and there are ready-made specifications for DN50 to DN300. Don't change the size yourself, the sudden change in pipe diameter will produce local turbulence and pressure drop, which is unfavorable to gas transportation.
Three Most Common Mistakes to Make When Installing
Forgot to remove the transport tie rod. When the expansion joint leaves the factory, the tie rod is locked to prevent the bellows from being strained during transportation and hoisting. Some people weld it directly to the pipeline with a tie rod. As a result, the bellows can't expand or contract at all, and the thermal expansion and contraction of the pipeline are all suppressed on the bellows, ranging from deformation to cracking. Remember that after installation, the tie rod nut should be loosened to the specified position, leaving the working stroke.
Install the guide tube in reverse direction. The guide tube must be directed toward the inflow direction of the medium, otherwise the air flow will directly scour the inner wall of the bellows, and the wear speed will double. There are not many impurities in the gas pipeline, but the flow rate is fast, and without the protection of the guide tube, the bellows can be worn out in two or three years. The judgment method is very simple: there are flow arrows on the expansion joint cylinder, so you can't go wrong if you install them against the arrows.
Use an expansion joint as a grounding jumper. Static electricity in gas pipeline must be grounded separately, and electrical corrosion will occur when the current passes through the bellows, which will form pitting pits on the corrugated surface and gradually expand into cracks. This is the invisible cause of many leaks that owner units tend to ignore. Copper braided belt should be used as a jumper between the flanges at both ends of the expansion joint, and the current should be bypassed. The bellows is only responsible for absorbing displacement.
After installation, pay attention to how to adjust the tie rod nut. The cold tightness must be calculated according to the difference between the installation temperature and the operating temperature of the pipeline, otherwise the displacement exceeds the design value after operation, and the fatigue life of the bellows will drop sharply. Ask the manufacturer for specific values, don't adjust them by feeling.
How many years do you change it?
Don't listen to the "maintenance-free for life" nonsense. The life of gas bellows depends on the actual displacement, the number of pressure fluctuations and the corrosiveness of the medium, and there is no fixed replacement year. However, the appearance must be inspected once a year, focusing on whether there are rust spots, cracks or deformations on the surface of the bellows.
If there are frequent earthquakes or large pipeline vibrations in your area, preventive replacement is recommended every 5-8 years. Why? Because the fatigue life of bellows is calculated by the number of cycles, earthquakes and vibrations can greatly increase the number of cycles and accelerate bellows fatigue. The cost of preventative replacement is much lower than the cost of accidental gas outage repairs.
Shine the bottom of the bellows with a flashlight. If there is light transmission, it has cracked, so it must be stopped and replaced immediately. In addition, check the welds and flange sealing surfaces at both ends. These three locations are high leak areas. The flange sealing surface is scanned with a leak detector, and the weld seam is permeable tested.
Failure cases seen over the years
Ninety percent of the selection is wrong. Some people use non-metallic expansion joints (fabric fibers) in high-floor gas pipelines, but they are pecked by birds and punctured by decoration nails, which is very dangerous. Fabric fiber expansion joints are designed for flue gas pipelines, which can't withstand gas pressure, let alone the impact of sharp objects.
Some people want to buy bellows without deflectors cheaply, and the bellows are worn out after two years of operation. There is a trace amount of dust in the gas, and the high-speed airflow washes the bellows with dust. No guide tube is equivalent to streaking naked.
Remember, high-rise gas metal expansion joint is not a standard part, and must be designed according to the actual displacement, pressure and medium. If you are not sure which bellows to use for your own pipeline, send the caliber, pressure and direction diagram to the manufacturer and ask them to suggest the selection. Professional manufacturers will calculate the number of bellows layers, wall thickness and length based on pipe layout and displacement data, rather than fooling you with a general model.