Find out first: Shelf life, design life and actual service life are three different things
Many customers come up and ask, "How many years is the shelf life of this expansion joint?" To be honest, there is no unified standard in the industry. The manufacturer's "shelf life of 1 year" is usually the warranty period-that is, in case there is a defect from the factory, I will replace it for you, which is completely different from how long the metal bellows can last. The design life is calculated according to the number of fatigue times. For example, the design life of general-purpose corrugated expansion joint can reach 1000 times or even higher under standard working conditions. However, when it is actually used in cement industry or flue gas pipelines of power stations, problems may occur in three to five years. What reference value do you say this nominal number has?
So don't be fooled by the propaganda of "how many years is the shelf life". It was measured under ideal conditions in the laboratory, with the temperature, pressure and medium all fixed, which was completely different from the scene. Two days ago, I met a desulfurization project. They used the corrugated pipe matching the double-sealed single-axis circular baffle door. The manufacturer claimed that the shelf life was 5 years, but it leaked in 2 years. Why? Because that concentration of chloride ion in the medium is far above the design value.
Hardcore factors that really determine the life of metal bellows: materials, working conditions, media
In terms of materials, the corrosion resistance of the 304 and 316L is much worse. It is used behind the desulfurization flue gas baffle door, and it will be perforated for a few years without selecting the right material. For every 100℃ increase in temperature, the fatigue life of bellows may decrease by an order of magnitude. Chloride ions in the medium? The risk of stress corrosion cracking in stainless steel has skyrocketed. There is also installation deviation. Many directly buried expansion joints or external pressure single axial expansion joints fail early because the pre-offset is not adjusted properly.
Corrugated expansion joints used in power station industry are common in 10-15 years if they are properly designed and clean in medium. Metal corrugated expansion joints in cement industry have to be monitored in 5-8 years because of large dust and large temperature fluctuation. High temperature axial type expansion joints are used in steam pipelines. If they are started and stopped frequently, the fatigue life may be only a few years. Rubber compensators and non-metallic expansion joints (fabric fibers) are greatly affected by aging, and their shelf life is usually 2-3 years, so they have to be replaced. These data are not absolute, but can be used as a reference baseline.
Don't wait until it leaks to replace it-how can you tell that the bellows is dying?
There are no obvious corrosion pits, cracks and bulges on the surface of the bellows. Displacement check: whether the pre-tension amount during installation is still there, such as whether the tie rod nut position of the large tie rod expansion joint has abnormal movement. Measured stiffness: If the bellows feels soft or stiff, it means that the material is fatigued. Leak detection: Negative pressure leakage is a fatal signal for vacuum special hose or double hinge expansion joint of air-cooled island vacuum pipe. Doing these checks regularly is a hundred times more reliable than looking at the shelf-life label.
How often is it appropriate to check it? It is recommended to do an appearance inspection at least once every six months and a comprehensive inspection during overhaul. If the working conditions are harsh (such as behind the desulfurization flue gas baffle door and the tail of the cement kiln), shorten it to once a quarter.
Want to extend the shelf life? Installation and maintenance are more important than model selection
When installing, pay attention to the direction of the guide tube (the specific function of the expansion joint guide tube), and don't install it backwards. The adjustment of screw and tie rod (how to adjust the tie rod nut of expansion joint, and does the screw of expansion joint need to be disassembled) must be according to the instructions. If the pre-stretching amount is wrong, it will be wasted directly. Daily inspections focus on welds and bellows troughs, where dust accumulation and corrosion are most likely. For the expansion joint near the flue gas baffle door and the electric plug-in insulation door, also pay attention to whether the insulation layer is damaged and causes uneven heat and cold.
One customer installed the straight pipe pressure balanced expansion joint backwards, and the guide tube faced in the opposite direction of the medium flow, as a result, the bellows was eroded like a colander. This error can be completely avoided by proper installation.
Finally, let's tell you the big truth: There is no uniform shelf-life standard
Only life expectancy based on operating conditions. Don't be superstitious about the "shelf life of how many years" advertised by manufacturers, which is the ideal data of the laboratory. In actual procurement, it is recommended to write the minimum cycle times and warranty conditions into the technical agreement for complicated structures such as bellows matched with double-sealed single-axis circular baffle doors or straight-pipe pressure-balanced expansion joints. Really wonder how long your bellows will last? Find someone who knows the job to calculate the fatigue life, or do an online monitoring, which is much more useful than looking at the production date.
After all, the shelf life of expansion joint metal bellows is a false proposition. If you use it as a fixed number, you will suffer sooner or later. It is better to put some thought into model selection, installation and daily inspection so that the equipment can really last.