First, find out first: how many degrees can the metal expansion joint carry?
Many people come up and ask, "What is the temperature resistance of your expansion joint?" This question can't actually be answered in one sentence. The temperature resistance of metal expansion joint depends on three factors: material, structure form and working condition of bellows. The upper limit of the long-term service temperature of ordinary stainless steel (such as 304) in oxidizing atmosphere is only over 800℃, but in power station boiler or cement kiln system, the flue gas temperature always exceeds 1000℃. At this time, it is necessary to change Inconel, GH series superalloy or add lining insulation layer. Our product line has "high-temperature axial expansion joint" specifically for high-temperature scenes, which is used to deal with such extreme working conditions.
What is the difference between 800℃ and 1000℃? To put it bluntly, the strength of 304 stainless steel will drop sharply above 800℃, the grain boundaries will start to oxidize, and the bellows will break as soon as they are pinched like brittle plastic. However, nickel-based alloys such as Inconel 625 can maintain good tensile strength at 900℃, and with the multilayer bellows structure, they can withstand the thermal displacement of the main steam pipeline of the power station.
Second, the core factors affecting temperature resistance: the material is the foundation, and the structure is the skeleton
Metal expansion joints carry high temperatures, and materials are the first threshold. Austenitic stainless steels (such as 321 and 316L) perform acceptably below 600℃, but start to creep when they exceed it. Nickel-based alloys (such as Inconel 625) can hold up above 900 °C. But materials alone are not enough, and the structural design is equally critical-for example, the detail of "diversion tube" is ignored by many people. The guide tube can isolate the high-temperature medium to directly flush the bellows, and at the same time form a heat insulation layer inside. When you look at the product information of "Corrugated Expansion Joints for Power Station Industry" and "Metal Corrugated Expansion Joints for Cement Industry", both emphasize the design of deflectors and insulation layers. In addition, the multi-layer bellows structure can also disperse thermal stress, which is more stable than single layer temperature resistance.
Here, many peers only mark the temperature resistance of the material, but forget the influence of the wall thickness and wave pitch of the bellows. When the wall thickness increases by 0.5mm, the thermal fatigue life can be doubled, but the ability to compensate for displacement will decrease, which is why different working conditions should be matched with different structures. For example, "external pressure single axial expansion joint" is suitable for large axial displacement scenarios, while "compound hinge transverse type" specializes in lateral displacement. Materials determine life and death, and structures determine life, both of which are indispensable.
3. Temperature scenarios in different industries have completely different selection ideas
Two days ago, I met a customer who made cement lines. He came up and asked if there was an expansion joint that could carry 1150℃. I said that it is definitely not possible to put pure metal bellows directly, so I have to combine the scheme: metal bellows (such as high-temperature axial type) is used for the outer layer, non-metallic thermal insulation lining is added inside, and even "non-metallic expansion joint (fabric fiber expansion joint)" is used as the thermal compensation section. And what about the power station industry? The temperature of the main steam pipeline is 560-600℃, and the pressure is high. At this time, the "corrugated expansion joint for power station industry" is usually designed with heat-resistant steel + large wall thickness. Another example is the desulfurization flue gas baffle door. Although the temperature is not high (80-150℃), it is highly corrosive. Temperature resistance is not the main contradiction, but corrosion resistance is. So, don't just focus on temperature, you have to consider pressure, medium, and displacement.
And guess what? There is a customer who does waste incineration. The flue gas temperature fluctuates drastically, 700℃ during the day and 400℃ at night. The ordinary "universal corrugated expansion joint" leaks in half a year. Later, we equipped a "straight pipe pressure balance expansion joint", plus a double-layer heat insulation sleeve, which hasn't been changed after three years of use. Temperature is not a single variable, the frequency and amplitude of fluctuations are the invisible killers.
4. The three easiest pits to step on during model selection
Just look at the design temperature and ignore the actual fluctuation. Some pipeline temperature rises and falls periodically, resulting in fatigue cracking of bellows, which is not insufficient temperature resistance, but thermal fatigue. The second pit: regard the limit temperature as the working temperature. Some manufacturers name "temperature resistance 1000℃", which may be a short-term peak. If you use it for a long time, it will be wasted in half a year. Pit 3: Ignore cold-end compensation. When the high-temperature pipeline is started, the expansion joint may have to bear low-temperature stress first, especially the "directly buried (fully buried) expansion joint" that works underground. The temperature gradient is large, and the temperature difference between the inner and outer walls can crack the bellows. Remember: When selecting the model, list the temperature, pressure and displacement conditions clearly, so that we can match you with the structure of "external pressure single axial type" or "double hinge transverse type".
By the way, the function of the expansion joint tie rod is to prevent over-stretching, and the direction of the arrow must not be reversed during installation-the direction of the arrow of the expansion joint refers to the flow direction of the medium, and the guide tube cannot protect it if it is installed backwards. This detail has been repeatedly emphasized in the FAQ, but some people at the scene still pretended to be wrong. Alas, put more thought into it.
5. Proper maintenance, doubling life is not a dream
No matter how high the temperature resistance of the metal expansion joint is, it is afraid of scaling and dust accumulation. Especially in cement industry and power station flue gas system, dust is sintered on the bellows surface at high temperature, which hinders heat dissipation, local temperature rises, and accelerates failure. Regularly check the wear of the guide tube, clean up the dust, and see if the tie rod nut is loose (the function of the expansion joint tie rod is to prevent over-stretching). In addition, the direction of the arrow must not be reversed during installation-the direction of the arrow of the expansion joint refers to the flow direction of the medium, and the guide tube can not protect it if it is installed backwards. If you do this, your high-temperature axial expansion joint will last for ten or eight years.
Don't wait until the bellows cracks before replacing it, when system downtime losses are dozens of times more expensive than expansion joints. Usually, scan the surface temperature with an infrared thermometer during inspection. If the local temperature is more than 30℃ higher than the design value, quickly check whether the insulation layer has collapsed. With these details in place, not to mention ten years, fifteen years is not impossible.