Last winter, the cement factory in Shandong almost had a big accident
The flue gas pipeline has been in operation for less than three months,Non-metallic expansion joints (fabric fiber expansion joints)The skin cracked a big hole, and the heat wave was rolling in the workshop, and the temperature directly soared to more than 60 degrees. The old master squatted at the scene and watched for a long time with a cigarette in his mouth. Finally, he suppressed a sentence:"I don't understand the expansion coefficient."
I was particularly impressed by this. Many people choose compensators, and their eyes only focus on how much temperature they can withstand and how many MPa they can withstand, and they think that the parameters are similar. However, in fact, what really makes the equipment scrapped in advance is often the one that is neglectedExpansion coefficient of non-metallic compensator。 Today, let's start with this case of almost scrapping to see how critical this parameter is.
What is the expansion coefficient? To put it bluntly, it is the magnitude of thermal expansion and contraction
Non-metallic materials are not the same as metals. Like what we often useNon-metallic expansion jointThe skin is a multi-layer fabric fiber plus rubber or fluororubber composite, and the thermal expansion and contraction amplitude is much larger than that of metal. For example: a 10-meter-long pipe, with a temperature rise of 100℃, the metal expansion joint may only extend by 10mm, but the non-metal model can extend by more than 30mm. If you miscalculate this number, the compensator will not actually compensate enough-either stifle the pipe or tear the skin.
And guess what? The one next to itRectangular non-metallic expansion jointIt is because the expansion amount is under-calculated by 20% in the design, and the air leaks in three months. The old master said that when the equipment was removed, the skin was full of cracks, like a cracked riverbed.
Three "Invisible Killers" Affecting Expansion Coefficient
Which factors will changeExpansion coefficient of non-metallic compensator？ I've summarized three, and you compare them:
- Temperature range.The low temperature section is different from the high temperature section. For example, used inDesulfurization flue gas baffle doorLater, the flue gas temperature fluctuates back and forth from 80℃ to 180℃, and the thermal expansion rate of the skin material is nonlinear-the expansion coefficient is 0.008 at 80℃, and it may become 0.015 at 180℃. It's twice as bad.
- Media Properties.The flue gas with acid will make the non-metallic layer age, and after the material hardens, the expansion coefficient will decrease instead. There's a power plant project, usedHigh temperature axial expansion jointThe non-metallic section next to it, inspected after one year of operation, found that the skin had hardened and the actual expansion was 15% less than when it left the factory.
- Install preload.We found at the scene that some customers putrubber compensatorIt is pulled too tightly and crushes the material when it expands thermally. This matter is the same as screwing-too tight will easily slip.
Speaking of which, I have to mention the national standardJB/T 12235-2015，Non-metallic expansion jointThe reference values of linear expansion coefficients at different temperatures are clearly given in the technical requirements of. But many people don't look at it and directly copy their experience. As a result... Alas, all the rework is blood and tears.
How to calculate the compensation amount when selecting a model? Actually, it's just three steps
It is not complicated, but you must not be lazy. First calculate the total thermal displacement of the pipe: the length of the pipe multiplied by the temperature difference and then multiplied by the linear expansion coefficient. For example, if the pipe is 30 meters, the temperature rise is 120℃, and the expansion coefficient is 0.012 mm/m·℃, the total elongation is 43.2 mm. At this time, you have to choose a model with a compensation amount greater than this number, and it is best to leave a 20% margin.
Like the ones we produceUniversal corrugated expansion jointAlthough it can also be used, the non-metallic model is more flexible andLarge diameter thick wall expansion jointSpecial-shaped pipes that can't be installed are particularly suitable. Especially for flue gas pipelines with many turns and limited space, usingNon-metallic expansion jointInstead, it is more worry-free.
But be careful, don't just look at the expansion coefficient table given by the manufacturer. In actual working conditions, the coefficient will drift after skin aging. Last year, there was a power station project, which was calculated according to the standard during installation. As a result, after one year of operation, it was found that the offset was nearly 20% worse than the design value. Therefore, it is recommended that you make a cold and hot mark after installation, and measure it again after three months to compare the actual offset with the design value. If it is much worse, find the manufacturer to adjust it quickly-don't wait for the pipe to blow up before you regret it.
Finally, a "stupid way" of picking
When it comes to pipeline safety, the details are really killing. If you are choosingNon-metallic compensatorIn addition to looking at the parameters, it is best to let the manufacturer provideExpansion coefficient of non-metallic compensatorMeasured curves of rather than a standard table. In addition,Rubber PTFE compensatorAndPTFE compensatorThe expansion characteristics are also different, so don't mix them.
Remember that cement factory in Shandong? Later, I changed the skin with a new skin, re-checked it according to the measured expansion coefficient, and added 20% margin. It's been two years now, and nothing has happened. The old master said that if I had known earlier, I would have taken a look at the national standard, and I would not have spent 30,000 more rework fees.
So, don't bother. Choosing the right expansion coefficient works better than anything else.