How exactly is the flue metal expansion joint used? Don't step on these pits
To be honest, who in our business hasn't been tricked by flue metal expansion joints a few times? Two years ago, I went to a power plant for a return visit. The customer said that the newly installed expansion joint leaked after only three months. When I removed it, the bellows was worn like paper. After asking around, I found out that the selection was wrong and the installation was reversed. Do you think it's irritating? Today, let's talk openly about how to use the flue metal expansion joint well, and point out all those common pits to you.
First, find out which metal expansion joint you should choose
Flue, the temperature is high, the smoke is corrosive, and the pipeline is always turning. There is trouble behind choosing the wrong model. You should keep in mind that the metal expansion joints commonly used in flues are mainlyMetal rectangular expansion jointAndCorrugated expansion joint for power station industry。 Rectangular ones are generally used on large-diameter square tubes such as boiler outlet and desulfurization tower inlet; Circular ones are more common in the connecting sections between chimneys and induced draft fans.
In addition, if the flue gas temperature exceeds 600℃, then you have toHigh temperature axial expansion jointOrMetal Corrugated Expansion Joints in Cement Industry— — Don't think that it is only used in the cement industry, but the high-temperature flue gas of power plants and steel plants can also be applied. When selecting the type, you should also card the pressure level. Most of the flues are slightly positive or negative pressure. UseUniversal corrugated expansion jointYou can make do with it, but don't save that little money to buy a non-standard goods. You will know what a headache is if you leak back.
(Want to check the specific dimensions? We have "Metal Expansion Joint Weight Table" and "Expansion Joint Model and Dimensions" Q&A on our site. You can check your pipe parameters.)
2. Before installation, don't use those pull rods and arrows as decorations
I have met many customers who tightened the tie rod nut tightly during installation and used it directly as a pipe-do you think it's irritating?Tie rod nut for expansion jointThey are pre-adjusted when they leave the factory to prevent damage to the bellows during transportation and hoisting. After installation in place, be sure to loosen the nut to the designated position according to the manufacturer's instructions, or simply remove the pull rod for transportation (depending on what you have in handLarge tie rod expansion jointStillCompound hinge transverse expansion joint）。
The body of the expansion joint is usually marked with an arrow, which isDirection of guide tube。 The arrow should point to the direction of the medium flow, that is, the smoke follows the arrow. If you install it backwards, the deflector doesn't hold the airflow, and the smoke flushes the bellows directly, which will wear out in a few months. Regarding the function of the deflector, we have a special talk in our Q&A, so you can flip through it.
By the way, there is a lot of dust in the flue.Rectangular non-metallic expansion joint(For example, fabric fiber), then be careful not to let the dust collapse. But we are talking about metal today, so we focus on whether the metal parts are bumped and the anti-corrosion layer is damaged.
3. Three places that are most prone to problems in operation
Installing it is only the first step, and it is only when you use it. I have summarized the three most overturned points in the use of flue metal expansion joints:
First, stuck.After the flue is heated and expanded, if the pipes on both sides do not reserve enough displacement space, or the compensation amount is small when the expansion joint itself is selected, the bellows will be forcibly pulled to death or even bulged. What do you change at this timeCompound straight pipe bypass pressure balanced expansion jointIt's also useless. The root cause is that the design is not right.
Second, corrosion.Sulfide in flue gas condenses in the low temperature section, which will form sulfurous acid, which has strong pitting corrosion to stainless steel bellows. Don't think that everything will be fine with 304, it depends on the flue temperature and humidity. If it doesn't work, go onPTFE-lined hoseOrPTFE compensator— — However, note that fluoroplastics can withstand limited temperature, so you have to check the working conditions.
Third, leaks.The flange surface is not properly padded, the bolts are loose, or the bellows is fatigued and cracked, which are all sources of leakage. You should regularly use soapy water or ultrasonic leak detection, especially don't be lazy when the machine is shut down for maintenance.
4. Maintenance is actually about two things: watching and listening
Many masters say that expansion joints are "maintenance-free", but I think that's a lazy statement. You go to the site at least once a month:
- Look at the appearance: Are there any cracks, pits and rust spots on the surface of the bellows? Especially the positions of the crests and troughs, which are the fatigue stress concentration areas.
- Look at displacement: Mark the positions in cold and hot states. If you find that the expansion joint has been pressed to the bottom or pulled to the limit, you have to change it quickly.
- Listen to the sound: If you hear a "hissing" sound during operation, it is likely to leak; If you hear metal rubbing, it may be that the tie rod or guide bolt is loose, or the expansion joint is twisted.
In addition,Service life of expansion jointGenerally, the design takes 10 years, but if the actual working conditions are poor, it will have to be changed in 5 years. For example, the coal quality has high sulfur content and frequent start-and-stop, so the life of bellows will be discounted. We have "Expansion Joint Service Life" Q&A on our site, you can refer to the empirical data inside.
Don't weld the bad expansion joint to save money and continue to use it. That's laying a mine. Change when you need to,Universal corrugated expansion joint、Large diameter thick wall expansion jointWe all have it in stock. If the model doesn't match, ask for technical support to help you match it. How to use the flue metal expansion joint well, to put it bluntly, is to choose the right type, install it in place, and check it frequently. Such a simple thing, don't let it become a big trouble.