Some time ago, a buddy who did desulfurization project came to me and said that the expansion joint used in their factory had leaked in less than half a year. When I removed it, it was a mess of internal corrosion. After asking around, I found that many peers had stumbled on the wet desulfurization pipeline. What's the problem? In fact, I just don't understand the real demand of non-metallic expansion joints in wet environment. Today, let's break it up and talk about this topic.
How "ruthless" is the wet desulfurization pipeline? — — Working condition analysis
The flue gas pipeline of wet desulfurization looks low in temperature (usually 60-120℃), but the medium is full of "ruthless characters": a large amount of sulfuric acid, sulfurous acid, and gypsum slurry, with a pH value as low as 2-3. With high humidity and solid particulate matter, the metal expansion joint can't bear it at all in this environment-pitting corrosion and intergranular corrosion will be scrapped in a few months. Non-metallic expansion jointsNon-metallic expansion joint (fabric fiber expansion joint)) With the advantages of corrosion resistance, wear resistance and absorption of multi-dimensional displacement, it has become the mainstream choice.
But don't think you can use it just by buying one. I have seen many cases in which manufacturers installed general-purpose non-metallic expansion joints on the wet system, and the skin bulged and acid leaked in less than half a year. To put it bluntly, if the material and structure are not selected correctly, they will still be scrapped in advance.
How to choose core materials? — — Don't just look at "temperature resistance"
The skin of a non-metallic expansion joint is not a layer of cloth, but a multi-layer composite material. Fluororubber, silicone rubber, and polytetrafluoroethylene (PTFE) membranes are stacked together, each performing their respective duties. However, under the wet desulfurization condition, the two most critical indicators areAcid corrosion resistanceAndResistance to hydrolysis。
On our stationNon-metallic expansion joint (fabric fiber expansion joint)Products, tetrafluorine film (PTFE) is commonly used for anti-corrosion layer, and glass fiber or aramid fiber is used for reinforcement layer. Tetrafluoride membranes are chemically inert and can keep acids and slurries out. But there is a pit here: some manufacturers replace it with ordinary neoprene in order to reduce costs. After installing it for a month, there was no problem, but when the condensate acid was repeatedly infiltrated, the rubber molecular chain broke and directly cracked-that scene, tsk, was like shredded paper.
Rectangular or circular? — — Structural selection has a doorway
In wet desulfurization system, the flue gas pipeline commonly has rectangular cross section, especially the inlet and outlet position of desulfurization tower. That's when you have to use itRectangular non-metallic expansion joint。 Its characteristic is that it can absorb multi-dimensional displacement-axial, transverse and angular displacement can withstand.
And this kind of position is usually matched in front and rearFlue gas baffle doorOrDesulfurization flue gas baffle doorUsed to isolate smoke. When installing, pay attention to the spacing between the expansion joint and the baffle door, as well as the limit design. If the limit is not done well, the expansion joint will be over-stretched or compressed, the skin stress will be concentrated, and the life will be cut in half directly.
guide tube。 There is generally a guide tube (also called a guide plate) inside the expansion joint, which is used to guide the flow of the medium and avoid direct washing of the skin. However, the material of the guide tube is very critical under wet working conditions-if it is stainless steel, it is recommended to thicken or replace it with corrosion-resistant alloy (such as 316L or duplex steel). Otherwise, the guide tube will be perforated by acid corrosion first, and the medium will directly wash the skin, which will soon be game over. We wrote in the question and answer on "The Specific Function of the Expansion Joint Guide" that the failure of the guide is equivalent to the skin streaking.
The "hidden rules" of installation and maintenance
Many on-site construction workers are used toCorrect installation method of expansion joint of large tie rodTo operate, but the non-metallic expansion joint has no tie rod at all, it is supported by flanged connections and frames. There are two details when installing, and I've seen things go wrong with my own eyes:
- Sealing gasket is not cheap.The sealing gasket of the side frame must be made of acid-resistant rubber, and some construction sites use ordinary asbestos pads, which are corroded by acid in half a year, and the flange leaks slurry and air, so it is too late for maintenance.
- The drain holes must be opened downward.There will be condensed liquid and dust accumulation inside the expansion joint, and the drainage hole is not oriented correctly, so the accumulated liquid can't be drained away, which will soak the bottom of the skin for a long time to accelerate aging. Before I was in a power plant, I saw workers opening the drainage hole to the side, and as a result, the accumulated liquid formed an "acid pool" at the bottom, and the skin rotted from the inside out.
For maintenance cycle, refer to JB/T 12235-2015 standard, it is recommended thatSemi-annual inspection。 Focus on: whether there are cracks, bulging, discoloration on the skin surface, and whether there are any signs of leakage at the welds and flange corners. If you find a problem, don't drag it. Changing the skin is much more cost-effective than changing the whole expansion joint.
After all, this is a "consumable"
Non-metallic expansion joints are essentially consumables under wet working conditions, but with the right material and structure, it is not a problem to use them for three to five years. If you have encountered the situation that the expansion joint is perforated by acid corrosion at the project site, you might as well look back at the original skin material report-there is a high probability that the fluororubber layer is too thin (some factories only paint 0.2mm), or the tetrafluorine film is not well attached (blistering and delamination).
Sooner or later, the money saved in pipeline safety has to be doubled back. You think about it, is this the truth?