What exactly does a metal mesh do in a non-metal compensator? — — Structural disassembly and typical damage phenomena
Non-metallic compensators, also called non-metallic expansion joints, are common in the industry are fabric fiber expansion joints. As soon as many people hear about non-metal, they think that the whole equipment is made of cloth. Actually, it's not. Inside it is hidden a layer of metal mesh – usually stainless steel wire mesh or galvanized wire mesh, sandwiched between layers of fabric and insulating cotton.
What the hell is this metal mesh for? Simply put, it's a "skeleton". The fabric layer of the non-metallic compensator is responsible for sealing and temperature resistance, and the insulating cotton is responsible for heat preservation, but the overall compression, tension and shear resistance are all supported by this layer of metal mesh. Without it, the pipe expands and contracts thermally, and the fabric layer is directly deformed and torn; When the smoke rushes, the compensator is like a deflated ball.
What about typical damage phenomena? I have seen a lot of things on the scene over the years: partial fracture of metal mesh, enlarged mesh, detachment of welds, corrosion and perforation. The most ruthless time, I saw a rectangular non-metallic expansion joint in the cement factory. The metal mesh was directly disconnected from the middle, and a large bag bulged out of the fabric layer, which collapsed when I pressed it with my hand. And guess what? At that time, the operator thought it was the aging of the fabric layer, and almost only changed the layer of cloth.
Why is it always metal mesh that is bad? Three culprits: mechanical scraping, high-temperature oxidation and medium corrosion
Here comes the problem-in the same working condition, the fabric layer is still fine, but the metal mesh collapses first. Why?
First, mechanical scraping.Many non-metallic compensators are installed at the corner of the flue or the inlet and outlet of the fan, and the airflow directly washes the metal mesh with dust particles. Especially behind the desulfurization flue gas baffle door of thermal power plant, in the area with large dust content, the metal mesh is worn like sandpaper, and it is worn out in a few months. I have seen a working condition, the wind speed is 25m/s, the dust concentration is 30g/Nm³, and the metal mesh is exposed in three months.
Second, high-temperature oxidation.The upper temperature resistance limit of metal mesh depends on the material. Ordinary stainless steel 304 starts to oxidize and peel above 650℃. If the actual working conditions exceed the design temperature for a long time, or there is short-term overtemperature (such as when the boiler starts and stops), the oxide layer on the surface of the metal mesh will fall off, the matrix will become thinner, and the strength will drop by a cliff. Some non-metallic sections matching corrugated expansion joints used in power station industries have been changed three times in two years because the local high temperature is not considered.
Third, media corrosion.The desulfurized wet flue gas contains sulfuric acid and hydrochloric acid condensate, and the pH value is as low as 2-3, which ordinary stainless steel mesh can't bear at all. If you have to choose 316L or duplex stainless steel, the cost will go up, but the life can be prolonged by more than 5 times. Unfortunately, many selection people only focus on the corrosion resistance of the fabric layer, ignoring that the metal net is the first link to be broken down by corrosion.
When the metal mesh breaks down, the consequences are worse than you think-a chain reaction from air leakage to overall structural instability
The metal mesh is broken. You thought it was just a leak? That's naive.
In the first step, the metal mesh breaks locally, causing the fabric layer in this area to lose support. The fabric layer begins to show fatigue cracks and slowly leaks air. As soon as the air leakage volume comes up, the system resistance changes, the fan current fluctuates, and the energy efficiency decreases.
In the second step, the air leakage continues, and the high-temperature flue gas directly contacts the heat insulation cotton at the fracture of the metal mesh. The heat insulation cotton is blown away and ablated by the air flow, and the heat insulation performance of the compensator is lost. The temperature of the external skin has soared, and accidents of burning people have not happened before.
The third step, and the most frightening, is that after the metal mesh fails in a large area, the non-metallic compensator loses its constraint as a whole. Pipe thermal displacement is no longer effectively absorbed, and the stress is directly transferred to the flange bolts and adjacent equipment. The seal at the flange fails, and in severe cases, the whole compensator falls off, and the displacement of the pipeline exceeds the standard, even causing unplanned shutdown of the unit. Last year, a power plant failed to deal with the broken metal mesh of a non-metallic expansion joint in time, and the metal rectangular expansion joint next to it was deformed. The maintenance cost 400,000 yuan.
How can you judge that the metal mesh is no longer working on the spot? Don't ignore a few unremarkable details
Don't wait for the smoke to stop and check. During daily inspection, these signs should be kept an eye on:
- Local bulge or depression.Touch the surface of the non-metallic compensator with your hand. If a certain area is obviously softer than others, or there is a bump, 80% of the metal mesh is broken, and the fabric layer is hard-holding alone.
- There is rust oozing from the surface.Especially for non-metallic expansion joints, if yellow-brown or reddish-brown water stains appear on the surface of the fabric layer, that is, the rust caused by the corrosion of the metal mesh has penetrated the outer layer. Don't believe the "that's dust" nonsense.
- Unusual noise.When you hear a "snap" or metal friction sound during operation, it is likely that the broken metal mesh wire head scratches the inner wall or the guide tube. This problem often occurs in the non-metallic section near the double hinge expansion joint of the air-cooled island vacuum pipe.
- Run parameter exception.The fan current increases inexplicably, or the negative pressure fluctuation of the system becomes large, and the damage of metal mesh leads to the increase of air leakage.
What if it's broken? Emergency repair vs overall replacement, what to pay attention to when selecting (combined with the non-metallic expansion joint of this station)
It is found that the metal mesh is broken, and there are two situations first:
Emergency repair.If the damage range is small (such as the size of a palm) and it is not a critical load-bearing area, the method of local repair welding metal mesh + covering reinforcing layer can be adopted. However, note that repair welding must be carried out in a shutdown, ventilated, and flammable gas-free environment. Moreover, the repaired life generally doesn't last more than half a year, so it can only be regarded as a temporary transition.
Overall replacement.If the fracture area exceeds 30%, or multiple damages, don't hesitate to replace the whole non-metallic compensator directly. At this time, the selection will have a long memory:
- Confirm the working temperature, dust concentration and media corrosiveness. Stop using 304 net to deal with desulfurization flue gas, go directly to 316L or Hastelloy net.
- If space allows, selectRectangular non-metallic expansion jointWhen, the number of layers of metal mesh can be thickened or a layer of anti-wear liner can be added. The product information of rectangular non-metallic expansion joint in this station has reinforced design for high dust conditions.
- For high temperature operating conditions, such asNon-metallic expansion joint (fabric fiber expansion joint)At the outlet of the boiler, the metal mesh material should be heat-resistant stainless steel, and at the same time, attention should be paid to the external insulation layer to reduce the surface temperature of the metal mesh.
- Don't forget to check the deflector and baffle door status. If the desulfurization flue gas baffle door is not closed tightly, the leaked acidic gas will accelerate the corrosion of the metal mesh, and it is useless to replace the compensator with a new one.
Preventing problems before they occur: Practical suggestions for extending the life of metal mesh from material selection to installation and maintenance
How to make metal mesh last two more years?
Material selection stage.Don't just look at the word "non-metal" and think that metal mesh doesn't matter. When purchasing, the supplier is clearly required to provide the material, wire diameter, number of mesh holes and welding process of metal mesh. It is best to make targeted selection according to working conditions. For example, this site'sUniversal corrugated expansion jointAlthough the main body is metal, the non-metal segments can likewise be customized to metal mesh specifications.
Installation phase.Ensure the coaxiality between the non-metal compensator and the pipeline, and avoid excessive local stress on the metal mesh caused by installation deviation. In addition, the adjustment of the tie rod nut should strictly follow the instructions-how to adjust the tie rod nut of the expansion joint? Refer to the question and answer of this site, the pre-stretching value should be accurate.
Run Maintenance.Clean the compensator surface area of ash regularly, especially acidic dust. During inspection, scan the surface temperature of different areas with an infrared thermometer gun. If the temperature difference exceeds 30℃, it indicates that the insulation layer may collapse or the metal mesh may be deformed. If you find minor problems, make them up in time, and don't delay them until overhaul.
In the final analysis, the metal mesh of non-metallic compensators is the "hero behind the scenes", which is invisible at ordinary times and cannot be dropped at critical times. Next time you encounter a compensator failure, don't just stare at the fabric layer-peel it open and look inside, and the metal net may be the real murderer.