Non-metallic expansion joint modification: don't wait for shutdown to regret, these practical experiences are worth seeing
Two days ago, I passed by a power plant and saw that a non-metallic expansion joint beside the desulfurization flue was broken, and the smoke came out straight, making the scene gray. The operation and maintenance master squatted to the side and smoked, saying that it had been reported for repair, but the spare parts had to wait for at least three days. Three days? How much load is enough to lose and how much fine is enough in three days? Non-metallic expansion joints, unlike metal expansion joints, will not collapse directly for you to see if they are broken, but air leakage, dust accumulation and thermal insulation failure will finally force you to stop. Wait until something goes wrong before changing, who will bear the loss of construction period? Today, let's talk about the pits and practical experience of transformation.
1. Why do non-metallic expansion joints need to be modified? Talk about the pain points that can't be avoided
After a few years of running, it is the norm for the fabric fiber layer to age. As soon as the acidic condensate in the desulfurization flue gas is bubbled, the ordinary silicone cloth begins to delaminate and bulge after only one year. High temperature ablation is even worse – and guess what? In some power plants, the flue gas temperature rushes to 450℃, and generally non-metallic expansion joints (fabric fiber expansion joints) are directly carbonized. There is also the problem of dust accumulation: the folds of non-metallic expansion joints are filled with dust, the elasticity is gone, the compensation amount is discounted, and finally the flange bolts are loosened, and the air leaks into a fountain. To put it bluntly, it won't work if you don't change it. If you don't change it, you will bet on downtime.
Why didn't you do it in one step in the first place? Tsk, cost. Many projects are priced down when bidding, so choose a "general-purpose type" to make do. As a result, major surgery was required after two years of operation. Renovation is not about burning more money, but about calculating the general ledger-it takes five years for one renovation, which is much more cost-effective than changing it twice in three years.
2. Find out the working conditions before the transformation: don't take parameters as a child's play
Temperature, pressure, medium composition, displacement amount, one cannot be less. Take the working condition behind the desulfurization flue gas baffle door as an example. The medium contains SO₂, HF and condensed water, and the pH value is as low as 2. At this time, ordinary silicone cloth is a consumable. Depends on the composition of the medium, and then choose the matching fluororubber or PTFE compensator material. In addition, don't forget to ask whether it is a rectangular or circular pipe-the interface size and flange bolt hole distance between rectangular non-metallic expansion joint and circular baffle door, if you are wrong, you can't install it. We encountered a project before, and the customer only gave the outer diameter of the pipeline, but not the circular diameter of the flange bolt hole distribution. As a result, when we went to the site, we found that the holes didn't match, so we temporarily expanded the holes, and the flange strength was discounted.
And the displacement amount. Don't just look at the design value, but the actual measurement. Flue thermal expansion tends to be larger than designed, especially open pipes with direct sunlight. It is recommended to use a laser rangefinder or displacement sensor to run a full working condition record, and do not estimate.
3. Material selection and upgrading: What is the difference from fabric fiber to composite layer?
The non-metallic expansion joint (fabric fiber expansion joint) in our station has an outer glass fiber cloth + silicone rubber coating, and the temperature resistance is within 300℃. But if your flue gas temperature exceeds 400℃, you have to change to high-temperature-resistant ceramic fiber cloth or add a layer of insulation. Similarly, the corrosion in the desulfurization system is heavy, so you can consider the idea of lining PTFE hose, and lining the non-metallic expansion joint with a layer of PTFE. Don't be superstitious about "universal type", there is no universal material.
The flue gas temperature at the kiln tail of a cement plant often reaches 380℃, and the expansion joint of the original ordinary silica gel cloth will be brittle and cracked in three months. Later, it was replaced with ceramic fiber cloth + fluororubber composite layer, and it took two years to show slight aging. The cost has more than doubled, but the downtime has dropped from four times a year to once, making the account cost-effective.
The direction of material composite is also worthy of attention. Now some manufacturers make the PTFE compensator into an inner liner, and then wrap a layer of rubber compensator on the outside, which is corrosion-resistant and vibration-reducing. However, note that the overall compliance will decrease after multi-layer composite, and the displacement compensation ability should be re-checked.
4. Structural transformation: How to choose between rectangular and circular, single-layer and multi-layer?
Some sites originally used metal rectangular expansion joints, but because of corrosion and frequent replacement, will it be better to replace them with non-metal ones? Answer: Look at the installation space. Rectangular non-metallic expansion saves energy and absorbs large displacements, but has weak pressure bearing capacity, so it is only suitable for low-pressure flues. If it is a high-pressure or negative-pressure system, it is recommended that double-hinge transverse expansion joints or external-pressure single axial expansion joints be matched with non-metallic sections-for example, metal for straight pipe sections to ensure pressure, and non-metallic for vibration elimination at elbows or equipment interfaces.
The renovation near the baffle door also has to pay attention to the linkage interference. If there is an expansion joint between the double-sealed single-axis circular baffle door and the manual plug-in insulation door, leave enough space during installation to prevent the compensator from being squeezed when opening and closing the door. We have seen a project in which the non-metallic expansion joint was directly flattened after the baffle door was closed, the seal failed, and the smoke flowed back.
Single or multi-layer? Multi-layer construction can improve pressure and temperature resistance ratings, but the fit between each layer is key. If there are voids between the layers, aging will be accelerated. It is recommended to choose the multi-layer composite parts pre-pressed in the factory, and don't stack them yourself on the spot.
5. Pit for installation and commissioning: the tie rod, screw rod and flow direction mark are not sloppy
Many on-site modifications leak because the transportation screw is not disassembled during installation. The tie rod nut in the middle of the expansion joint must be adjusted in place to ensure the amount of pre-tension or pre-compression. Referring to what was mentioned in the previous question and answer, the direction of the arrow of the expansion joint points to the direction of the flow of the medium, and if it is installed backwards, it will directly lead to wrinkle tearing. There is also the connection between the flange and the pipeline. The tightening torque of the bolts should be uniform and screwed diagonally, otherwise the sealing surface will be crooked and air leakage. We have a trick: after tightening, use a flashlight to shine from one side to see if the gap is even, and adjust if it is not even.
After installing the non-metallic expansion joint, the limiting device (transport screw and tie rod positioning block) should be dismantled in time. Some operation and maintenance personnel are afraid of misoperation, so they leave the limit without dismantling. As a result, the expansion joint loses its compensation function, and the pipeline stress is concentrated on the weld joint, which is more dangerous.
Don't save trouble in the debugging phase. After smoking, run for one hour, stop to check whether the flange bolt is loose and whether there is any abnormal bulge or wrinkle on the surface of the expansion joint. If you find the problem early, you can re-tighten it with a wrench. When the leak is large, it will be late.
Modification is not part replacement, it is system-wide diagnosis. Only by thoroughly figuring out the working conditions, selecting the right materials, setting up the structure and standardizing the installation, can the transformation of non-metallic expansion joints make you run the next overhaul cycle with peace of mind.