How to choose, install and maintain the non-metallic compensator for denitrification flue? A hands-on guide

1. How "tricky" is the working condition of denitrification flue? Understand why non-metallic compensators are the first choice

The denitrification flue has a high temperature (usually 300-400℃, and some of it may rush above 500℃), and the flue gas still carries sulfide, ammonia and water vapor, so the corrosiveness is unambiguous. What is even more headache is that the displacement of thermal expansion and contraction of the pipeline is large, and the cross section of the flue is mostly rectangular, so the conventional metal expansion joint can't bear these composite stresses at all. You let the metal bellows absorb lateral and angular displacements at the same time? It fatigues and cracks every minute to show you.

What about that?Non-metallic compensatorThat would come in handy. Here, "non-metal" mainly refers toFabric fiber expansion jointAs a representative flexible structure, the main body is laminated with composite materials such as glass fiber, silicone rubber and PTFE. It does not have the "fear of twist" weakness of metal bellows, can absorb axial, radial and angular displacements at the same time, and is corrosion resistant, thermally insulated and lightweight. I have encountered many projects. The original metal rectangular expansion joint leaked in less than two years, so it was replaced withRectangular non-metallic expansion jointAfter running for five consecutive years without problems.

So don't worry about it-non-metallic compensator is the optimal solution for denitrification flue with low positive pressure (generally-5kPa to 5kPa), large size and multi-directional displacement. Let's elaborate on how to pick, install and raise.

Second, the key to selection: from rectangular non-metallic expansion joints to fabric fiber expansion joints, which one is more resistant?

Rectangular non-metallic expansion jointSpecially corresponding to rectangular flue, flange interface, easy to install;Non-metallic expansion joint (fabric fiber expansion joint)It is more versatile, circular and rectangular can be done, and the layer structure can be customized. The denitrification flue is mostly rectangular in section, so rectangular non-metallic expansion joints are used most.

• Temperature: The temperature of the denitrification inlet section is as high as 400℃. High-temperature resistant silicone rubber + glass fiber composite layer must be selected, and PTFE anti-corrosion film must be added to the outer layer. Temperature over 500℃? Then you have to discuss with the manufacturer to add ceramic fiber insulation.
• Medium corrosivity: The flue gas contains SO₂, HCl and NH₃, and the material whose inner layer is directly in contact with the flue gas must be tetrafluorine (PTFE) or fluororubber. Don't use ordinary rubber cheaply, it will be brittle into slag in three months.
• displacement amount: The total amount of multi-directional displacement is calculated for flue thermal expansion. The displacement values in X, Y and Z directions are given during type selection. The manufacturer determines the wavenumber (or the number of wrinkle layers) of the expansion joint according to these data.
• Pressure/differential pressure: The denitrification flue is generally slightly negative pressure or slightly positive pressure, but if there is positive pressure fluctuation caused by induced draft fan, it is necessary to confirm that the expansion energy saving bears a pressure difference of at least ±10kPa.

In addition, when you encounter a flue with a large cross section (such as 3m ×4m), don't hold it hard. You can consider setting up multiple compensators in sections, or consult the manufacturer for non-standard customization. Don't do your own blind calculations, send the working conditions parameters to the manufacturers and let them come up with a plan-they do this work every day.

3. Installation and pit avoidance: coordination of flue gas baffle door, expansion joint pull rod and guide tube

First, the expansion joint and the smoke baffle door should be separated by a sufficient distance.The baffle door is to cut off the smoke, and impact airflow will be generated when opening and closing. If the compensator is too close to the baffle door (less than 1 meter), the airflow will directly scour the inner wall of the expansion joint, which will accelerate the wear failure. Usually the spacing is at least 1.5~2 meters, and a section of straight pipe transition is added in the middle. As for the selection of baffle doors, this site has ready-madeFlue gas baffle door、Round Flap Door (Double Seal)as well asDouble-sealed single-axis circular baffle doorWhich is suitable for denitrification system.

Second, the adjustment of the expansion joint tie rod should leave a margin.The installation tie rod is generally in a pre-stretched or pre-compressed state when leaving the factory, which is used to compensate for cold shrinkage. For on-site installation, first loosen the tie rod nut to the initial length required by the design, and then tighten the locking nut according to the manufacturer's instructions after installation in place. Never screw it to death-otherwise the compensator will have no room to move when the pipe expands thermally and will tear directly. The adjustment method can be turned over this siteFAQ"How to adjust the expansion joint tie rod nut", there are detailed steps.

Third, the guide tube is a life-saving piece, so don't save it.The guide tube is installed on the inner side of the expansion joint, and its function is to guide the flue gas to flow smoothly and prevent the vortex current from washing the inner wall of the compensator. For the denitrification flue, which contains particulate matter (fly ash and catalyst dust), the guide tube must be equipped. Moreover, the length of the guide tube should cover the peak range of the expansion joint, which is generally 200~300mm longer than the total length of the expansion joint. For specific functions, please see the article "Specific Function of Expansion Joint Guide Tube" on this site.

4. Maintenance and lifespan: Don't wait for a leak before replacing it. Keep an eye on these points for daily inspection

The non-metallic compensator of the denitrification flue of a power plant was left unattended for three years. As a result, it suddenly tore during operation, and the flue gas leaked out, resulting in the shutdown of the furnace, resulting in a direct loss of millions. After the inspection, it was found that two months ago, there was a phenomenon of partial coating peeling and fiber fluffing-as long as you have a regular look, there will be no big accident.

• Outer skin: See if there are bulging, cracking, and powdering. In particular, the silicone coating will gradually age at high temperature. Once a crack appears, it should be replaced immediately (if it is not a whole replacement, only the skin layer can be replaced).
• Flange connecting bolt: Vibration and thermal cycling can easily cause bolts to loosen. Inspect all bolts with a torque wrench and tighten to the design torque at each shutdown for maintenance. Don't underestimate loosening, it can trigger a leak and then corrode the flange sealing surface.
• Intactness of guide tube: Observe whether the guide tube has fallen off or deformed through the manhole or inspection port. Once the guide tube fails, the flue gas directly washes the main body of the expansion joint, and the service life is shortened by at least half.

In terms of life, under normal working conditions (temperature ≤350℃, no strong acid dew point corrosion), the non-metallic compensator can be used for 6~8 years. If the humidity content of the flue gas is high and there is condensation, the life will be reduced to 3~4 years. It is recommended to have a full examination every two years and replace the outer skin every four years (the inner skeleton is usually fine).

5. Quick answers to frequently asked questions: deformation, leakage and loose bolts, how to deal with them on site

Q1: What should I do if there is local bulge in the non-metallic compensator?

The bulge is mostly caused by interlayer peeling, which may be caused by high temperature to fail the adhesive. Small-scale bulge (area

Q2: The flange surface leaks, and it leaks after tightening the bolts?

Check the sealing gasket. Graphite winding pad or PTFE clamping pad are commonly used in denitrification flue. If the gasket is aged or installed in misalignment, it is useless to tighten it. Remove the flange, clean the sealing surface, replace the gasket with a new one and reinstall it. Pay attention to the tightening sequence of the bolts: diagonally and alternately in three times to the torque value.

Q3: How to adjust the expansion joint tie rod nut on site?

This question is answered in detail in FAQ. Simply put: first loosen the locking nut, adjust the position of the middle nut to make the length of the tie rod reach the design value (the manufacturer will give two sizes: cold state and hot state), then lock the nut, and then check whether the force of each tie rod is uniform. Instead of adjusting one by one, adjust symmetrically.

Q4: Can you just use a rubber compensator instead?

Don't think about it when the temperature exceeds 200℃. This site'srubber compensatorAndRubber PTFE compensatorIt is suitable for flue at room temperature or below 100℃, but denitrification section is not good. But can be used for dust removal pipeline in low temperature section after denitrification.

Alas, in the final analysis, the compensator is "three-point selection and seven-point installation and maintenance". Choose the right and install it, it can work quietly for seven or eight years; If you choose the wrong choice and pretend to be sloppy, you will be given a look in three months. Next time you do the denitrification flue project, take this guide for comparison and check, and basically you won't step on pits.