Step 1: Find out the working conditions of the flue-temperature, pressure, medium and displacement must not be less
Before making the selection, first find out the bottom of the flue. When you get the requirements, don't rush through the samples. Ask yourself four questions first:
- How hot is the temperature?Continuous operating temperature, instantaneous peak temperature, both must be required. Don't just look at the average, many faults are due to "occasional overtemperature".
- How stressful?Positive or negative pressure? The desulfurization flue is usually a slightly negative pressure, but the induced draft fan outlet may have several hundred Pa positive pressure. Although the pressure is small, it directly affects the number of layers and the clamping structure of the skin layer.
- What is the medium?Is the concentration of flue gas, sulfur-containing wet flue gas and dust high? Are there any corrosive ingredients? This determines whether the skin material should be acid-resistant and hydrolysis-resistant.
- What is the displacement amount?Axial, transverse and angular displacements are given in three directions respectively. The design institute drawings are generally marked. If not, you can estimate it according to the flue direction and temperature difference.
Tips:Experienced masters will do this-write the temperature, pressure and media composition on a strip of paper and stick it next to the flue, take photos and send it to the manufacturer. The more complete the information, the more accurate the selection. Two days ago, I met a customer and only gave him the temperature. As a result, the skin cracked after three months of installation. After checking, I found out that there was a small amount of sulfuric acid mist in the medium.
Step 2: Choose the right skin material according to the working conditions-fluororubber, silicone rubber or PTFE?
The core of the skin expansion joint for flue is that layer of "cloth". There are actually three categories of common skin materials on the market:
- Fluororubber (FPM/FKM): Temperature resistance 200~250℃, corrosion resistance, oil resistance first-class. It is suitable for desulfurization and chemical flue in power plants, especially for wet flue gas containing sulfur. Disadvantages? It's expensive, but it has a long life.
- Silicone Rubber (VMQ): Wide temperature resistance range (-60℃ ~ +250℃), good elasticity, but average acid and alkali resistance. Suitable for dry hot air flues, or places that require frequent telescoping.
- Polytetrafluoroethylene (PTFE): Temperature resistance-200℃ ~ +260℃, almost any medium is not afraid, but the elasticity is poor, usually used as an inner lining layer or as an anti-stick layer.
How to choose? Simple and crude:There is corrosive medium + high temperature → fluororubber or PTFE composite; Pure high temperature corrosion-free → silicone rubber; Low temperature humidity sulfur → fluororubber.By the way, we have two kinds of products, "Non-metallic Expansion Joint (Fabric Fiber Expansion Joint)" and "Rubber PTFE Compensator", which are actually different formulas of skin expansion joints. You can take a closer look at the parameter comparison inside.
Can it be multi-layer composite? Of course you can. Many skin expansion joints are sandwich structures-the outer silicone rubber is weather-resistant, the middle PTFE is seepage-resistant, and the inner fluororubber is corrosion-resistant. However, it should be noted that the more layers, the worse the compliance, and the displacement compensation ability will be discounted.
Step 3: Determine the structural form-rectangular or circular? Refer to our rectangular non-metallic expansion joint
Flues are mostly rectangular in section, so rectangular skin expansion joints are most commonly used. But don't think a rectangle is a box welded frame,The structural form directly affects the sealing performance and the ease of installation.
What you see"Rectangular non-metallic expansion joint"Generally composed of metal frame, skin, thermal insulation cotton, pressure plate and bolts. The key point lies in the connection mode between the frame and the flue wall:
- Flanged connection: Most commonly, a circle of flange is welded at the flue opening, and the expansion joint is clamped with the flange. Easy to install, but the flange surface must be flat.
- Plug-in connection: Holes are left in the flue wall, and the expansion joint is inserted and then welded. Suitable for space-constrained places, but troublesome to replace later.
- Unilateral fixation + sliding: When the long flue needs to absorb both axial and transverse displacements at the same time, one side is fixed, and the other side is equipped with sliding supports. At this time, the candidate circular skin expansion joint may be more cost-effective-the circular force is uniform, but if the site is a rectangular flue, the square-to-round transition section has to be used.
Note:Don't make the rectangular expansion joint too wide in order to save money. When the width exceeds 1.5 meters, the skin will sag by its own weight, which will cause wrinkles and affect the life span. Our rectangular non-metallic expansion joint product page has the conversion table of specific dimensions and displacement, just follow the selection.
Step 4: Do these 3 checks before installation, otherwise there will be constant trouble in the later period
Equipment is in hand, don't rush up. Spending half an hour doing three things can save a week of rework:
Check ①: Check product specifications
Confirm the diameter, pressure class, skin material and displacement direction according to the nameplate and contract. Pay special attention to"Direction of Displacement"-Does the direction of the arrow on the expansion joint refer to the direction of media flow, or the direction of displacement? As we said in the FAQ, the expansion joint arrow usually refers to the installation direction (media flow direction), but some manufacturers mark the displacement direction,Getting it backwards will make you play backwards.
Check ②: Check the flatness of the frame
The diagonal error of the rectangular frame shall not exceed 3mm, and the warpage of the flange surface shall not exceed 1mm. The method is very simple: stick a ruler to the plug seam. If it is uneven, level it with a gasket first. Don't expect the bolts to be hard screwed-hard screwing will pull and deform the frame, and the skin will be unevenly stressed, which will leak in two or three months.
Inspection ③: Verify bolt holes and interference space
Are the bolt holes drilled on site aligned with the expansion joint flange holes? After the expansion joint is installed, are there any pipes and brackets around it to prevent its normal expansion and contraction? EspeciallyLarge tie rod expansion jointThe rotation space of the pull rod is very critical, so don't let the pull rod die. If there is not enough space, we can consider our double hinge transverse expansion joint, which occupies a small axial space.
Step 5: Routine Maintenance Do this to double your life-with a pit avoidance guide
Installed and finished? That's a big loss. The flue skin expansion joint is most afraid of two things:Ash accumulationAndLocal overheating。
- Clean up dust accumulation regularly: The surface of the skin, especially the folds, is easy to accumulate dust, and there is moisture + acidic substances in the ash, which will slowly corrode the skin. Give it a blow of compressed air every month or sweep it with a soft brush. Don't use a wire ball, it will scratch the surface.
- Check for collapse of insulation cotton: The inside of the skin is generally filled with ceramic fiber cotton, which will sink after long use, resulting in a cavity at the top, and heat directly radiates the inner layer of the skin. Every year, when the furnace is shut down for maintenance, open the observation port to feel whether the cotton has collapsed, and fill it up a little if it collapses.
- Monitor the torque of the compression bolt: The bolt will be loosened due to thermal cycle. Check it once every three months and tighten it again with a torque wrench according to the torque value given by the manufacturer. Don't screw it-one of our customers once added a socket to an ordinary wrench, which directly screwed the skin pressure plate into deformation.
Pit Avoidance Guide:
- Pit 1: Replacing skin expansion joints with metal expansion joints? Where the flue temperature changes drastically and there is much dust, the metal bellows is prone to fatigue cracking, and the skin expansion joint is the correct solution.
- Pit 2: Only tape when the skin is broken? Temporary emergency is fine, but don't think about making do with it for half a year. Once the skin is broken, the acidic gas in the smoke will drill into the insulation layer and corrode the whole frame. At that time, not only the skin will be replaced, but the whole expansion joint.
- Pit 3: Ignore the deflector. If the dust content of the flue gas is large, remember to install a guide tube inside the expansion joint to protect the skin from being directly washed by particles. Our article "The Specific Function of the Expansion Joint Guide Tube" makes it very clear.
Tsk, in the final analysis, selection, installation and maintenance, every step hides a doorway. If you follow these five steps, I can't guarantee that you will use them forever, but at least there will be no major problems for three to five years. If you are still worried, go directly to the information of "Rectangular Non-metallic Expansion Joint" and "Non-metallic Expansion Joint (Fabric Fiber Expansion Joint)" on our station, and all parameters and cases are available. Let's go!