Step 1: Understand what "metal built-in flue expansion joint" is-what is the difference between it and ordinary expansion joint? Don't buy the wrong one
Don't be in a hurry to place an order. The "metal built-in flue expansion joint" you want to install is similar to our common onesUniversal corrugated expansion jointIt's not the same thing. To put it bluntly, ordinary expansion joints are mostly external, directly strung in the middle of the pipeline, and rely on the bellows itself to absorb the displacement. But the built-in flue expansion joint is different-its bellows is "hidden" inside the flue, and there is a layer of guide tube outside to protect it, so that the flue gas will not directly flush the bellows when it flows through. This is like wearing a body armor on the bellows, which is resistant to high temperatures, wear-resistant and not easy to accumulate dust.
Then when must you use this built-in? For example, desulfurization flue of power plant and blast furnace gas pipeline of iron and steel plant have high medium temperature and large dust content, and ordinary expansion joints are worn out in a few months. Replace withCorrugated expansion joint for power station industryOrHigh temperature axial type expansion joint Not necessarily right. What you are looking for is a model specifically marked with "built-in structure" or "with guide tube". Don't buy it as a general-purpose type, otherwise it will be useless to install it.
Step 2: Find out these three hard indicators before selecting: temperature, pressure and displacement. If one is less, it will be easy to overturn
Selection is not complicated, but there are many pits. Two days ago, a customer asked with the flue gas temperature parameters, saying whether stainless steel 304 can be used at 300℃? I almost couldn't hold it back-over 250C you have to consider heat resistant alloys. Remember three numbers:
- Temperature: What is the maximum temperature of the medium? If it is smoke, you have to consider the instantaneous overtemperature. Typically, 304 is used for less than 200 °C, and 316L or Incoloy 800 is used for more than 300 °C. Don't just look at the design temperature, ask about the actual operating peak.
- pressure: The flue system is mostly slightly positive or negative pressure, but don't be careless. For example, the pressure fluctuation at the inlet of desulfurization tower can reach ±5kPa. If you chooseExternal pressure single axial expansion jointIts pressure bearing capacity is different from that of ordinary axial type, so you have to check the design pressure.
- displacement amount: What are the axial, transverse and angular displacements? Generally, the built-in flue expansion joint is mainly compensated in the axial direction, but when the pipe direction is complicated, the lateral displacement may also be large. You chooseCompound hinge transverse expansion jointStillThe pressure balance expansion joint of curved tube depends on the displacement direction.
Write these three parameters on paper and send them to the manufacturer. Don't just give one drawing, it may miss the label. I have encountered it before. The customer said "do it according to the picture", but the temperature was not written on the picture. When I went to the scene, it was measured at 450℃, and the bellows was directly deformed. Tsk, that's a miserable one.
Step 3: Do not be careless in unpacking inspection-look at the interface, measure the size, check the guide tube, and replace it immediately if you find defects
The goods arrived? Don't rush to the car. Unpack it first and check it all the same. The metal built-in flue expansion joint you get should contain these parts: bellows, guide tube, end tube (or flange), tie rod nut (if it is a model with tie rod).
- Interface Size: Inner diameter, outer diameter, flange bolt hole distance, take a tape measure. The outer diameter of the end pipe of some manufacturers will be smaller than the inner diameter of the flue, which is for the convenience of installation, but the deviation cannot exceed ±2mm. Once, someone didn't measure it, and when he hoisted it, he found that the flange hole didn't match, so he returned it to change it, and the construction period was delayed for a week.
- guide tube: This is the core of the built-in structure. Is the thickness of the guide tube sufficient? Are there any burrs? Are the welds smooth?Specific Function of Expansion Joint Guide TubeIt is diversion and anti-scour. If the cylinder wall is thin or the weld has cracks, it won't take long to be worn out.
- Bellows surface: See if there are scratches and pits. The wall thickness of bellows is usually only 0.5~1.5mm, and once it is damaged, it is the stress concentration point, and its life is greatly reduced.
- tie rod nut: If a tie rod is configured, check that the nut is loose. The function of the tie rod is to prevent the bellows from over-stretching, but it is easy to shake and fall off during transportation.
If you find any defects, take pictures immediately and contact the manufacturer for an exchange. Don't think of "use it first and talk about it later". When something goes wrong, the whole flue will have to be dismantled.
Step 4: Welding installation is particular-how to align the built-in structure? When will the tie rod nut be removed? Step by Step
Okay, the site conditions are in place, start installing. Remember: Metal built-in flue expansion joints can't be welded directly like regular pipes, it has a whole process.
1. Centering
First hoist the expansion joint in place and adjust the position so that its center line coincides with the center line of the flue. The error is controlled within ±3mm. Why? Because there is a gap between the built-in guide tube and the inner wall of the flue, if it is deviated, the guide tube may get stuck, or the flue gas may directly bypass the guide tube and flush the bellows.
2. Spot welding fixation
Use spot welding to spot the end pipe and flue first, do not weld fully. Then measure the dimensions and alignment again, and confirm that it is correct before welding. Symmetric segment welding is adopted during welding to reduce thermal deformation. The wall thickness of the flue is generally thin (4~6mm), and J422 welding electrode or argon arc welding can be used, but the current should not be too large to prevent burning through.
3. When will the tie rod nut be removed?
This is the most frequently asked question by newbies. Remember: When the expansion joint is transported and installed, the tie rod nut is tightened to act as a temporary fixing and prevent the bellows from being stretched or compressed. Once installed in place,Before the piping system is pressurized or heatedThe tie rod nut must be loosened to leave space for displacement. Loosen by how much? Generally, it is adjusted to the "free state", that is, the bellows is neither pulled nor compressed. Specific operation: First, loosen the nut for a few turns, then gently push and pull the expansion joint with your hand, feel that it can move freely, and then tighten the locking nut, but don't lock it. If you forget to dismantle it, or if you don't dismantle it enough, the bellows can't be deformed, which is equivalent to white installation, and it will even crack the pipe.
4. Check the direction of the tube
The built-in deflector usually has an arrow to indicate the flow direction, ensuring that the direction of the arrow and the flue flow direction are consistent. When installed backwards, the bell mouth of the deflector will form resistance to the airflow and aggravate the front end wear.
When the installation is complete, mark around the expansion joint to indicate "No treading" and "Tie rod loosened". There are many people on the construction site, so who will screw the nut back if it can't be guaranteed.
Step 5: Acceptance and routine maintenance-no air leakage doesn't mean it's okay, it's safer to check these places regularly
After the installation is finished and the pressure test does not leak, will everything be fine? Of course not. Metal built-in flue expansion joints are "maintenance-free" devices, but maintenance-free does not mean "no inspection". And guess what? Many manufacturers terminate the warranty period not because the product is broken, but because users don't find the initial problems in time.
- Cold state inspection: Are the tie rods all loosened? Are the anti-retreat spacers of the nuts properly installed? Are there any obstacles around the expansion joint that prevent displacement?
- Thermal re-examination: After the system heats up to normal operating temperature, check again. See whether there is any abnormal discoloration on the surface of the bellows (discoloration indicates local overheating), listen to whether there is any abnormal noise (may be that the internal guide tube is loose), and measure whether the displacement at the flanges at both ends is within the design range.
Routine maintenance, to put it bluntly, is as follows:
- Appearance inspection: Once a month, see the bellows for cracks, corrosion spots and bulges. Especially the expansion joint near the flue elbow, the smoke and dust washing is the most severe.
- Guide tube wear: After the built-in guide tube is worn out, the smoke will directly impact the bellows, resulting in rapid failure. Therefore, during maintenance, it is best to reach in with an endoscope to see the inner wall of the guide tube.
- State of tie rod nut: After running for a long time, the nut may loosen or rust to death, apply grease regularly to ensure that it can be adjusted when needed.
- insulation layer: If there is insulation on the outside of the expansion joint, pay attention to the insulation material not to compress the bellows, otherwise it will affect the displacement. Some construction maps save trouble, and the insulation cotton is stuffed full. As a result, the bellows is so hard that it can't be moved, which is equivalent to white installation.
Follow this guide you have on hand and you can avoid 90% of all installation accidents. The remaining 10%? That depends on field experience and luck. If you are unsure, ask the manufacturer's technology directly.