Structural disassembly of flue metal expansion joint: five steps to understand how to use each component

Two days ago, a buddy who did power plant maintenance asked me, saying that there was always a problem with the expansion joint on their flue. When I removed it, I saw a bunch of parts in it, and I didn't know which one piped which one. In fact, the structure of the flue metal expansion joint is not so mysterious, and the core is only a few iron knots. Today, let's not talk falsely, just go to the steps, and you can find out by following the operation.

Step 1: Know the four "iron knots" in the flue metal expansion joint first

Whether you bought a general-purpose corrugated expansion joint or a customized corrugated expansion joint for power station industry, you will probably see these four guys after disassembling them:

• BellowsIt is that metal tube that telescopes like an accordion, the core of the core. The flue gas temperature change and pipe displacement are absorbed by it.
• guide tube: A straight tube in the bellows, some with and some without. More on Step 3 later.
• Tie rods and nuts: Several screws threaded on the outside of the bellows, with nuts, and some with gaskets. This thing is not for you to twist randomly, and the operation sequence is particular.
• End tube (take-over): A short pipe with both ends connected to the flue, welded flange or directly welded to the pipe. Some expansion joint end tubes are also reinforced with ring plates.

You remember these four, and it will be easier to handle later. Many novices get blind as soon as they come up. In fact, staring at bellows and tie rods is enough.

Step 2: How to choose bellows? Single layer, multi-layer, thick wall, don't mix it up

Bellows is the "heart" in the flue metal expansion joint structure. If you choose the wrong one, it's all in vain. There are roughly three types of bellows on the market: single layer, multi-layer and thick wall. How to pick? It depends on your working conditions.

Single layer bellows-use when the pressure is low, the temperature is not high, and the medium is clean. For example, general ventilation ducts can save enough money. But if you use it in desulfurization flue or cement industry, the flue gas carries dust and corrosive gas, and the single layer will soon be wasted.

Multi-layer bellows – where the pressure is a little higher, or where a greater amount of compensation is required. The multilayer structure has good flexibility and can absorb more displacement. Like the corrugated expansion joints used in the power station industry, many of them are multi-layer designs.

Thick-walled bellows-wear-resistant, erosion-resistant, suitable for high-dust, high-flow conditions. For example, metal corrugated expansion joints in cement industry, or large-diameter thick-walled expansion joints, have a wall thickness of even more than 3mm. However, note that the stiffness of thick walls is large, and the compensation ability will decrease, so it needs to be selected according to calculation.

Before selection, it is best to get the stiffness data of the bellows (generally provided by the manufacturer), otherwise the thrust of the pipe will exceed after installation, and the bracket will be deformed.

Step 3: Is the deflector installed or not? Location and direction are particular

Many people think that the diversion tube is optional. Let me tell you, whether to install it or not, how to install it, directly decide how long the expansion energy saving will last. You can understand by referring to the question and answer on "Specific Function of Expansion Joint Guide Tube" in our station.

What circumstances must be installed?When there are dust, particles and liquid droplets in the flue gas, the guide tube can guide the airflow to the center to avoid directly washing the bellows. You said that the desulfurization flue and the cement kiln tail do not install the guide tube? The bellows wore out in three months.

What circumstances can you not pretend?Clean gas, low flow rate, low temperature environment, such as air conditioning pipes, install it to increase resistance, which is useless.

Don't reverse the direction of installation!The guide barrel usually has an arrow pointing to the direction of the medium flow. Whichever way you let the arrow go, the bellmouth of the deflector goes. If it is installed backwards, the airflow will be directly poured into the inner wall of the bellows, and dust accumulation, wear and vibration will all come. Remember: the medium goes in from the large port and out from the small port (or vice versa, depending on the manufacturer's design). If you are not sure, compare your expansion joint, and the sequence of takeover on the end tube can also judge the flow direction.

Step 4: Don't screw rods and nuts-construction determines your operation sequence

Tie rods and nuts are the most vulnerable places in the flue metal expansion joint construction. I have seen many on-site workers who screwed the nut to death when they got it. As a result, there was no displacement at all when the expansion joint was installed, and the pipe was forcibly broken.

Correct sequence of operations:

1. Before installation, the nut on the tie rod is generally installed near the end plate (transportation screw state), the purpose is to compress and lock the bellows to prevent damage during transportation. You can't move in this state.
2. Put the expansion joint in the middle of the pipe, and after welding or flange connection, start adjusting the tie rod.
3. Loosen the locking nut (some call the back cap) first, then adjust the adjustment nut to get the bellows back to free length (or pre-stretch/pre-compress as per design). How long it needs to be drawn depends on the installation size given by the manufacturer.
4. After adjusting in place, lock the back cap again. Note: Do not screw to death with brute force, leave a little clearance to allow the tie rod to move freely with the thermal expansion and contraction of the pipe. The role of the tie rod is to withstand the thrust (blind plate force) generated by the internal pressure of the pipe, not to let you crush the bellows to death.

Q: Does the screw of the expansion joint need to be removed? A: The transport screw should be removed, but the tie rod nut cannot be removed, it is a permanent piece. Specifically, you can turn over the question and answer on our station "Does the screw of the expansion joint need to be disassembled?", which is more detailed.

Step 5: Different flue working conditions, the structure should be "changed"

If the structure of the same flue metal expansion joint is changed to different working conditions, the parts will have to change accordingly. Let's take a look at the actual case:

• High temperature flue (> 600℃): The bellows must be made of heat-resistant alloy (such as Inconel), and the wall thickness should be thickened. Also consider exterior insulation. OurHigh temperature axial expansion jointThat's what we do.
• Desulfurization flue (wet flue gas, containing acid): Stainless steel 316L or more corrosion-resistant for corrugated pipe material, the guide tube is best lined with PTFE or rubber. You go and seePTFE-lined hoseAndNon-metallic expansion jointThey are specifically designed to deal with corrosion.
• Cement industry (high dust, large displacement): The guide tube must be thickened and made into a wear-resistant liner. The corrugated pipe chooses a multi-layer structure, and the compensation amount is large. LikeMetal Corrugated Expansion Joints in Cement IndustryJust bring the guide tube and wear ring.
• Large pipes that need to withstand blind plate forces: Structures with tie rods must be used, such asStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint。 The number and thickness of the tie rod should be calculated according to the pressure, and you can't pat your head.

You see, with the same structure, in a different working condition, the material of the bellows has changed, the guide tube has been thickened, and the tie rods have increased. This is called "dressing up". When you choose a model next time, compare it with this idea, and you will basically not go awry.

All right, after five steps, you'll figure out the foundation of the flue metal expansion joint construction. Next time you encounter it again, just dismantle it and adjust it by hand. Don't just look at the drawings. Remember: the bellows is the core, the guide tube saves life, the tie rod is not screwed blindly, and the working condition determines everything. If you have any specific questions, we still have a variety of product information and questions and answers in our station. You will understand by turning it over.