How to choose the form of flue expansion joint? Follow these 5 steps to pick, no mistake

Step 1: Find out what is running in your flue-temperature, pressure, medium, none missing

When choosing flue expansion joint, the most taboo is to look at the sample and compare the price. Two days ago, I met a customer who was a power plant. When he came up, he asked me for a quotation of corrugated expansion joints. I asked him what the flue temperature was, and he said it was about 400℃. Again, there is sulfur in the flue gas, and desulfurization slurry reflux-then aren't you waiting for corrosion and perforation with ordinary metal corrugated expansion joints?

Therefore, the first step is to find out the operating parameters of the flue:

• Temperature: Is it normal temperature, high temperature (for example, the boiler outlet is above 800℃), or is there a sudden temperature change? High Temperature Environment PriorityHigh temperature axial expansion jointor a non-metallic compensator; For normal air ducts with normal temperature and low pressureUniversal corrugated expansion jointThat's enough.
• pressure: Positive or negative pressure? How stressful? In the negative pressure pipeline, the stiffness of the expansion joint should be enough, otherwise it will be deflated. Positive pressure pipelines should pay attention to blasting pressure.
• Medium: Is it a corrosive gas? For example, the wet flue gas after desulfurization contains chloride ions and sulfate radicals, so you have to useNon-metallic expansion joints (fabric fiber expansion joints)Or PTFE-lined metal expansion joints. If the medium contains solid particles (such as cement flue), the bellows must be protected by a guide tube. The role of the guide tube is to reduce particle erosion.

The first step of selection is not to place an order if the parameters are incomplete. No matter how urgent the customer is, it is necessary to confirm these numbers clearly.

Step 2: Choose the type by shape-circle or rectangle? Metallic or non-metallic? Give you three criteria

Either circular or rectangular depending on the flue section shape. However, whether you choose metal or non-metal, you have to screen according to the following three items:

• Criterion 1: Look at displacement。 Metal expansion joints are good at absorbing axial displacement (tension, compression), but a large transverse displacement will pull the corrugation. Instead of metal expansion joints (such asRectangular non-metallic expansion joint) is soft, can simultaneously absorb lateral and angular displacement, and save installation space. If your flue is a rectangular air duct, with many turns and complicated directions, it is more cost-effective to go directly to non-metal.
• Criterion 2: Look at temperature。 If the temperature is lower than 600℃, the metal expansion joint is fine, just use stainless steel 304/316L. Over 600℃? Then you have to use high-temperature alloy or internal insulation layer, and the cost will rise. At this time, the non-metallic expansion joint has an advantage-its fiber layer can withstand the temperature of 1200℃, and the price is cheap. Such asNon-metallic expansion joint (fabric fiber expansion joint), power plant boiler export is often used.
• Criterion 3: See Sealing Requirements。 If there is poisonous gas in the flue, it will happen. Metal expansion joints are generally sealed by bellows, with many welds and many risk points. What about non-metallic expansion joints? One layer of cloth as a whole, as long as the flange is pressed tightly, the air leakage rate is extremely low. Therefore, desulfurization flue and chemical plant exhaust flue are basically non-metallic.

Small displacement, high temperature, rectangular air duct, closed eyes to select non-metal; Large displacement, high pressure, circular pipeline, metal corrugated expansion joint is more reliable.

Step 3: Match the expansion joint for baffle door, pay special attention to these details (double seal, single axis double baffle door matching scheme)

I've seen this pit many times. If the flue system has itFlue gas baffle doorEspeciallyDouble-sealed single-axis circular baffle doorOrSingle-axis double-flapper door, the fit between the expansion joint and the baffle door must consider "spatial displacement compensation".

And guess what? Many design institutes install the expansion joint directly on the flange before and after the baffle door. As a result, as soon as the baffle door moves, the expansion joint bears extra torque, and it won't take long to crack. The correct way to do this:

• Install an expansion joint at the inlet and outlet of the baffle door to compensate for the thermal expansion of the pipe and the installation deviation of the baffle door itself.
• In the case of a double-sealed baffle door (which requires filling with sealing air), the expansion joint must be able to withstand the pressure differential of the sealing air. At this time the candidateMetal rectangular expansion jointOrCompound hinge transverse expansion jointIt's more suitable, they are stiff and won't be blown up.
• If two baffle doors are installed side by side (such as a single-axis double baffle door scheme), the middle expansion joint should be selected that can absorb bidirectional displacement, such asStraight pipe pressure balanced expansion jointIt comes with its own balance structure and will not transmit thrust to the baffle door.

attention: The gasket between the flange of the baffle door and the flange of the expansion joint must be temperature-resistant and corrosion-resistant, otherwise the seal will fail and the whole system will leak.

Step 4: 3 Things to Do Before Installation-Check Dimensions, Check Guide Direction, Pre-Stretch or Pre-Compress

Right selection, wrong installation, for nothing. Did You Do These 3 Things Before Expansion Joint Installation?

• First, check the dimensions: Don't laugh, I have seen workers put the expansion joint of DN600 into the pipe of DN800, shoot it in and weld it to death. Check the flange bolt hole distance, sealing surface width and total length, and compare them with the design drawings one by one. EspeciallyNon-metallic expansion jointIf the frame size error exceeds 2mm, it may not fit.
• Second, check the direction of the guide tube: The function of the expansion joint guide tube is to guide the flow direction of the medium and protect the bellows. Its arrow direction must be consistent with the flow direction of the medium! If it is installed backwards, the smoke backwards will directly flush the bellows and wear out in a few months. There is a question and answer on this site that specifically said this-Specific Function of Expansion Joint Guide Tube, which mentions that the arrow direction is the key to the installation.
• Third, pre-stretching or pre-compression: This depends on the temperature at the time of installation. For example, the design temperature of the pipeline is 200℃, but you install it at 0℃ in winter, and the metal expansion joint needs to be pre-compressed (to compensate for future thermal expansion); If it is a hot installation (such as the pipe is already running), then it has to be pre-stretched. For specific numerical calculation, refer toWhat is the difference between metal expansion joint and corrugated expansion jointThe stiffness formula in that question and answer. If you are really unsure, ask the manufacturer directly for pre-installed data.

Pre-tensioning/pre-compression is achieved by adjusting the tie rod nut.How to adjust the tie rod nut of expansion joint？ It's simple: first loosen the locking nut, turn the adjustment nut with a wrench to compress or stretch the bellows to the specified amount, and then lock it tight. However, be careful that the tie rod nut should not be removed after it is adjusted, it is used to limit excessive deformation.

Step 5: Don't just look at the appearance in acceptance-is the tie rod nut adjusted or not? Is the arrow in the right direction? Make it clear all at once

When the equipment arrives at the site, the acceptance is not just to see if the paint has fallen off. You score four steps:

• 1. Look at the tie rod nut: Many manufacturers screw the tie rod nut to death when delivering to prevent deformation during transportation. But you have to loosen it before installing it, allowing the expansion energy saving to stretch and contract freely. If you forget to adjust, the expansion joint is stuck by the tie rod during operation, which can't compensate for the displacement, and the pipe will be directly damaged.Does the screw of the expansion joint need to be removed?Usually it is only necessary to loosen the nut, but in the case of a transport screw (temporary fixture) it must be removed.
• 2. Check Arrow Direction: Check that the flow direction arrow on the expansion joint body coincides with the flow direction of the pipe media. Inconsistency? Rework it on site, no negotiation.
• 3. Check the guide tube: If it is a metal expansion joint, has the guide tube been deflated? Are there any gaps? Once the guide tube is deformed, the airflow will howl, which may also lead to vibration fatigue of the bellows.
• 4. Clean the sealing surface: The flange sealing surface of non-metallic expansion joint cannot have oil stain and welding slag. Rubber compensators or PTFE compensators are more delicate, and even a small particle can cause a leak.

After acceptance, don't forget to do pressure test (generally 1.5 times working pressure) and leak inspection. The flue is filled with high-temperature toxic gases, so leaking is no small matter.

Well, after these five steps, the selection, installation and acceptance of flue expansion joints will basically not make big mistakes. Finally, a long-winded sentence: The working conditions of different projects vary widely. If you are unsure, it is best to directly ask the manufacturer for technical communication-like oursCorrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry、Desulfurization flue gas baffle doorThese products have mature cases, and it is much more reliable to directly apply ready-made experience than to talk on paper.