Round flue expansion joint selection and installation, you can't go wrong by following these steps

Round flue expansion joint selection and installation, you can't go wrong by following these steps

Two days ago, I met a buddy who was doing a desulfurization project. He called me anxiously and asked me, "Should the round flue expansion joint be metal or non-metal? It leaked after two months of installation. Did you buy the wrong one?" This is actually very typical. Wrong selection and irregular installation, no matter how good the expansion joint is, it can't bear it. Today, let's break up the chat. Follow these steps, and basically you won't flip over.

Step 1: Start by figuring out exactly what kind of expansion joint you need for your round flue – metal or non-metal?

Although they are all round flue expansion joints, the temper of metal and non-metal is completely different. Metal corrugated expansion joints (such as the one on our stationCorrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry) High pressure resistance, high temperature resistance, suitable for high temperature and high pressure steam pipeline or flue gas pipeline. Non-metallic expansion joints (also called fabric fiber expansion joints, orNon-metallic expansion joint (fabric fiber expansion joint)) is mainly corrosion resistant, large compensation and light installation, especially suitable for flue gas systems containing acidic media such as desulfurization and denitrification.

How to choose?You first look at the medium temperature and composition.

• If the temperature exceeds 400℃ or the pressure exceeds 0.1MPa, honestly choose metal corrugated expansion joints, such asHigh temperature axial expansion jointOrUniversal corrugated expansion joint。
• When the temperature is 200 °C or lower, the flue gas contains sulfur and humidity, and a large amount of compensation is required, a non-metallic expansion joint is preferably selected. Note that the non-metallic expansion joint implements the national standard JB/T 12235-2015.
• Another situation: you need both high temperature resistance and corrosion resistance? That can be consideredPTFE-lined hoseOr the expansion joint of the composite structure, but this situation is relatively rare. Generally, non-metal is enough for the net flue gas after desulfurization.

Step 2: Measure the accurate dimensions and parameters, and don't let the expansion joint become a "shrinking joint"

Pipe outer diameter (or inner diameter), wall thickness, installation length. But the amount of light is not enough, you still have to figure out these parameters:

• Axial displacement:Exactly how many millimeters do pipes expand and contract after thermal expansion? Doesn't count? The thermal elongation on the design drawings is removed or estimated using the formula Δ L = α · L · Δ T (α is the coefficient of linear expansion, carbon steel is about 12×10 ^-6/℃).
• Lateral and angular displacements:If the pipe has a bend or a different axis, choose a model that can absorb multi-directional displacement, such asCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。
• Pressure Class:The round flue is usually low pressure (0.05~0.1MPa), but don't guess. Ask what is the static pressure at the outlet of the fan. If the pressure is slightly higher, considerStraight pipe pressure balanced expansion jointOrExternal pressure single axial expansion joint。

Tips:When measuring the dimensions, it is recommended that you measure three positions (0°, 120°, 240°) to take the average value, because there will be some ellipticity when round flue welding. Otherwise, it will be embarrassing if you buy it back and find that it can't fit in.

Step 3: Check the checklist before installation-the pull rod, guide tube and direction arrow should not be leaked

After the expansion joint arrives, don't rush up. First check the following against the checklist:

• Tie rods and nuts:The role of the tie rod is to prevent the expansion joint from being over-stretched or compressed during transportation and installation, and some also come with an adjustment nut. You flip through the Q&A "How to Adjust the Expansion Joint Tie Rod Nut" on our site, which has detailed steps. Note: Before installation, the tie rod nut is generally in the "pre-tightened state" to ensure that the length of the expansion joint meets the design value.
• Guide tube:The function of the guide tube (refer to the question and answer of the specific function of the expansion joint guide tube on our station) is to guide the airflow and protect the bellows from being washed by high-speed media. The direction of the deflector must be consistent with the direction of media flow during installation!
• Directional arrows:Expansion joints are usually marked with arrows indicating the direction of media flow. If it is installed backwards, the guide tube will block the airflow, the bellows will also be washed, and the life will be greatly reduced. Alas, I've seen no less than ten cases of this thing acting backwards.

In addition, check for any bumps on the flange surface and scratches on the sealing surface. If it isrubber compensatorOrRubber PTFE compensatorIt also depends on whether the rubber is aged and hard.

Step 4: On-site installation and practical operation-screw screw, adjust clearance, and leave pre-deformed doorway

At this stage, it was time to see the true skill.

1. How to screw the screw?First pre-tighten the nut by hand until the flange fits, then tighten diagonally with a wrench, with even force. Don't screw to death at once, gradually apply force in 3~4 times. Tightening too tightly will crush the bellows of the expansion joint to death, and it won't be able to compensate for the displacement-so what's the use of it?

2. How to adjust the gap?After the expansion joint is installed, the clearance required by the design (usually the cold tightness value) should be reserved between the pipes at both ends. For example, if the design says cold tightness 50mm, you have to use a jack or inverted chain to pull the pipe apart 50mm, and then put the expansion joint in. This is done so that the expansion joint is in the optimal working position when hot. Many masters call it "pre-deformation", which to put it bluntly means "pull it in the reverse direction first".

3. Don't forget to dismantle temporary support!See clearly: There may be temporary supports or positioning screws for transportation on the expansion joints, which must be removed after installation and pipe fixation. If it is not dismantled, the expansion joint will be locked, and the pipe will burst in minutes after heat expansion. And guess what? Some construction units really forgot to dismantle it, and it leaked less than a week after startup.

Step 5: Check the key points after running-teach you to judge whether the expansion joint is "dying"

After the expansion joint is installed and the pipeline is put into operation, don't think that everything will be fine. Regular inspections are important. Look at these signs:

• Bellows Surface:If there are obvious indentations, cracks or corrosion spots in metal bellows, it means that the life is almost the same. If there is bulging, delamination or leakage in the non-metallic fabric layer, change it quickly.
• Loose tie rod nut:Twist it by hand. If it is loose, it means that the displacement of the pipe is outside the design range, and the length of the tie rod needs to be readjusted.
• Water seepage or smoke from the flange:This one is the most intuitive. Once the gasket is found to fail, the machine has to be stopped.
• Unusual sound:When you hear a "quack" or "fizz" sound during operation, it may be that dust or scale in the bellows causes stuck, or it may be that the expansion joint has fractured by fatigue.

Note:If you find that the expansion joint has the above problems, don't make do with it. Once the circular flue expansion joint fails, it can be shut down to reduce the load, and the smoke gas backward flow and equipment damage in the worst case. It is recommended to refer to the Q&A of "Service Life of Expansion Joint" on our station, and make a regular replacement plan according to the working conditions.

BTW: Don't confuse baffle doors with expansion joints, they do their own jobs

"Can you putFlue gas baffle doorUsed as an expansion joint? " I said never. Flap doors (incl.Round Flap Door (Double Seal)、Double-sealed single-axis circular baffle dooretc.) is to cut off or regulate the flue gas flow, which by itself cannot absorb pipe displacement. Expansion joints are specially used to absorb thermal expansion and contraction. If the pipeline needs to be cut off and compensated at the same time, it has to be used in combination with "baffle door + expansion joint", and each performs its own duties. For example, on the inlet and outlet flue of the desulfurization system, usually one side isDesulfurization flue gas baffle door, one side is a round flue non-metallic expansion joint. Don't try to save trouble and just pretend to be one. When the time comes, it will be too late to cry.

Well, add a reminder to the above five steps. If you follow it, there will be no big problems in the selection and installation of the round flue expansion joint. If you are still unsure, go directly to the product information and FAQ on our site. The terms are the same, so there will be no confusion.