Welding flue corrugated expansion joint selection and installation, step by step to teach you to avoid those pits

Step 1: Find out the temper of your flue first-temperature, pressure and medium can't be missed

To be honest, many people came up and asked me, "Master, give me a welding flue corrugated expansion joint. What is the diameter?" Let me ask you, you haven't even figured out what the hell is running in your flue, so it's useless to buy it back and put it on.

Let's talk about temperature first. Flue temperatures range from tens of degrees to hundreds or even thousands of degrees. If you choose ordinary stainless steel, it will crack in a month or two under high temperature, which is called a misery. Pressure too, positive or negative pressure in the flue? Almost, the expansion joint may bulge or deflate.

And the medium-are there any corrosive components in the smoke? For example, the wet flue gas after desulfurization contains sulfur and chlorine, which is extremely corrosive. At this time, you have to consider lining PTFE or high nickel alloy. Two days ago, I met a buddy in a cement factory, and the flue was full of dust. As a result, I chose a general-purpose corrugated expansion joint, which was worn out in three months. You say it was wrong or not?

Tips:Make a table of temperature (maximum/minimum), pressure (design pressure/working pressure), and media composition (especially sulfur, chlorine and dust concentration). Take this table to find the manufacturer, at least you won't step on a big pit.

Step 2: Choose Waveform or Material? Seat according to the working conditions, don't let the expansion joint be "laid off" in advance

Okay, the parameters are figured out, then pick the model. How to choose waveform and material? I'll give you a local method directly:

• Low pressure, low temperature (≤300℃), no corrosion→ Universal corrugated expansion joint, cheap enough.
• High temperature (300~800℃)→ High-temperature axial expansion joint, or lined with stainless steel and thermal insulation layer.
• High pressure or large displacement→ Consider double hinge transverse expansion joints, straight tube pressure balance expansion joints, or external pressure single axial expansion joints.
• Corrosive flue gas (e.g. desulfurization flue)→ Either lined with PTFE or high-nickel alloy (such as Incoloy 825), and be sure to add a guide tube to protect the bellows.
• Rectangular flue→ Metal rectangular expansion joint, don't buy a round one.

Do you know the difference between U-shaped wave and omega-shaped wave? Omega-shaped wave has high withstand voltage, but small compensation displacement; The U-shaped wave has a large displacement but a low withstand voltage. You calculate according to the actual displacement, don't just look at the price.

Alas, many people are stuck in the "cheap picture" when selecting the model. As a result, it leaked within two years after installing it, and the rework cost is ten times more than the price difference. I've seen this too much.

Step 3: Finish these things before installation, so as to save rework later

When the expansion joint arrives at the site, don't rush to unbox and weld. Let's do these things first:

1. Check Model Number and Size— — Caliber, length, flange bolt hole spacing, item by item. Especially for flange connection, is the sealing surface consistent?
2. Check transport limiting device— — When the expansion joint leaves the factory, it is usually locked by a transport rod or nut to prevent deformation during transportation. Don't disassemble it before installation, wait until it is installed and then release it.
3. Clean up pipe interfaces- -The pipes at both ends of the welded flue corrugated expansion joint must be flat, burr-free and oil-free. The gap of the counterpart should be uniform, and the amount of misalignment should be ≤1mm, otherwise the stress will be uneven when welded, and it will crack soon.
4. Pre-stretched or pre-compressed— — According to the design compensation amount, if the difference between the temperature during installation and the operating temperature is large, it is necessary to pull/press a certain amount in advance. For specific values, see the installation instructions provided by the manufacturer.

And guess what? I have seen a construction site where the installation team didn't dismantle the transport nut and welded it directly. After the pipeline heated up, the expansion joint couldn't move at all, and the pipeline was deformed. Isn't this a typical "saving trouble" and being pitted?

Step 4: Welding flue corrugated expansion joint, follow these 5 steps to install correctly

Welding is a technical activity, which directly determines the life of the expansion joint. Don't let the welder just click twice, follow the steps:

• 1. Positioning spot welding— — Place the expansion joint in the middle of the pipeline, symmetrically spot weld 4~6 points, and fix the position. Note: Do not weld through the end tube of the expansion joint during spot welding, otherwise it is easy to concentrate stress.
• 2. Symmetric Segment Welding— — Sectional jump welding from the middle to both sides, the length of each section does not exceed 200mm, reducing heat input. It is best to use argon arc welding for primer and manual arc welding for cover. The welding current should not be too large to prevent overheating and burning through.
• 3. Pay attention to the direction of the arrow-There are generally arrows on the expansion joint to indicate the flow direction of the medium. Never weld it backwards! If the guide tube is welded backwards, it will not protect the bellows. There was a question and answer before that specifically said this, "The direction of the arrow of the expansion joint refers to". If you don't understand, go back and turn it over.
• 4. Weld Treatment— — Clean the coating and splash after welding, and the weld height should not exceed 1.5mm. If possible, do non-destructive testing (PT or MT), especially in high temperature and high pressure conditions.
• 5. Remove transport limits— — After all welding, loosen the transportation rod or nut, so that the expansion energy saving can expand and contract freely. Note: Some tie rods are permanent (such as large tie rod expansion joints), so don't dismantle them, only adjust the nut clearance.

Note:During the welding process, it is necessary to avoid welding slag splashing on the surface of bellows. Bellows are thin-walled parts, and a small pit may become the starting point of stress corrosion.

Step 5: Don't leave in a hurry after installation. Acceptance and daily maintenance are the keys to life extension

Is it all right when it's installed? Beautiful thought! Without acceptance and maintenance, the expansion joint will still die early.

Acceptance points:

• Visually inspect whether there are cracks, pores and biting edges in the weld?
• Check the expansion joint with a level ruler for skew?
• Turn the tie rod nut to make sure it moves freely (if there is a tie rod).
• If conditions permit, do a pressure test, press to 1.5 times the design pressure, and hold the pressure for 30 minutes without leakage.

Routine maintenance:

• Regularly check the bellows surface for corrosion, wear and cracks. Especially near the weld and at the trough of the corrugation.
• Check whether the tie rod nut is loose, and if loose, adjust it according to the method of "How to adjust the tie rod nut of the expansion joint".
• For expansion joints with guide tubes, pay attention to the scour marks on the inlet side. If the wear is serious, replace or add anti-wear plates in time.
• Make a displacement record every six months to see if the actual displacement of the expansion joint is within the design range. If an abnormal displacement is found, there may be a problem with the pipe support.

To put it bluntly, welding flue corrugated expansion joints is like people, and they have to have regular physical examinations. If you treat it well, it will last you for ten or eight years. You don't care about it, and they will strike for you in two or three years. Is that the truth?

Okay, that's it for today. If you are encountering difficulties in selection or installation, you are not sure whether to use the general-purpose type or the high-temperature axial type, or you want to know how to match the metal rectangular expansion joint with the flue baffle door, please feel free to ask. Anyway, you remember: find out the parameters first, don't be cheap in selection, don't save the installation process, and don't be lazy in maintenance-these four are done, and the expansion joint will basically not drop the chain.