Selection and installation of square metal expansion joint of flue, three steps

First, don't rush to buy, find out what it looks like

When it comes to flue square metal expansion joints, is your first reaction "a square box with corrugated pipe"? Pretty much, but the devil is in the details. To put it bluntly, this thing is a compensator used on a square flue, which specifically absorbs the displacement of thermal expansion and contraction.

Structurally, it mainly consists of three parts:Bellows、End tube (flange or direct section)、guide tube。 Some also come with tie rods or ring plates. Bellows are the core, generally pressed with stainless steel 304 or 316L, with wall thicknesses ranging from 0.5mm to 3mm. The end pipe material is usually the same as the pipe, Q235B or weathering steel. What about the guide tube-here is a key point: it is installed on the inner wall of the bellows, and its function is to guide the flue gas downstream, prevent the high-speed airflow from directly washing the corrugations, and prolong the life. The direction must be consistent with the flow direction of the medium, otherwise it will be in vain.

Alas, two days ago, I met a customer and bought a non-metallic expansion joint and installed it on a square flue. As a result, it collapsed after two months of use, because the medium temperature was high and the non-metallic couldn't bear it. So remember: Flue square metal expansion joint is suitable for high temperature, high pressure, corrosive flue gas, such as desulfurization flue in power station, steel and cement industries. We also have them in our stationMetal rectangular expansion jointAndRectangular non-metallic expansion jointProduct information, but today is dedicated to metal.

Second, the selection stares at these three parameters, can't be wrong

There are a lot of parameters, such as waveform, wavenumber, stiffness... Don't dizzy, you only need to stare at three:Dimensions、Temperature、displacement amount。 Take care of these three, close to ten.

• Dimensions:It is not just to look at the inner diameter of the flue opening. You have to count the installation space and flange bolt hole spacing. For example, the square flue is 800×600mm. You have to confirm whether the flange is flanged or inserted. Is the number of bolts correct? From our libraryMetal rectangular expansion jointThe sizes range from 200×200 to 5000×5000, but the specific selection should provide the flue section size and pipe wall thickness.
• Temperature:What is the flue gas temperature? Normal temperature? 300℃？ 600℃？ Temperature determines the material of the bellows and the need for insulation. Generally, 304 is used below 300℃, 316L or 321 is used over 300℃, and high-temperature alloys have to be considered if they are higher. Attention! The higher the temperature, the displacement capacity of the expansion joint will decrease, and the temperature correction coefficient should be used when selecting the model.
• Displacement Amount:This is the easiest to overlook. You want to calculate the amount of thermal expansion of the pipe: how much is the axial elongation? How much is the lateral yaw? For example, a 10-meter-long steel flue has a temperature rise of 200 °C and an axial expansion of about 24 mm. If there is a bend in the flue, you have to calculate the lateral displacement. When selecting the model, leave 20% margin for the nominal displacement. For example, to calculate the axial displacement of 20mm, you choose one that can compensate for 25mm. In the stationUniversal corrugated expansion jointAndHigh temperature axial expansion jointParameters can also be referenced, but squares should be accounted for separately.

If the displacement is large, you can chooseCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion joint-Of course these are circles, but squares have similar structures. You directly tell the manufacturer the working condition parameters and let them help you calculate them.

3. Confirm these 5 key points before installation. Don't wait for installation before disassembling

When the goods arrive, before installing, you better take a piece of paper and go through the following 5 items:

• Direction of guide tube:See the arrow? There are generally arrows on the guide tube to indicate the flow direction of the medium, or there are marks on the product nameplate. No arrows? Then you look at the bell mouth of the guide tube, and the big mouth faces the direction of the medium. Installed backwards, the bellows are hit by the airflow and perforated in a few months. We have an article on our site calledSpecific Function of Expansion Joint Guide TubeYou can check it out.
• Tie Rod Adjustment:When many square expansion joints leave the factory, the tie rod nut is locked to prevent transportation deformation. Don't rush to dismantle it when installing it. Install it on the pipe first, tighten the flange bolts, and then adjust the tie rod nut. How to tune it? refer toHow to adjust the tie rod nut of expansion jointThat question and answer: Loosen the locking nut, let the tie rod be in a free state, and ensure the expansion and energy saving free expansion and contraction. If the tie rod is used for limiting position, it should be adjusted to the specified length according to the design drawing.
• Pre-stretch/pre-compression:This one is more professional. Simply put, if the pipe installation ambient temperature is lower than the operating temperature, the expansion joint needs to be pre-stretched to compensate for thermal expansion; Otherwise, pre-compression. The amount of pre-stretch is generally marked by the manufacturer when leaving the factory, or told you in the installation instructions. Just do it. Don't pull blindly by your feeling, you will get tired if you pull too many ripples.
• Hoisting Protection:The square expansion joint is large in volume and heavy in weight, and the wire rope cannot strangle the bellows when hoisting. Use soft slings, or pad planks. Once the bellows is deformed, the local stress is concentrated, and the life is discounted.
• Grounding jumper:If the flue has anti-static requirements, the flanges at both ends of the expansion joint should be jumped with copper wires to form a passage. Otherwise, static electricity will accumulate and ignite ash powder.

4. Don't go in a hurry after installation. Check these three details during trial operation

Installed? Before turning on, take another 10 minutes to look at these three points:

• Flange Seal:After half an hour of running, sweep the flange connection with a thermometer gun. Found unusually low temperatures somewhere? It means there is an air leak. Or use soapy water to detect leaks. If it is a metal wound spacer and the compression force is not enough, re-tighten the bolts. Pay attention to symmetrical screwing, do not twist the flange to deform.
• Is the displacement normal:See whether the deformation direction of the bellows of the expansion joint is consistent with the design. For example, when axial compression is designed, it is stretching, indicating that the guide tube is installed backwards or the tie rod is not correctly adjusted. Shutdown adjustment.
• Abnormal noise vibration:Listen closer, is there a "hissing" sound? That's the airflow washing out the ripples. During normal operation, the expansion joint should have a slight and uniform displacement, but it should not shake violently. In case of jitter, check whether there is air column resonance or the stent stiffness is insufficient. Install limiting devices if necessary.

And guess what? There was a feedback from a power plant that the expansion joint sounded. I went to see that the tie rod was not loose, and the bellows couldn't move, so I just held it back. Loose nut, problem solved.

5. Daily maintenance Look at these two points, and it is not a problem to use it for several years

• External inspection:Once a quarter, see the bellows surface for cracks, corrosion spots, and depressions. Especially the weld position is most prone to fatigue cracks. If slight corrosion is found, remove rust and touch up paint in time. If the corrosion is serious, such as thinning more than 30% of the original wall thickness, change it quickly. Don't save this money, the leak affects the whole system.
• Guide Condition:It is normal for the deflector to wear out, but don't let it wear out the bellows. If possible, look at the end of the guide tube with an endoscope once a half year. If the guide tube is found to be detached or severely deformed, replace it. In addition, if the flue square metal expansion joint is in low temperature and humid environment for a long time, pay attention to anti-corrosion. From our stationCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustrySelection cases can also be referred to, all of which are actual combat experience.

Selection and installation are in place in one step, so you can worry less later. If you are unsure of the size or displacement, ask the manufacturer directly, send the flue path map to them, and let them come up with a plan. Save the back and forth of buying the wrong model.