First, find out what the expansion joint on the bypass flue of economizer carries exactly-temperature, pressure and displacement must be missed
The economizer bypass flue is in a pretty bad condition. If you think about it, the flue gas comes directly from the furnace, and the temperature can always be 300 to 400 degrees Celsius, and it may even rush to 500 degrees Celsius. Although the pressure is not high, the flue section is large, and the rectangular structure is stressed enough. More importantly, when the boiler starts and stops and the load changes, the flue will expand and contract thermally, and the displacement is not a little. If you don't understand these parameters thoroughly when choosing expansion joints, you are laying mines for yourself.
So, when you get a project, don't rush through the product brochure in the first step. Give me these three numbers first:Design temperature, design pressure, compensation amount in each direction(axial, transverse, angular). Temperature determines whether you use metal or non-metal, pressure determines the number of layers and thickness of the corrugation, and the amount of displacement determines the number of waves of the corrugation or the depth of folding of the fabric layer. If one is missing, your later quotation will be inaccurate, let alone the selection.
Two days ago, a buddy told me that he directly copied the specifications of the previous project to save trouble, but it leaked after two months of installation. Why? Because that project is the low-temperature flue gas after desulfurization, his project is the bypass original flue gas, and the temperature difference is 200 degrees. Tsk, the lesson is deep enough, isn't it?
Practical Selection: Metal Rectangle or Non-Metal Fabric? According to these four steps
OK, I got the parameters, so the next step is to choose one of the two:Metal rectangular expansion jointStillRectangular non-metallic expansion joint？ Actually, it's not so entangled. You can't go wrong by following the following four steps.
Step 1: Look at the temperature
If the medium temperature exceeds 400℃, the non-metallic fabric basically can't handle it (unless you add multiple layers of insulation, but the cost goes up). At this time, honestly choose the metal rectangular expansion joint, 304 or 316L stainless steel corrugation, and the temperature resistance can reach eight or nine hundred degrees. When the temperature is below 400℃, non-metals have an advantage-cheap, corrosion resistant, and large compensation.
Step 2: Look at the stress
If the pressure exceeds 0.1MPa (about 1kg), the non-metallic expansion joint has to be reinforced or metal instead. Why? Because the fabric layer has poor pressure capacity, it is easy to bulge and tear. If it is a negative pressure condition, both will work, but non-metal is lighter and hassle-free to install.
Step 3: Look at the displacement and installation space
Need a large amount of compensation (such as more than 50mm in the axial direction)? Non-metallic expansion joints can compensate more in the same space because the fabric can be creased more. Metal rectangular expansion joint depends on corrugation, wave height and wave pitch have limits, and may not be as flexible as non-metal when the space is limited.
Step 4: Look at the corrosiveness of the medium
Flue gas contains sulfur and chlorine? The skin materials of non-metallic expansion joints (such as fluororubber and polytetrafluoroethylene) have much stronger corrosion resistance than stainless steel. However, it should be noted that the frame of the non-metallic expansion joint is metallic, and the flange surface also needs anti-corrosion treatment.
If you are not sure, ask the manufacturer directlyNational Standard for Non-Metallic Expansion Joints JB/T 12235-2015AndMetal rectangular expansion jointThe sample can be known by checking the parameter table. In addition, this site hasHigh temperature axial expansion jointAndCorrugated expansion joint for power station industryYou can also refer to the product information.
3 Most Common Mistakes to Make When Installing, The Last One Can Cost You Money
Choosing the right expansion joint doesn't mean everything will be fine, the installation link is the real killer. You'd better take a notebook and write down the following three pits.
Error 1: Forgot to remove transport screw or fixture
How many times have I said this, but someone still commits it. When the expansion joint leaves the factory for the convenience of transportation, the corrugation or fabric will be compressed and locked with a screw. These fixings must be removed after installation! If it is not disassembled, the expansion joint has no displacement ability at all. When the pipe is hot, it will either pull the corrugations or tear the flue. So the first thing to do during acceptance is to check if all the tie rod nuts have been loosened.
Error 2: The installation direction is reversed (especially the guide tube)
Expansion joints are usually marked by arrows Direction of media flow。 If you install it backwards, the deflector becomes a baffle, and the smoke directly washes the inner wall of the corrugation, which wears out quickly. Note: Both metal rectangular expansion joints and non-metal expansion joints have guide tubes, and the direction must be consistent with the airflow. There are information on the siteSpecific Function of Expansion Joint Guide TubeAfter reading the question and answer, you will understand why you can't play backwards.
Mistake 3: Not enough cold-drawn displacement (the hardest hit area for losing money)
This can really make you lose your underwear. For example: the design thermal expansion of the pipe is 80mm, and you weld the expansion joint tightly without any pre-stretching. After operation, the expansion joint is forcibly elongated, the corrugation plastic deformation, and the sealing surface is cracked. The proper thing to do is to pre-stretch (or pre-compress) the expansion joint according to the ambient temperature at the time of installation, and the amount of cold draw is usually about half the thermal displacement. For specific values, refer to the design drawings, or let the manufacturer calculate. There was no climbing distance left, and in the end the entire flue connection was torn apart, which was not something that could be solved by a few thousand dollars.
What about that? After entering the field, first check whether the model, size and compensation direction of the expansion joint are consistent with the drawings. During the installation process, a special person stares at the cold pull displacement and the removal of the fixing parts. Finally do a hydraulic test (or air tightness test), brush the weld and flange surface with soapy water, and redo it if it bubbles. Don't bother, one rework is enough for you to drink a pot.
Which points do you look at in daily inspection? Here's a checklist for you
It's installed and running, and you can't be a hands-off shopkeeper. The daily inspection focuses on these places:
- Presence or absence of surface deformation or bulging: If there are obvious twists, scratches and depressions in metal corrugations, stop the machine for inspection quickly; If the non-metallic skin bulges, it may be that the inner insulation has failed or the pressure is too high.
- Is there any leakage at the flange connection: Light the gap around the flange with a flashlight, or wipe it with a paper towel to see if there is any black ash. If you find air leakage and ash leakage, tighten the bolts first, and if it doesn't work, you have to change the sealing gasket.
- Is the guide tube stuck: If there is a deflector, you can gently poke it with a long rod (be safe!), to see if there is any displacement blocked. If the deflector falls off, it is not welded firmly during installation and has to be re-welded.
- Is the tie rod nut loose: Regularly check that the locking nut on the tie rod is not loose. If loose, the expansion joint may produce abnormal vibration or abnormal noise.
- Whether the insulation layer has fallen off: If the insulation cotton outside the non-metallic expansion joint is damaged, the heat will directly ablate the skin, and the life will be greatly reduced.
The economizer bypass flue expansion joint is a protector if you choose it right, and a time bomb if you choose it wrong. You can basically avoid 99% of the pits by following the four-step selection, three installation taboos and an inspection list mentioned today. With 1% left, that's probably luck – but luck will also be on the side of the well-prepared, right?