Find out the "temper" of your flue first
When choosing the material of flue expansion joint in power plant, I am most afraid of turning over the sample and looking at the price as soon as I come up.
"Master, give me a corrosion-resistant expansion joint. How much does it cost?" When I heard it, my head was headache. What's the temperature of your flue? What is the composition of smoke? Is it stressful? He couldn't say anything. And the result? It leaked within two months of installation, and it was dismantled and replaced. Not to mention the delay in the construction period, the labor cost alone was enough to buy several.
Therefore, the first step is not to choose materials, but to find out the "temper" of the flue. You can't miss any of the three indicators:Temperature, Corrosiveness, Pressure。 With these three settled, the aftermath will naturally go smoothly.
How to touch? It is very simple: turn the design drawings and check the running parameters. If the drawings are lost, measure them on the spot-take a thermometer to hit the temperature of the outer wall (pay attention to safety), and then look for the smoke composition data in the operation record. If it doesn't work, take a picture and ask someone who knows what it takes. Don't guess. Wrong guesses are all useless.
Step 1: Choose the broad category by temperature-metal or non-metal?
Temperature is the most straightforward screening condition.
If flue temperatureBelow 200℃, thenNon-metallic expansion joints (fabric fiber expansion joints)Orrubber compensatorI can basically handle it. The advantages of non-metal are that it is cheap, light to install, and good vibration and sound insulation. The temperature of desulfurization flue (wet flue gas) in many power plants is only 50 or 60 degrees, and non-metallic expansion joints are completely sufficient.
But if the temperatureOver 200°C, even to 400℃, 600℃? Then the non-metals won't work – the cloth will burn and the rubber will carbonize. Must get on at this timeMetal corrugated expansion joint。
In the flue of the boiler outlet, the flue gas temperature is often above 350℃, so you have to useHigh temperature axial expansion jointOrCorrugated expansion joint for power station industry。 Their core is a thin-walled stainless steel bellows that can withstand high temperatures and absorb displacement.
Tips:If you are unsure of the temperature boundary value, you can remember it like this: 200℃ is a hurdle, the following is the world of non-metals, and the above is the stage of metals. But don't be rigid-some new non-metallic composite materials can also carry up to 250℃, but the price is not cheap, so it's better to go directly to the metal to save worry.
Step 2: Determine the specific grade according to the corrosivity-ordinary flue gas vs desulfurization wet flue gas
Temperature delineates a broad category. Next, take a closer look at corrosiveness. This is the tough battle to choose materials.
Ordinary flue gasAndDesulfurization wet flue gas。
Ordinary flue gas mainly contains sulfur dioxide and nitrogen oxides, with high temperature but little moisture and medium corrosiveness. In this case, the metal corrugated expansion joint304 stainless steelThat's enough; If it's slightly more serious, replace it with316L(Contains molybdenum, which is better against chloride ion corrosion).
But desulfurized wet flue gas is completely different. And guess what? After the flue gas is desulfurized by wet method, the temperature drops to 40 or 50 degrees, the humidity is close to saturation, and residual sulfur dioxide and chloride ions will formSulfuric acid dew point corrosionAndChloride ion stress corrosion。 Ordinary 304 went up, and it leaked within a few months.
- Upper nonmetallic: withNon-metallic expansion joint (fabric fiber expansion joint)Pair with acid-resistant rubber lining, or use it directlyRubber PTFE compensator、PTFE compensatorThe corrosion resistance of PTFE is almost invincible.
- Upper metal + inner liner: likePTFE-lined hoseThe outer surface of the metal bellows is covered with a layer of PTFE lining, which not only ensures strength but also prevents corrosion. However, note that PTFE lining can only be used in low temperature (
Also, don't forget the matching onesDesulfurization flue gas baffle doorIts material also needs to match the expansion joint, otherwise the interface will be corroded preferentially.
Step 3: Pick the structure by looking at the displacement requirements-don't just stare at the material, the wrong structure is completely useless
The material was selected correctly, but the structure was not selected correctly, so it still leaked.
axial(telescoping),landscape orientation(left and right),angular direction(deflection). Different displacements have different requirements on the structure of expansion joints.
- If it is mainly axial displacement (such as thermal expansion and contraction of a straight pipe section), useUniversal corrugated expansion jointOrExternal pressure single axial expansion jointJust do it. Note: Large diameter flue (more than 1 m) should be usedLarge diameter thick wall expansion jointOtherwise, the stiffness is not enough.
- If the flue needs to absorb both axial and lateral displacements, it needs to beCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 For example, the inlet and outlet pipelines of induced draft fans have large vibration and complicated displacement. This structure can effectively reduce the force on the equipment.
- If it is a vacuum pipe of air-cooled island, which has many displacement directions and requires zero leakage, it should be usedDouble hinge expansion joint for air-cooled island vacuum pipeline。
- AndDirect buried (fully buried) type expansion joint、Rotary compensator、Sleeve type pipe expansion jointAnd so on, all designed for special working conditions. You just have to remember:Define the direction and magnitude of displacementThen look at the product parameter table to find the matching structure.
Note:Some on-site personnel tried to save trouble by replacing all positions with the same expansion joint. As a result, there was no lateral compensation at the corner, and the bellows was directly twisted and cracked by hard pulling. The matter of structure selection is really not "universal can be used".
Step 4: Don't forget to seal, divert and install – the details determine life
This last step is the easiest to overlook, but it is precisely the key to "how long it will last and how long it will last".
Let's talk about the guide tube first。 You might ask: What does a deflector do? Simply put, it is installed inside the expansion joint, and its function is to let high-speed flue gas flow along the cylinder without directly washing the bellows. There is no guide tube, and the flue gas carries particulate matter, which wears out the bellows in a few months. Therefore, you must ask clearly when selecting:Is it equipped with a deflector?Our products such asCorrugated expansion joint for power station industryThe deflector is standard, but many small factories will take it off in order to save costs, so you have to keep an eye on it.
Talk about sealing。 The sealing of non-metallic expansion joints relies on multi-layer fabric and rubber sealing tape, but if the sealing tape is not pressed tightly during installation, or the direction is reversed, it will still leak. The sealing of the metal expansion joint depends on the welding quality of the bellows itself and the sealing gasket of the end flange. The gasket material must also match the smoke-ordinary asbestos gasket will soak in wet smoke, so it must be usedPTFE gasketOrGraphite gasket。
Finally, the installation details: For example, should the screw of the expansion joint be removed? That's a classic question. Tie rods and nuts for transportation must be removed or loosened after installation, otherwise the expansion joint will not be able to expand and contract freely. We have a customer who forgot to remove the tie rod nut after installation. As a result, as soon as the pipe heated up, the expansion joint bulged directly. So remember:After installation is completed, after confirming the displacement direction, loosen or remove the tie rod nut。 For specific operation, please see our previous answer "How to adjust the tie rod nut of the expansion joint".
In addition, the flange bolts should be symmetrically tightened and the torque should be uniform; Non-metallic expansion joints should avoid sharp objects scratching the fabric layer; Grounding protection should be done when welding the metal expansion joint to prevent the bellows from being damaged by electric welding.
Well, after saying so much, it is actually one sentence:Select the material of flue expansion joint of power plant, and take the four steps of temperature, corrosiveness, displacement and details without jumping a step.Don't be too troubled. Spend half an hour more in the early stage and half a month less in the later stage. If you are still unsure, just ask the parameters, and we will help you match them-after all, there are more than 30 expansion joint products in our station alone, and there is always one that can match the temper of your flue.