Find out what "temper" is in your flue first-temperature, medium, and pressure are all necessary
When choosing the pressure level of the flue expansion joint, the first step is not to turn through the product manual, but to squat on the edge of the flue and ask yourself three questions: What is running in this thing? How hot is the temperature? How stressful?
Don't laugh. Two days ago, I met a customer, and when he came up, he said that he wanted a "universal corrugated expansion joint with a pressure of 1.0MPa". As a result, when he asked, he found out that it was a desulfurization flue, the temperature was only 80℃, and the medium was highly corrosive. And guess what? The ordinary bellows he chose couldn't withstand the corrosion at all, and it pierced within two months. Therefore, it is important to figure out the "temper" first.
- Medium: Flue gas, steam, corrosive gas or dust? If it is wet flue gas after desulfurization, it has to be consideredNon-metallic expansion joint (fabric fiber expansion joint)Or the metal hose lined with PTFE, the pressure grade can't only look at the nominal, but also the material tolerance.
- Temperature: The higher the temperature, the lower the permissible stress of the material. Such asHigh temperature axial expansion jointThe design temperature can reach above 600℃, but the pressure level has to be discounted. Remember a simple common sense: for every 100℃ increase in temperature, the allowable pressure drops by about 20% ~30%.
- pressure: Don't just look at the "nominal pressure" on the nameplate. You have to figure out whetherDesign pressure、Test pressureStillPermissible working pressure— — If these three words are mixed up, it will be easy to roll over later.
If flue pressure fluctuates greatly (such as boilers that start and stop frequently), it is recommended to choose a model with a pressure level one grade higher than the design pressure, and it is always right to leave some margin.
Identify several key parameters of the pressure rating: design pressure, test pressure, allowable working pressure, don't get confused
Many purchase orders say "pressure grade 1.6MPa", but no one tells you, is this 1.6 design pressure or test pressure? A word difference, the life of a piece of equipment.
- Design pressure: The highest pressure that the flue system may reach during normal operation. This is the benchmark number for the selection. For example, the design pressure of corrugated expansion joints used in power station industry is usually multiplied by 1.1 times according to the maximum pressure at the boiler outlet.
- Test pressure: The pressure used for pressure testing before leaving the factory or after installation is generally 1.5 times the design pressure. Attention!The test pressure is not the allowable working pressureYou can't run under test pressure for a long time, otherwise the fatigue life of the bellows will be greatly shortened.
- Permissible working pressure: Expansion at specific temperatures energy saving long-term safely withstand pressure. This parameter is usually linked to temperature, and the allowable working pressure of the same expansion joint at 200°C and 400°C may be twice different.
What about that? Remember an iron rule:When selecting the model, use the design pressure to compare the "allowable working pressure-temperature curve" in the product sample。 For example, if you chooseUniversal corrugated expansion joint"DN200, PN1.0" is written on the sample, but you should see clearly that it is at room temperature. If the flue temperature is 350℃, the actual allowable working pressure may only be 0.6MPa.
Tsk, don't you think it's a little twisty? Don't be afraid, the next step will teach you directly to screen against models.
Compare product models and screen them step by step: from general-purpose corrugated expansion joints to power station industry-specific types, who is suitable for you?
Now that you have the three numbers of temperature, medium and design pressure in your hand, the next step is to "take the right seat".
Step 1: Look at the media and temperature, and lock in the categories
- Normal temperature, no corrosion, pressure ≤1.0MPa →Universal corrugated expansion joint, the best value for money.
- High temperature (> 450 °C) or containing corrosive media →High temperature axial expansion jointOrNon-metallic expansion joint。 Non-metals have good corrosion resistance, but the pressure grade is usually low (generally ≤0.1MPa), which is only suitable for low-pressure flues.
- Severe working conditions such as power station, desulfurization and dust removal →Corrugated expansion joint for power station industryOrDesulfurization flue gas baffle doorMatching expansion joint. The pressure grade of this kind of product is usually designed according to the national standard GB/T 12777, with higher pressure resistance, and also with guide tube and liner.
Step 2: Look at the pressure values and determine the specific series
For example, if you chooseStraight pipe pressure balanced expansion jointProduct models usually have "PN0.6", "PN1.0", "PN1.6" and other logos. Compare your design pressure directly and choose the level that is closest to and not less than the design pressure.
If the flue is square (such as the desulfurization tower inlet), selectMetal rectangular expansion joint。 The pressure level marking method of rectangular expansion joint is different from that of circular joint. It is usually marked by "working pressure + allowable displacement", such as "0.1MPa/±50mm". Don't take the standard of a round tube to set a square tube, it will be wrong.
Step 3: Special working conditions, don't guess yourself
Vacuum pipes? chooseSpecial hose for vacuumThe pressure level is labeled as "vacuum degree", not pressure. High temperature steam? chooseSleeve type pipe expansion jointOrRotary compensator, their pressure rating can be done very high (above 2.5MPa), but the installation method is completely different.
Well, getting to this point, you can basically lock in 2~3 model candidates. But don't rush, the last step is a warning – see how others choose wrong.
What should I do if the pressure level is not selected correctly during installation and commissioning? — — Two common rollover cases give you a reminder
Some old man installed a dust collector at the inlet of the power plantCompound hinge transverse expansion joint, selected according to the design pressure of 0.25MPa. Results When the pressure test reached 0.4MPa, the bellows bulged and deformed directly. Why? Because the test pressure is 1.5 times the design pressure ≈ 0.375MPa, but in the actual pressure test, the worker directly used the system test pressure of 0.6MPa (the intermediate pipeline valve was not closed tightly). In the end, I was forced to changeLarge diameter thick wall expansion jointOnly when the pressure level is increased by one gear can it be held.
Be sure to confirm the test pressure value before installation and isolate other pipelines。 The expansion joint is not a valve and cannot be overpressure tested.
The flue of a chemical plant was usedNon-metallic expansion jointThe withstand voltage is nominal 0.05MPa. However, in actual operation, the water hammer effect occurs due to the start and stop of the fan, and the instantaneous pressure rushes to 0.12MPa, and the fabric layer is directly torn. Later replaced withrubber compensator(pressure level 0.2MPa) to solve the problem.
Dynamic pressure spikes are more deadly than steady-state pressure。 If your flue has a pump, fan, or quick-switch baffle, it is recommended to choose a higher pressure rating or add a buffer.
To say a thousand words, to choose the pressure level of the flue expansion joint, the core is four words:Tailoring。 Don't buy low-voltage cheap, and don't blindly go to high-voltage (it's quite expensive). Following these four steps, it was very steady.