Step 1: First find out the working conditions of the flue-high temperature, corrosion and large displacement. Why is the expansion joint necessary?
Think about it, what's running in the flue? Hundreds of degrees of smoke are still sandwiched with sulfide and water vapor, and the pipe wall expands and contracts by tens of millimeters at every turn. Two days ago, I met a customer, and the flue was not equipped with expansion joints. As a result, the weld collapsed, and it was shut down for three days, losing hundreds of thousands. You say it was wrong or not?
Use flexible structure to absorb thermal displacement of pipe while withstanding high temperature and corrosion。 The worse the working condition, the more important it is. If expansion joints are not installed in power station boiler outlets, cement kiln tails and desulfurization systems, the pipes will either be cracked or bent. So the first step, you have to figure out the field parameters first-what's the temperature? How stressful? Is the media corroded? Is the amount of displacement axial, transverse or angular? Figure it out, and the rear selection will have the bottom.
Tips:If the flue gas temperature exceeds 400℃, it must be usedHigh temperature axial expansion jointOrCorrugated expansion joint for power plant industryOrdinary goods can't handle it.
Step 2: The core principle is just one sentence-how does the bellows "swallow" the thermal expansion and contraction of the pipe?
The core of the metal expansion joint is that ring of bellows. You think of it as a spring, but it's not for shock absorption, it's fordeformationOf. When the pipe is heated and elongated, the bellows is compressed; When the pipe cools and shrinks, the bellows is stretched. In this way, he stretched and contracted, "swallowing" all the displacement of tens of millimeters.
But flue conditions are no joke. Ordinary carbon steel bellows oxidize in a few minutes at high temperatures, so you have to use stainless steel, such as 304, 316L, or even heat-resistant alloys. Moreover, the shape, number of layers and wall thickness of the corrugation are particular. Single-layer bellows are suitable for low pressure, and multi-layer bellows can withstand higher pressures. And guess what? Used by many power station fluesExternal pressure single axial expansion jointThat is, the bellows is placed outside the shell, so that the high-temperature flue gas will not directly wash the bellows, and the service life will be long.
About stiffness, we have a special article on our siteStiffness and Calculation Formula of BellowsSimply put, the greater the stiffness, the greater the thrust, but the smaller the displacement compensation. It should be well balanced when selecting the model.
Step 3: Structural disassembly-bellows, guide tube, tie rod... What does each part do?
A complete flue metal expansion joint, not just bellows is done. Let's take a look at them:
- Bellows:Core piece, responsible for deformation, absorbing displacement.
- Guide tube:Installed inside the bellows, let the flue gas pass downstream, avoid high-speed airflow impacting the bellows, and also reduce wear. For specific functions, please refer to theSpecific Function of Expansion Joint Guide TubeThat article.
- Tie Rod/Screw:It is used to fix the bellows when leaving the factory to prevent deformation in transportation. Some should be removed after installation, and some should be kept (such asLarge tie rod expansion jointThe tie rod is used to withstand the internal pressure thrust and cannot be removed). Stop asking "Does the screw of the expansion joint need to be disassembled"-it depends on the working conditions!
- End tube/flange:Connect the pipes.
- Insulation layer/protective sleeve:High temperature flues require an outer layer of insulation to prevent scalding and heat loss.
Some flues are also equippedFlue gas baffle doorOrDouble-sealed single-axis circular baffle doorUsed to cut off the airflow. But that's another matter.
Note:If there is corrosive medium in the flue (such as wet flue gas after desulfurization), you must selectPTFE-lined hoseOrPTFE compensatorMetal bellows cannot withstand acid corrosion.
Step 4: Select the model and don't step on the pit-select the right model according to the on-site parameters (attached to the product comparison of this site)
Look at the direction of displacement, the temperature and the pressure。
For axial displacementUniversal corrugated expansion jointOrHigh temperature axial expansion joint； For lateral displacementCompound hinge transverse expansion joint； Used for angular displacementCurved tube pressure balance expansion jointOrStraight pipe pressure balanced expansion joint。 If the pipe is thick and short, useMetal rectangular expansion joint(For example, power plant desulfurization flue).
304 stainless steel is used below 350℃, 316L or 321 is used at 350~550℃, and Inconel alloy is used over 550℃. Pressure determines wall thickness and number of layers: low pressure (0.5MPa)Large diameter thick wall expansion joint。
The products in our station are very complete, such as:
- Corrugated expansion joint for power station industry— — Specially designed for boiler flue, high temperature resistance and fatigue resistance.
- Metal Corrugated Expansion Joints in Cement Industry- -Adapt to the high temperature and dust at the kiln tail.
- Desulfurization flue gas baffle door— — Used with expansion joints to isolate corrosive smoke.
Really unsure? Find a parameter table and compareModel and size of expansion jointAndMetal expansion joint weight table, can basically settle down.
Step 5: Installation and debugging in four steps-arrow direction, tie rod nut, pre-stretching... one can't be less
- Look at the arrow:There is an arrow on the expansion joint pointing in the direction of the medium flow. Never pretend backwards! When the deflector is reversed, it becomes a baffle, and the pressure loss soars.
- Adjustment rod nut:The tie rod nut was tightened at the factory, that is to prevent distortion in transport. After installation, loosen the nut to the design position (refer to the accompanying drawings). If it isLarge tie rod expansion jointDon't dismantle it randomly, the tie rod is used to offset the internal pressure thrust.
- Pre-stretch/pre-compression:Pre-stretching is required if the site ambient temperature is lower than the compensator design temperature; Higher than design temperature, requiring pre-compression. Don't know how to calculate? CheckHow to adjust the tie rod nut of expansion jointThat one.
- Fixing brackets must not be less:Set fixed brackets on both sides of the expansion joint, so that the pipeline displacement can be absorbed by the expansion joint, instead of letting the pipe frame carry it hard.
Note:Once the installation is complete, rememberRemoving transport screw(Not a tie rod), otherwise the expansion joint can't move, which is equivalent to white installation.
Step 6: Routine maintenance and troubleshooting-Small ways to extend your life
- Look at the bellows surface:Are there any cracks, corrosion spots, bulges? If you have one, replace it quickly, and don't wait for a leak to repair it.
- Look at the guide tube:If you find abnormal noise or vibration, it may be that the guide tube falls off or is worn out. refer toSpecific Function of Expansion Joint Guide TubeThat one.
- Look at the tie rod nut:Is it loose? Is there any abnormality in the amount of displacement? Use a ruler to measure the elongation and compare it with the design value.
By the way, there is another common misconception: some people use metal expansion joints as rubber compensators, and as a result, they break down in high temperatures for a few weeks. Rubber can only be used in low temperature and non-corrosive situations. Flue? Choose honestlyNon-metallic expansion joint (fabric fiber expansion joint)OrMetal rectangular expansion joint。
In case something goes wrong, such as a leak, don't worry yet. Check the order: first look at the weld (pipe body weld), then look at the root of the bellows, and finally look at the flange sealing surface. Eighty percent is fatigue crack of bellows, just change to the same model.
The life span is directly linked to the working conditions. In places where power stations and cement plants are running continuously, it is recommended to inspect them every two years and replace them every five years. The details can be checkedService life of expansion jointThat Q&A.