What exactly is a flange metal expansion joint? Find out the basic structure and principles first
To put it bluntly, the flange metal expansion joint is a component used to absorb thermal expansion and contraction, reduce vibration and noise in the pipeline system. The structure is not complicated: the middle section of bellows is welded with a flange at each end, and some of them have a guide tube, a tie rod or an outer sheath. Bellows are usually made of austenitic stainless steel (such as 304, 316L) by hydraulic or mechanical forming, and rely on the deformation of the corrugations to absorb displacement.
What about the principle? When the temperature is high, the pipe is elongated and the bellows is compressed; When the temperature is low, the pipe shrinks and the bellows is stretched. It's as simple as that. However, many people are planted in the feeling that the structure is "simple"-the selection does not depend on the working conditions and the installation does not follow the rules, and as a result, it leaks in two years.
The flange metal expansion joint is similar in structure to the [universal corrugated expansion joint] in our station, but the flange connection method determines that it is suitable for medium and low pressure and medium and low temperature scenarios. If it is high temperature and high pressure, you may want to look at [High Temperature Axial Expansion Joint] or [Straight Pipe Pressure Balanced Expansion Joint].
The easiest pit to step on when selecting a model: which one does you look at: pressure, temperature and displacement?
You have to look at all three, but many people only look at pressure and temperature and ignore the actual amount of displacement. For example, two weeks ago, a customer who was doing steam pipelines chose a PN16 flange metal expansion joint, and the temperature and pressure were satisfied. As a result, the bellows cracked after installing it for a few months. Why? Their pipeline is 80 meters long, the temperature difference is 150℃, and the calculated axial displacement is 40mm, but the design compensation of the product they selected is only 20mm. The bellows is over-stretched, and the fatigue life directly hits the fracture.
Calculate the thermal expansion of the pipe first. The formula is available online, but to put it simply, the steel pipe expands about 1.2mm per meter per 100℃. You calculate the total displacement and leave at least another 20% margin. Step 2: Check the design pressure temperature. Note that the pressure level of the flange metal expansion joint usually refers to the normal temperature, and the allowable pressure will drop when the temperature is high. For example, the strength of 304 stainless steel at 400℃ is only about 60% of that at normal temperature. At this time, you can consider [high-temperature axial expansion joint], whose bellows wall thickness and material have been optimized.
In addition, the type of displacement should also be distinguished. Is it axial, lateral or angular? Flange metal expansion joints most commonly absorb axial displacement. If the pipe has lateral offset, you have to choose [double hinge transverse expansion joint] or [curved pipe pressure balance expansion joint]. Don't take the general-purpose model to hard carry the lateral force, it will break once you carry it.
Installation That Thing: Do you remove the tie rod nut? Where is the guide tube?
Never screw down the tie rod nut before installation。 When shipped in the factory, the tie rod is used to prevent the bellows from deforming during transportation, and the nut is usually locked. Once installed in place, loosen the nut to the compensation required by the design, or remove it directly-see the product manual. If it is not loosened, the bellows will be locked, and there is no way to compensate for the displacement at all. When the pipe is hot and cold, the bellows or flange interface may be pulled out.
What about the direction of the deflector? Remember one thing:The arrows point to the direction of the medium flow。 The function of the guide tube is to guide high-speed fluid into the bellows, reducing erosion and vortex. If it is installed backwards, the medium directly impacts the bellows root and wears out quickly. There is a picture in our question and answer, which is very intuitive. You can see it by searching for "the specific function of the expansion joint guide tube".
During installation, the flange bolts should be tightened diagonally, and don't twist one side at once, otherwise the flange surface will be skewed, the sealing gasket will not be compressed firmly, and leakage will most easily occur on the steam pipeline. Use a torque wrench and twist it 2~3 times according to the standard value.
Compared with other expansion joints, what are the advantages and limitations of flange metal expansion joints?
Flange connection, easy to disassemble and assemble; Metal material, high temperature resistance (much stronger than rubber compensator, rubber PTFE compensator can't hold up to 150℃); The single wave compensation is large and the cost is relatively low. The [universal corrugated expansion joint] in our station is a typical representative.
What about limitations? First, it cannot be used in particularly corrosive media. If you have sulfuric acid condensate in your flue gas desulfurization system, you still have to use [non-metallic expansion joint (fabric fiber expansion joint)] or PTFE-lined products. Second, the flange metal expansion joint on large-diameter pipelines (such as a diameter of more than 1 meter) is too heavy and laborious to install. At this time, [metal rectangular expansion joint] or [sleeve pipe expansion joint] may be more suitable. Third, there is a limitation on the installation space, which requires a certain straight line segment to allow the bellows to deform.
Compared with [rubber compensator], the flange metal expansion joint has high pressure and temperature resistance, but the vibration reduction effect is a little worse. Compared with [non-metallic expansion joint], it can't absorb a lot of lateral displacement, but it has strong axial compensation ability. Therefore, don't choose one type, it depends on the working conditions.
It leaks after two years of use? Common fault causes and routine maintenance points
It leaked in two years, 80% of which is one of these reasons:
- Wrong selection: As mentioned earlier, the displacement is not enough or there is no margin for pressure and temperature. Solution? You can only change.
- Installation error: The tie rod is not loosened, the guide tube is installed reversely, and the flange bolt is not tightened diagonally. A reinstall will solve it.
- Water hammer impact: The condensed water in the steam pipe is pushed away by high-speed steam, which produces a huge impact force, and deflates the bellows at once. Installing a drain trap and a slow-start valve can alleviate the problem.
- Fatigue cracking: Frequent start-and-stop, drastic temperature fluctuation, bellows metal reaches the fatigue limit. At this time, either choose a low-fatigue design (such as multi-layer bellows) or control the number of starts and stops.
- Corrosion perforation: There are chloride ions or acid and alkali in the medium, and 304 stainless steel can't bear it. Replace 316L or [lined with PTFE hose] and other corrosion-resistant products.
Routine maintenance? Quite simple:Look at the appearance during inspection。 Are there obvious deformations, cracks, corrosion spots? Are there any leakage signs at the flange interface? Is there any abnormal howling (possibly a media leak) when listening to the sound? Once a year, check flange bolt torque when out of service. Don't wait until a leak is repaired, replacing an expansion joint is much cheaper than your labor cost of removing the pipe.
Flange metal expansion joint is not a black technology, but it can save worry for several years when used well. According to the five articles I mentioned, there are basically no major problems.