Export Metal Expansion Joints Are Not "Large Bellows"-Find Out Where They Are Used First
Two days ago, I met a customer. When he came up, he asked, "Your bellows are used for export. Give me a few." I asked him about the medium, pressure and temperature, but he didn't know anything. Alas, this is not buying expansion joints, this is buying time bombs.
Export metal expansion joints, not "large bellows" at all. It is a precise stress compensation element, which is specially used to absorb the displacement of pipeline due to thermal expansion and contraction, mechanical vibration and foundation settlement. Our station hasCorrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry、High temperature axial expansion joint…Each category corresponds to different working conditions. If you use the universal corrugated expansion joint to deal with the high-temperature flue gas pipe, it will crack in less than three months.
Does your pipeline run steam, smoke, corrosive gas or liquid? Is the displacement axial, lateral or angular? Will the ambient temperature exceed 600℃? If you don't understand these questions, all the rest will be useless.
The first step of model selection: pressure, temperature and displacement, one less of these three numbers is useless
Design pressure, design temperature, compensated displacement。 One less, the expansion joint is scrap iron.
The steam pipeline pressure of an export project is 1.6MPa, the temperature is 350℃, and the axial displacement is 30mm. The customer chose the universal corrugated expansion joint cheaply. As a result, the rigidity of the corrugated pipe was not enough, and it became unstable after half a year's operation. Later replaced withExternal pressure single axial expansion jointPeople specialize in carrying high pressure and high temperature, and they are fine until now. You see, if the parameters are wrong, even the gods can't save them.
Let's talk about displacement-don't just report a "total displacement". Pipes have thermal expansion and cold contraction, and you have to give maximum and minimum displacement, preferably in three dimensions. Otherwise, the manufacturer will design it for you according to the axial direction. As a result, it is found that there is still 10mm lateral displacement during installation, and the expansion joint will be twisted into a twist directly.
How to match the material and structure? High temperature axial type, general purpose type or direct burial type? Speak according to the working conditions
Material selection is a university question. The upper temperature resistance limit of ordinary 304 stainless steel is about 650℃. If it exceeds this number, you have to use 316L, Incoloy 825 or even Hastelloy. In the stationHigh temperature axial expansion jointCommonly used double-layer or multi-layer bellows, with high-temperature insulation layer in the middle, specially to deal with flue gas pipes.
For the structure, look at the direction and space of the pipeline:
- Axial displacement— — SelectExternal pressure single axial expansion jointOrStraight pipe pressure balanced expansion jointGood stability;
- Lateral or angular displacement— — UpperCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint；
- Direct buried pipeline-Must be usedDirect buried (fully buried) type expansion jointThe shell is treated with anti-corrosion and waterproof treatment;
- Large diameter and low pressure——Universal corrugated expansion jointEconomical and affordable.
Some people say "304 is enough", but as a result, the medium contains chloride ions and stress corrosion cracks. Especially for export projects, you should ask about the media composition, so don't save that little material money.
The three details of installation that are most easy to overturn: the direction of the guide tube, whether the tie rod nut is removed or not, and how to calculate pre-stretching
Installation is the big head. According to the Q&A on the site"Specific Function of Expansion Joint Guide Tube"The guide tube mainly prevents high-speed fluid from washing the bellows, and also plays a guiding role. However, the direction must not be reversed-the direction of the arrow must be consistent with the flow direction of the medium. If the guide tube is reversed, the bellows will be directly washed away.
Do you remove the tie rod nut or not? Look at the Q&A on the station"How to adjust the tie rod nut of the expansion joint"And"Does the screw of the expansion joint need to be removed?": The tie rod is a transport protection device and must be removed or loosened after installation. If it is not disassembled, the whole expansion joint will be locked and cannot be displaced. The stress of the pipeline will all be stuck on the bellows, and it will explode sooner or later. But note that for expansion joints with pressure balance (such asStraight pipe pressure balanced expansion joint), the tie rod is a structural part and cannot be disassembled. You have to look at the nameplate to confirm.
What about pre-stretching? When installing cold pipe, it is necessary to calculate the pre-stretching amount according to "installation temperature vs working temperature", so that the expansion joint has been partially elongated in the cold state, so that it can return to the free length during thermal expansion. Calculation formula: pre-stretching amount = axial displacement amount × (installation temperature-minimum temperature) / (maximum temperature-minimum temperature). If you are not sure, find the manufacturer to calculate it. Don't estimate it yourself.
Frequently asked questions about export projects: flue gas corrosion, vibration fatigue, deflector tube falling off, how to treat it?
- Flue gas corrosion: The flue gas contains sulfur and chlorine, and the temperature is high, so ordinary stainless steel can't bear it. Countermeasures: Choose corrosion-resistant alloys, or lining with non-metallic layers (such as those in the stationNon-metallic expansion joint (fabric fiber expansion joint)Specifically dealing with corrosive smoke).
- Vibration fatigue: Pipe resonance causes the bellows to bend repeatedly, resulting in micro-cracks. Countermeasures: Add damping devices, or use them insteadCompound hinge transverse expansion jointReduces stress concentration.
- Guide tube falling off: Poor welding or fluid impact causes the guide tube to fall off and jam the bellows. Countermeasures: Use double-layer guide tube or add fixing bolts.
Overseas projects often require resistance to earthquake and wind loads. From our stationSleeve type pipe expansion jointAndRotary compensatorPerforms well in earthquake resistance, but needs additional checking.
Regular maintenance is more important than buying expensive ones-replacing an expansion joint costs enough for ten years of maintenance
An export refinery bought an imported expansion joint at a large price. As a result, it was not maintained for three years, and the guide tube was rusted through, the bellows was corroded and perforated, resulting in a loss of more than 100,000 yuan during the downtime. The money for another expansion joint is enough for ten years of regular inspections.
So, uh,Export metal expansion jointThe maintenance is actually very simple: check the bellows surface every six months for cracks and corrosion pits; Check whether the draw rod and the guide tube are loose every year; Stress tests are done every two years. In harsh environments (such as flue gas pipes), the frequency doubles. Don't feel bad about the labor cost. The money for replacing an expansion joint is enough for you to do ten years of maintenance. This account will be calculated by yourself.
Inaccurate selection, messy installation, lazy maintenance, no matter how expensive the expansion joint is, it is useless. Read these five pits and double the life of your pipes.