Let's be clear: What counts as a non-ferrous metal expansion joint?
When many people hear the word "non-ferrous metal", they preconceived that stainless steel also counts. Not right. The matrix of stainless steel is iron, which is a ferrous metal. In the real sense, non-ferrous metal expansion joint refers to a kind of compensator in which the bellows material is mainly made of copper, titanium and nickel-based alloys (Hastelloy, Monel and Inco nickel). They are found in large numbers in seawater pipelines, wet chlorine environments, strong acid media, and sophisticated pipeline systems that require strict avoidance of magnetic interference. In these occasions, ordinary carbon steel can't last a maintenance cycle, and 304/316L occasionally stumbles, but non-ferrous metals can often run the whole device life stably.
Does that mean that as long as it is non-ferrous metal is stronger than stainless steel? Not necessarily. The water here is very deep.
Material determines the lower limit: Each material has its own life gate
Choosing a material is not about which one is expensive or famous, but about whether it can carry the medium in your pipeline. Titanium is most famous for its resistance to seawater corrosion and humid chlorine gas. It is almost the standard answer in the air-cooled island pipeline of Binhai Power Plant and the heat exchange system of chlor-alkali industry. But titanium has a temper-it is afraid of reducing acids, and it frowns when it encounters an environment with hydrochloric acid.
What about Hastelloy C-276? This thing has strong resistance to strong oxidizing acids, mixed acids and acidic media containing chloride ions. When the chemical industry deals with waste acid and high-concentration brine, it is the best choice. The disadvantage is that it is expensive, and it can make people's flesh hurt for half a day when it is priced by kilograms. Monel is a nickel-copper alloy that is unique to hydrofluoric acid and alkaline media, and is particularly resistant to stress corrosion cracking. As for pure copper, it has high thermal conductivity and non-magnetic properties, and is often used in instrument pipelines or heat exchanger inlets and outlets that require anti-magnetic interference.
If you choose the wrong material, no matter how cheap the expansion joint is, it will be a waste of money-not even a matter of spending money, but spending money to buy hidden dangers. It's not alarmism.
The structure follows the working conditions: the bellows is not a bare pipe
If you choose the right material, the thing is half done. The other half depends on the structure. Is Bellows Single Layer or Multi Layer? Do you want to add a deflector? Can you add a tie rod? These directly determine that the expansion energy saving cannot absorb lateral displacement and axial displacement.
Single-layer bellows has poor flexibility and strong pressure resistance, which is suitable for scenarios with high pressure and small compensation; Each layer of multi-layer corrugated pipe is thinner and easier to bend. When the compensation amount goes up, the pressure bearing capacity will be diluted, so it has to be made up by interlayer design. The deflector is also a key player-it blocks between the inner wall of the bellows and the medium, avoiding high-speed fluid or particulate media from directly washing the bellows. The universal corrugated expansion joint of this station is a standard multi-layer corrugated pipe plus guide tube structure, which can cope with the axial compensation of most thermal pipelines without problems. The compound hinge transverse expansion joint uses two groups of bellows and hinge structure to specially eat the transverse displacement on the L-shaped pipe section. If you choose the structure wrong, it is like asking a sprinter to run a marathon-not that he can't run, he can't run far.
Three accounts are calculated before selection: pressure, temperature and compensation amount
None of these three accounts could be saved. The working pressure determines the number of layers and wall thickness of the bellows, the working temperature determines the allowable stress of the material, and the compensation amount determines the wave number. Less calculation, the expansion joint on the pipeline is a hidden danger.
Especially the rigidity of non-ferrous metal bellows is completely different from that of stainless steel. The elastic modulus of titanium is about half that of 304 stainless steel. At the same size, titanium bellows will be much "softer" than stainless steel bellows, and the fatigue life curve will be completely different. This means that the formula of common stainless steel expansion joint cannot be copied when calculating the fatigue life, but must be calculated according to the E value of actual material, yield strength and durable strength parameters at design temperature. Some manufacturers take a set of general calculation programs to dominate the world, and the material parameters are casually applied. The final product may be fine on the test bench, and it will start to leak after being installed on the pipeline for two or three years.
Pit of installation and operation and maintenance: You don't necessarily step on it, but if you step on it, it will be a big pit
Expansion joints depend three parts on manufacture and seven parts on installation. Did you do the pre-stretch? Has the pre-stretch amount been checked according to the design value? Is the direction of the deflector set backwards? Are the pipe supports fixed according to the drawings? As long as there is a mistake in these links, the non-ferrous metal expansion joint will still have to be scrapped.
Two days ago, I met a customer. The titanium expansion joint leaked after half a year. When I cut it open, the inner wall was washed out of a trench by the medium. After asking a circle, the guide tube was added in the original design, but the direction was reversed during installation, and the medium directly washed the root of the bellows. Who do you say is to blame? The function of the guide tube is clearly written in the FAQ, that is, it is used to change the flow direction of the medium and protect the inner wall of the bellows. Installing it backwards means not installing it. Similar problems include: pre-stretching is not done, and the bellows is directly at the stretching limit after the pipeline is cold and tight; The bracket is not fixed, and the pipe system is fully pressed against the expansion joint by its own weight. These pits have nothing to do with the material, but with the working conditions, but they are ultimately reflected in the expansion joints.
Who to ask: The bar is much higher than you think
Not all manufacturers can weld nonferrous metals well. Titanium welding must be carried out under argon protection, and the weld will be brittle if the oxygen content exceeds the standard slightly; When Hastelloy is welded, the heat input is not well controlled, and the carbide precipitation directly reduces the corrosion resistance. These process thresholds are more than an order of magnitude higher than that of ordinary carbon steel. Coupled with heat treatment specifications and non-destructive testing standards (RT/PT, not just a casual appearance inspection), factories without relevant qualifications and experience accumulation will be useless no matter how beautiful the quotation is.
When choosing a supplier, don't just look at sample booklets and quotes. Focus on three things: What cases of similar working conditions have you done? Is there any acceptance record for the project site? Which set of standards and material parameters for fatigue life calculation of bellows come from?
The non-ferrous metal expansion joint is the "buffer" of the pipeline when used correctly, and the weakness of the system when used incorrectly. Which is more important, material or working condition? To put it bluntly, the material determines the lower limit, and the working condition determines the selection direction. The two are two sides of the same skin, which are inseparable. If you have to score a higher or lower-working conditions go first, and materials keep up. If the working conditions are not thoroughly understood, no matter how good the titanium material is, it can't save the field. Put these words here, and your friends who do pipes will taste them slowly.