How many times does the life of a metal expansion joint usually last? Don't get misguided by the word "times"

Two days ago, I received a phone call from a customer, who asked, "How many times do your metal expansion joints usually last? Give me the number." I said that you asked a good question, but we have to clarify one thing first-what exactly do you mean by "times"? Is it calculated on an annual basis or by scaling back and forth?

Find out one thing first: is life expectancy calculated by "times" or "years"?

Many people ask "how many years can it be used?" In fact, the design life of metal expansion joints is never calculated by years. What really determines its success or failure isnumber of fatigue cycles。 What is a fatigue cycle? That is, the bellows goes from the compressed state to the stretched state, and then returns to its original position. This is called a cycle. Design life is usually written as "1000 times" or "3000 times", which is the real number. Think about it, one piece of equipment may only be turned on and shut down dozens of times a year, while another piece of equipment fluctuates hundreds of times a day. After the same year, the "strain injury" of bellows is very different. So stop just asking "years", that's a false proposition.

Also called expansion joint, can the life difference be 10 times?

• Corrugated pipe material— — There is little difference between 304 stainless steel and 316L, but in high-temperature situations (such as flue gas pipes of power station boilers), you have to use high-temperature alloys (Inconel 625 and the like). When the material changes, the fatigue life can be differed by an order of magnitude.
• Size of compensation amount— — The larger the compensation margin given in design, the higher the stress generated by a single cycle, and the naturally shorter the life. For example, it is also a general-purpose corrugated expansion joint. If the compensation amount is only 20mm, it may easily run to 3,000 times, but if it insists on eating 50mm, it will crack after 500 times.
• Operating Pressure and Temperature— — For every 100°C increase in temperature, the creep rate of the material doubles. The high pressure also puts the bellows wall under additional stress, accelerating crack initiation.
• Mounting skewed or not— This one is the easiest to overlook. If the on-site installation is hard-pulled, resulting in the initial bending or twisting of the bellows, the fatigue life will plummet. After sales, we encountered a metal corrugated expansion joint in the cement industry, with a design life of 2,000 times. In fact, it leaked after less than 800 times of use. When it was removed, it was misaligned during installation.

What exactly is the common fatigue design life in the industry?

Different product types are given different times. I'll list a few typical ones:

• Universal corrugated expansion joint— — The most common kind, the design fatigue life is usually 1000~3000 times. How do you get this in the middle? Depends on the working conditions. If the displacement of the pipeline is not large and the temperature does not exceed 200℃, 1000 times is enough; But if you want to leave some safety margin, run 3,000 times.
• Corrugated expansion joint for power station industry— — Especially the high-temperature axial expansion joint, the design life is often more than 5,000 times. Why? The starting and stopping times of power station units are limited, but the running time is long, and the bellows are in high temperature for a long time. If the number of fatigue cycles is not high, thicker materials and better waveforms have to be used.
• Straight pipe pressure balanced expansion joint— — The internal structure of this kind of product is complicated, with bellows and balance rings, which are greatly affected by pressure fluctuations, and the design life is generally set at 1000~2000 times. If the actual working pressure is unstable, the life will be discounted.
• Compound hinge transverse expansion joint— — It is mainly used in large lateral displacement occasions, when the bellows is subjected to bending and torsion, and the fatigue life is usually only 500~1000 times. But from another angle, it solves the problem of large displacement that other products can't solve, and its short life is worth it.

To put it bluntly, the number of design times is only a "theoretical value", and whoever takes it as an imperial edict will suffer.

The actual working condition is the only criterion to test the truth

No matter how good the design is, you have to look at your "face" when you enter the scene. To give a few real cases:

• Metal Corrugated Expansion Joints in Cement Industry— — The hot air duct on the cement production line has high dust concentration and hard particles. Once the guide tube is worn, the high-speed airflow directly washes the bellows, and the life can be half lower than the design value. We recommend thickening the guide tube when selecting the model, or simply using double guide tubes, which can carry for 3~5 years.
• Expansion joint for desulfurization flue gas baffle door— — The flue gas in the desulfurization system is moist and acid, which is extremely corrosive. Even if you use 316L stainless steel, it can't bear the long-term acidic environment. Some customers feedback that "it leaked after half a year of use". In fact, it is not that the quality of the expansion joint is not good, but that the material selection does not match the corrosion conditions. In this case, it is recommended to use PTFE-lined or more corrosion-resistant materials.
• Double hinge expansion joint for air-cooled island vacuum pipeline— That's interesting. The temperature of air-cooled island vacuum pipeline is stable (60~80℃ all the year round), without drastic fluctuation, and the displacement is mainly caused by installation cold shrinkage, and the number of back-and-forth cycles is very few in actual operation. Therefore, this kind of expansion joint can often run the full design life, or even overdue service. We have done statistics, and the bellows are still in good condition after using the double hinge expansion joint of air-cooled island vacuum pipe for more than 10 years.

Tsk, you see, it is not unusual for the same product to have a difference in lifespan several times in different places.

When should I change? Don't wait until you leak

If you really wait until the bellows cracks and the medium is sprayed out before replacing it, the price will be great. Be careful during daily inspections:

• Guide tube falling off— — Listen to the sound. If there is an abnormal metal impact or clattering sound in the pipe, 80% of it is that the guide tube has fallen off. As soon as the guide tube drops, the airflow rushes straight to the bellows, and it takes less than a month to wear out.
• Cracks appear on the bellows surface-Look carefully, especially at crests and troughs, the crack starts at the surface and may be as thin as a hair at first. It's easy to spot with a flashlight or a spray of penetration detector.
• Significant decrease in stiffness— — Shake the expansion joint with your hand. If it feels softer than before, or the limiting device on the pipe support is abnormally displaced, it means that the elasticity of the bellows has degraded. At this time, the fatigue life has basically come to an end.

Regular inspections are much more reliable than staring at "design times". How often do you check that? Make a comprehensive appearance inspection at least once every overhaul cycle (usually 1~2 years).

Selection suggestion: Don't blindly pursue "long life"

Some customers said "I want the most durable" as soon as they came up, and as a result, they chose the high-temperature axial expansion joint for use in normal temperature water pipelines. The price is twice as expensive, but also because the stiffness is too high to absorb the displacement, the flange is broken. So matching the working conditions is king:

• High temperature occasion(For example, power station boiler and heating furnace outlet) → SelectHigh temperature axial expansion jointThe material is high-temperature alloy, and the design life is more than 5000 times.
• large displacement occasion(For example, long straight pipe section, pipe gallery) → SelectCompound hinge transverse expansion jointOrCompound straight pipe bypass pressure balanced expansion jointAlthough the single cycle life is low, it can carry the lateral displacement of tens of millimeters, and the comprehensive cost performance is high.
• Corrosive media(such as desulfurization flue gas and hydrochloric acid pipeline) → SelectPTFE-lined hoseOrPTFE compensatorDon't rely on stainless steel.
• Vacuum pipe(e.g. air-cooled island) → SelectDouble hinge expansion joint for air-cooled island vacuum pipelineOrSpecial hose for vacuumThe wall thickness of the bellows should be sufficient to prevent instability.

How many times does the life of a metal expansion joint usually last? There is no standard answer, but you just throw the working parameters (temperature, pressure, displacement, media corrosiveness, start-stop frequency) to our technology, and we can calculate a reliable expected cycle number. Don't be dead by those "typical life 1000~3000 times" data frames on the Internet. After all, your pipeline is not a standard part.