1. There is no fixed answer to the life of metal bellows expansion joint: design, working conditions and materials have the final say
Two days ago, a customer called and asked, "How many years will your bellows expand and save energy?" I asked him, "What working conditions do you use it in? What is the temperature? Is the medium corrosive?" He was stunned. To be honest, there is no standard answer to this question. Metal bellows expansion joint life? It is not a true or false question at all, but a comprehensive question determined by design, working conditions and materials.
The same general-purpose corrugated expansion joint, placed on a clean steam pipe, may run steadily for ten years; It is not unusual to throw it into the dusty environment of the cement industry and scrap it in three years. So don't believe those sales talk that pat your chest and say "guarantee for 10 years"-those are theoretical values measured under certain conditions.
Five core factors affecting life: temperature, pressure, medium, displacement frequency and installation quality
If the expansion joint of metal bellows fails, 90% of them can't escape these five pits.
- Temperature: If the design temperature exceeds 10℃, the fatigue life may be directly discounted by 50%. For example, corrugated expansion joints are used in power station industry. If high-temperature axial expansion joints run above 600℃ for a long time, the material must be heat-resistant alloy, and ordinary stainless steel can't last a maintenance cycle.
- pressure: The higher the internal pressure, the greater the circumferential stress on the bellows wall. Frequent pressure fluctuations can easily lead to low-cycle fatigue.
- Medium: Chloride ions and sulfides in the chemical industry are invisible killers. PTFE-lined hoses or PTFE compensators are specially designed to deal with corrosive media, but if you touch hydrochloric acid with ordinary stainless steel, the life is calculated by day.
- Displacement frequency: A pipeline that starts and stops several times a day has a much shorter life than a pipeline that runs continuously. Frequent expansion and contraction is equal to repeatedly bending the wire, and it will break sooner or later.
- Installation quality: Most overlooked. During installation, the tie rod bolts are not removed, or the pre-offset is not done correctly, and the expansion joint directly enters the limit state before it runs. I saw a scene two days ago, and the screw of the expansion joint of the big tie rod was not disassembled, but it was directly welded to death-that would compensate for a fart.
3. Life difference in different application scenarios: from high temperature of power station to cement dust, to chemical corrosion
Come on, let's talk about a few practical scenarios.
Power station industry: Boiler outlet, turbine bypass, high temperature, high pressure, and frequent start-and-stop. In this scenario, the power station industry uses corrugated expansion joints and air-cooled island vacuum pipeline double-hinged expansion joints, and the design life is usually about 10 years. However, in fact, due to overtemperature and overpressure, it is the norm to change a batch in 5-8 years.
Cement industry: There is much dust, strong grinding property, and high temperature. If the metal corrugated expansion joint in cement industry is not added with wear-resistant guide tube, the corrugated pipe will be impacted by particle flow and perforated quickly. The specific function of the guide tube is to protect the bellows body, but the guide tube itself is also a wear part.
Chemical industry: Corrosive media are enemy number one. PTFE-lined metal hoses and rubber PTFE compensators are standard here. But even with a PTFE compensator, the liner needs to be checked regularly to see if it has fallen off-once the medium penetrates the metal matrix, the rate of corrosion will make you doubt your life.
Flue gas desulfurization system: The non-metallic expansion joint (fabric fiber expansion joint) behind the desulfurization flue gas baffle door has a long life, but the acid dew point corrosion of the metal corrugated expansion joint is particularly serious in the low temperature and high humidity environment of wet desulfurization. We advise customers to give preference to rubber compensators or non-metallic compensators in this case.
4. Common failure modes: fatigue fracture, corrosion perforation, instability deformation, which one have you encountered?
For so many years, what I have seen the most isFatigue fracture。 Under the cyclic displacement of the bellows, micro-cracks gradually appear in the stress concentration areas of the peaks and valleys, and finally there is a click. And guess what? Many owners have to wait until they leak before changing, so why did they go early?
Secondly,Corrosion perforation, especially point corrosion. Stainless steel is most afraid of chloride ions. Even if the concentration is very low, it will penetrate quickly at the stress concentration point. Therefore, on chemical pipelines, we recommend using vacuum special hoses or directly buried (fully buried) expansion joints plus external protection.
Unstable deformationWhat? That's caused by too much pressure or skewed installation. The bellows is like being twisted and twisted, with uneven wave pitch and bulging directly in severe cases. Structures with restraints, such as double hinge transverse expansion joints and straight tube pressure balance expansion joints, are designed to prevent instability. However, some masters regard the tie rod as a reinforcement when installing it, which should be dismantled or not, and the result becomes an accomplice.
5. Practical operation method of prolonging life: one-stop selection, installation, inspection and maintenance
To say a thousand words, if you want to prolong the life of metal bellows expansion joint, you have to start from the root.
Don't make do with the selection: The working condition parameters must be given accurately. For high temperature, choose high-temperature axial expansion joint, for corrosion, choose PTFE-lined or rubber compensator, and for large diameter and high pressure, choose large-diameter thick-walled expansion joint. Don't buy a general-purpose model cheaply to handle harsh conditions-that's equivalent to running a rally with a Xiali.
Installation should be standardized: How to adjust the expansion joint tie rod nut? It's all clearly written in the instructions, but field workers often get it wrong. Remember: the transport tie rod should be removed after installation, and the adjustment tie rod should adjust the pre-bias according to the compensation amount. Does the screw of the expansion joint need to be removed? Depending on the situation, but by no means means welded to death.
Patrol inspection cannot be omitted: Look at the bellows surface every month for cracks, corrosion spots and deformation. Focus on checking whether the guide tube is loose and whether the tie rod is bent. The national standard JB/T 12235-2015 for non-metallic expansion joints also stipulates the periodic inspection cycle, so just follow it.
Maintaining records is done: What day it was changed, what model, and the change of working condition parameters-take a note, and there will be a basis for the next change. Products such as rotary compensators and sleeve pipe expansion joints are relatively simple to maintain, but don't ignore them.
The life of a metal bellows expansion joint is like asking how many years a car can run- -how you drive it, where you drive it, and how you maintain it. Choose the right product, install the right position, and check it out regularly. There will be no problem for ten or eight years; On the contrary, it is not unusual to lie down in a nest in three or two months. What scenario are you using for the equipment at hand? Comparing the above few, you probably have an idea in your mind.