In a corrosive environment, why can't ordinary expansion joints bear it?
Acid-alkali media in chemical plants, wet flue gas in desulfurization systems, chloride ions in seawater... If these places are equipped with universal corrugated expansion joints, they will show you a look in a few months. The weld seam rust first, the corrugated valley bottom starts pitting, and then the perforation leaks. What's the problem? Wrong material? Structural design not keeping up? Or did you choose the wrong model at all?
The general type is mostly stainless steel 304 or 316L, which can be pitted quickly when the chloride ion concentration exceeds 500ppm and the temperature is above 60℃. The concentration of sulfuric acid and sulfurous acid in desulfurization flue gas is high, and ordinary stainless steel can't even handle a maintenance cycle. To put it bluntly, corrosion is not just a matter of the kind of medium. The combination of temperature, flow rate, concentration and pressure is the real killer.
Material is the last word: stainless steel, PTFE-lined or PTFE?
Some products in the station, such as PTFE-lined hoses and PTFE compensators, are specifically designed to deal with strong corrosive media. The PTFE layer lined with PTFE is extremely chemically inert and can withstand most acids and bases. But don't think everything will be fine if you choose a 316L. Last year, there was a customer with dilute sulfuric acid in the pipeline, and the temperature was 80℃. He thought 316L was enough, but it leaked in half a year. After checking, the chloride ion concentration is 1200ppm, and the critical concentration of pitting corrosion of 316L has long been exceeded.
What about 254SMO and Hastelloy? The cost has gone up, is it worth it? Depends on the actual working conditions in your pipeline. If the medium is concentrated hydrochloric acid or high temperature chloride, Hastelloy C276 is the positive solution; If it is a wet desulfurization environment containing Cl⁻¹, 254SMO is more cost-effective than Hastelloy. The key is to give all the media composition, temperature and pressure data, and let the manufacturer help you calculate it. Don't choose your own head.
Deflector, number of corrugated layers, liner-these are details that are easily overlooked
I've seen too many people just look at the material, not the structure. What is the specific function of the expansion joint guide tube? Refer to the question and answer in the station, it not only protects the bellows from erosion, but also reduces direct contact between the media and the bellows in corrosive media. The material and thickness of the guide tube should also be corrosion-resistant, otherwise it will break first and the bellows will be finished.
Also, are multi-layer bellows more resistant to corrosion than single layers? Not necessarily. If there are gaps between multiple layers, the residual corrosive substances will dry up and concentrate when the machine is shut down. As soon as the next start-up temperature rises, the local concentration will increase sharply and corrode from the inside to the outside. Therefore, the multilayer structure must be treated well between layers, and even sealed and welded. There are also liners, such as PTFE liners, which both resist corrosion and reduce flow resistance. Tsk, there are a lot of doors here.
Common pits in model selection: only look at corrosion resistance, not pressure and temperature combination
Two days ago, a customer selected a PTFE-lined compensator according to the chemical medium. As a result, the operating temperature exceeded 200℃, and the PTFE layer was directly softened and failed. The price is not low, but the lifespan is short. Corrosion-resistant metal corrugated expansion joints are not a panacea, and the temperature, pressure, displacement and cycle times have to be calculated together. The "stiffness and calculation formula of bellows" and "model and size of expansion joint" often talked about in the station can come in handy at this time.
For example, high-temperature and high-pressure steam pipelines can't be lined with PTFE at all, so high-temperature resistant alloy plus multi-layer corrugated structure must be selected, and the fatigue life must be checked. Another example is the vacuum pipeline, which must not only be corrosion-resistant, but also external pressure-resistant. Ordinary bellows will deflate. The vacuum special hose in the station is aimed at this working condition, lined with corrosion-resistant material and strengthened wall thickness, which is not only anti-corrosion but also anti-deflation.
Practical application-from desulfurization flue gas to chemical pipeline, how to land?
In the desulfurization system of the power station, the wet flue gas contains a large amount of sulfuric acid and sulfurous acid, which can't be done by ordinary stainless steel. The desulfurization flue gas baffle door in the station and the corrugated expansion joint used in the power station industry, with corrosion-resistant design, can last for seven or eight years. Dual-seal construction with corrosion-resistant coatings and liners for baffle doors, very low leakage rate. The expansion joint is made of high nickel-based alloy or lined with polytetrafluoroethylene, and the corrugated structure considers ash accumulation and condensate discharge.
Another example is the vacuum pipeline in the chemical industry, which is made of vacuum special hoses and corrosion-resistant materials, such as 316L outer mesh and polytetrafluoroethylene inner pipe, which is flexible and corrosion-resistant. There is also an air-cooled island system, with large vacuum pipeline displacement, double hinge expansion joint and weather-resistant coating, which has no problems for more than ten years. Choosing the right product combination is more effective than staring at an expansion joint alone. You can't expect one compensator to solve everything, can you?
Corrosion resistance is not the end point, system design is
To put it bluntly, corrosion resistant metal corrugated expansion joint is just one link. How to arrange pipe supports? If the brackets are not spaced correctly, the additional bending moment to the expansion joint will exceed the limit. How to adjust the tie rod nut? Referring to the question and answer in the station, the pull rod is used to absorb the internal pressure thrust, and it must be adjusted to the designed length during installation, otherwise it will either limit the displacement or cannot compensate. Is the expansion joint mounted in the right direction? The direction of the arrow must be consistent with the flow direction of the medium, and the guide tube will not play a role if it is installed backwards.
These details are not worthy, and no matter how good the material is, it is for nothing. Want to be worried? Find the manufacturer to give the working condition data to the whole-media composition, temperature range, pressure fluctuation, displacement, number of cycles, pipeline layout diagram, and let them recommend the model. Don't choose your own head, and don't just look at the price. Choose the right one time to save rework later. After all, the cost of shutting down production for pipeline leakage is much more expensive than an expansion joint.