First, if the inner wall is not smooth, what price will the pipeline system pay?
Let's tell you the truth first. Two years ago, there was a circulating water pipeline in a chemical plant, which used DN400 corrugated expansion joint, and it leaked after less than eight months of operation. When I removed it, I saw that a small hole had been worn through the bottom of the corrugated valley, and the inner wall was full of spots and rust marks. The director of the workshop is quite puzzled: the material is obviously 316L, and the medium is circulating water. How can it break so fast?
The reason is actually not complicated-the inner wall is too rough. The transition zone of peak and valley has forming stress, and the inner wall roughness is high. When the medium flows through, local vortex will be generated, and impurities and corrosive ions will remain at the bottom of the corrugated valley. Over time, pitting, intergranular corrosion and abrasion all come. Do you think this pot should be carried by "inner wall smoothness"?
The inner wall is not smooth, and the price is not just leaking in advance. Stress loss is the most immediate. The friction coefficient of rough inner wall is large. Under the same pipe diameter and flow rate, the resistance can be 15%-25% higher than that of smooth inner wall, and the energy consumption of pump will increase accordingly. Media retention is even more annoying-if there are residual materials inside the corrugation, it will either contaminate the next batch of products, or crystallize and solidify when the machine is stopped, blocking the expansion joint. There is also accelerated wear: particulate media will produce more severe erosion on rough surfaces, especially at elbows and corrugated troughs, and the wall thickness reduction speed can be several times worse.
Second, from waveform to inner wall: How is the smoothness of metal expansion joint achieved?
When the bellows is formed, there is a mold indentation on the inner wall. Is it enough to polish it?
It's not that simple. The forming methods of corrugated expansion joints include hydraulic pressure, rolling and spinning. The inner wall finish mainly depends on the surface quality of the billet pipe itself, forming process parameters, and subsequent polishing treatment. In general industrial bellows, the inner wall Ra value is usually between 1.6 and 3.2 μ m; If the requirements are higher, mechanical polishing and electrolytic polishing can be done, and the Ra value can be pressed below 0.4 μ m.
In the polishing process, electrolytic polishing is the most "reborn" step. It uses the principle of anodic dissolution to preferentially remove the slightly convex peaks on the metal surface, which not only reduces the roughness, but also forms a dense chromium-rich oxide film on the surface, and the corrosion resistance is also improved. This is not simply "looking bright", but actually changing the surface chemistry.
Another idea is to add lining. For example, there are PTFE-lined hoses and PTFE compensators on our website, that is, a layer of polytetrafluoroethylene (PTFE) is lined on the inner wall of the metal bellows. PTFE has an extremely low friction coefficient, strong chemical stability, and hardly sticks to any media. This solution is particularly suitable for highly corrosive environments at the expense of limited temperature and pressure resistance ratings. In addition, the guide tube is also a compromise scheme-a thin-walled light tube is added to the inner wall, so that the medium flows through the light tube without directly contacting the corrugation. The bellows body mainly bears the displacement, and the guide tube bears the medium erosion, which is the best of both worlds.
3. Actual gains from smooth inner walls: changes in flow rate, corrosion resistance and maintenance cycle
The inner wall is smooth, what substantial benefits can it bring?
Take the flow rate. With the same pump and tube diameter, Ra decreases from 3.2 μ m to 0.8 μ m, and the friction resistance along the way may decrease by about 20%. What does that mean? Either the operating pressure of the pipeline system is more stable, or it can run at higher flow rates at the same pressure drop, and the pipeline conveying capacity increases. This savings is considerable for long-distance conveying or large flow conditions.
The improvement in corrosion resistance is also noticeable. Where the surface is rough, it is easy to form an oxygen concentration battery, and the oxygen content at the gap is low, which becomes an anode and is preferentially corroded. After polishing, there are no microscopic gaps on the surface, the integrity of the passivation film is good, and the pitting potential can be improved a lot. Experimental data show that after electropolishing, the pitting corrosion resistance of 316L stainless steel in 3.5% NaCl solution is about 30%-40% higher than that of unpolished specimen.
Not to mention the maintenance cycle. The inner wall is smooth, the medium is not easy to adhere to scale, and the cleaning cycle is prolonged. Like the food and pharmaceutical industries, CIP/SIP cleaning is required every day. When the roughness is high, microorganisms are easy to hide, and the cleaning verification can't be passed. Do the inner wall well, the cleaning time is short, the effect is good, and the number of shutdown maintenance will naturally be less. Calculating accounts, this "invisible inner wall" has actually been saving money for enterprises.
4. Under different media and working conditions, how smooth should you choose?
Is the smoother the inner wall the better? Neither is it. Smoother means higher processing cost, and there is no need to pursue the ultimate in some working conditions.
Gaseous media, such as flue gas and compressed air, are relatively less sensitive to roughness. The viscosity of the gas is small, and the friction loss is not as obvious as that of the liquid. However, there is a problem with flue gas-it contains corrosive components such as sulfide and chloride ions, especially in the flue behind the desulfurization tower. As soon as the condensed water comes out, it is dilute sulfuric acid. At this time, not only the inner wall is required to be smooth, but also corrosion-resistant materials, such as the desulfurization flue gas baffle door of our website and the corrugated expansion joint used in the power station industry. Such working conditions generally choose higher-grade alloys or add lining.
Liquid media has to be serious about roughness. Liquids with high viscosity, such as crude oil and resin, have rough inner walls that will significantly increase the flow resistance and are easy to hang on the wall. In the case of high requirements, it is necessary for Ra to be below 0.8 μ m. If it is pharmaceutical grade or electronic grade pure water, the requirements are more demanding. The inner wall may be electropolished to Ra
Particulate media are the most inner-wall-picky. In the pneumatic conveying pipelines of cement plants and mines, the erosion of particles on the pipe wall is very serious. At this time, smoothness alone is not enough, it has to be designed in combination with hardness and wear-resistant lining. When selecting, don't just stare at roughness as an index. You have to count the flow rate, particle size, hardness and concentration, and make a comprehensive judgment.
5. Don't be fooled by "smooth with the naked eye": how to detect and accept the quality of the inner wall?
The smooth look of the eyes was completely different from the actual roughness.
For the quality acceptance of the inner wall, the data have to speak for themselves. The most commonly used tool is the surface roughness meter (also called roughness measuring instrument). The probe directly contacts the inner wall surface to measure parameters such as Ra and Rz. When measuring, pay attention to: the peaks, valleys and straight edges of the bellows should be measured separately, and you can't just pick the measurement places.
In addition to the roughness meter, endoscopy is also practical. Whether there are burrs, welding nodules, and scratches, pits and rust spots on the inner wall of the weld can be known at a glance by the endoscope. However, the endoscope can only see macroscopic defects, and the microscopic roughness still has to be measured by instruments.
More strictly, you can also do sapphire detection or electrochemical testing. Sapphire detection is to slide a precision-ground sapphire ball on the surface and record microscopic bumps through sensors; Electrochemical test is to judge the integrity of surface passivation film by potential polarization curve.
It is best to specify the roughness acceptance criteria and testing methods in the purchase contract. Otherwise, after the arrival of the goods, you can't produce evidence, and the manufacturer can't explain it. There is an old saying in our industry, which is called "don't look at the inner wall for acceptance, and run three more trips for maintenance". This is not rough, and the quality of the inner wall is really worth spending more effort to check.