Stainless steel heat exchange tubes with problems must be intercepted before leaving the factory and must not flow into the installation process. After all, heat exchange tubes need to work under pressure in an environment with alternating cold and hot temperatures, which may be corrosive. Even if a small problem is not handled properly, it may cause leakage and affect heat transfer after installation, and in severe cases, it may lead to tube rupture and cause safety accidents. So the first step is not to rush to fix or throw it away, but to first figure out “where it is not qualified and how serious it is”, and then according to the situation.
First, conduct precise testing to identify the root cause of any defects. For example, using a caliper to measure the outer diameter and wall thickness to see if it exceeds the tolerance (such as requiring a wall thickness of 2mm, but only 1.5mm in reality, or the outer diameter fluctuates and cannot penetrate the tube plate); Use a magnifying glass to examine the surface for deep scratches, cracks, pockmarks, and incomplete cleaning of oxide scales; Further performance tests should be conducted, such as water pressure testing (filling the pipe with water and increasing pressure to check for leaks and determine if there are any hidden seams or pores), spectral analysis (confirming if the material is correct, such as if 304 is being impersonated as 316L), and even intergranular corrosion testing (to see if the corrosion resistance is sufficient). Only when it is clear whether it is “unqualified size”, “surface defect”, “substandard performance” or “wrong material”, can targeted treatment be carried out. Otherwise, blindly tinkering will either cost nothing or leave hidden dangers.
If it is a minor surface defect, such as shallow scratches (depth not exceeding 10% of wall thickness), localized oxide skin, or small pits, it is highly likely to be repaired. For example, use fine sandpaper (400 grit or more) to polish along the grain of the pipe, removing scratches or oxide skin. Pay attention to even polishing, and avoid making one side thick and the other thin. After polishing, the wall thickness should still meet the requirements (such as originally 2mm, at least 1.8mm after polishing); Afterwards, passivation treatment is carried out by wiping with diluted nitric acid solution to form a dense oxide film on the surface, restoring corrosion resistance. Finally, the defect is checked with the naked eye and caliper to confirm that it is gone and the wall thickness is sufficient before continuing to use. But if it is a deep scratch (such as cutting through half of the wall thickness), surface crack, or if there are too many and too dense pockmarks, the wall thickness will not be enough after polishing. This cannot be repaired and can only be scrapped, because if it is forcibly used, weak areas will be first broken by pressure or corroded.
If the size is not up to standard, such as an outer diameter that is too large (unable to fit into the tube plate holes of the heat exchanger), a wall thickness that is too thin (unable to meet the design pressure requirements), or poor straightness (difficult to pass through the tube due to excessive bending), it is basically impossible to repair. Because the size is determined during the formation of the pipe, for example, if the outer diameter exceeds the tolerance, it may not be controlled properly during rolling or drawing, and it may not be possible to grind the coarse pipe finely in the later stage; The thin wall thickness is due to insufficient metal content during molding and the inability to make up for it; If the straightness is too poor, slight bending can still be adjusted with a straightening machine, but if the bending is too severe, it is easy to crack the pipe during straightening. Even if the surface is smooth and the material is the right, this size of hard damaged pipe cannot be used. It is a small matter that the pipe plate cannot be installed. If the wall thickness is not enough after installation, the pipe will burst under high pressure. Poor straightness will cause uneven fluid flow velocity inside the pipe and local overheating. Therefore, it can only be directly included in the scrap pile, and there is no need to think about fitting it together.
If the performance or material is not up to standard, such as water leakage during hydrostatic testing (indicating hidden seams and pores), spectral analysis revealing the wrong material (using 316L instead of 304), or failure in intergranular corrosion testing (insufficient corrosion resistance), regardless of how good the surface is, it must be scrapped and there is no room for repair. For example, if the material is wrong, 304 will quickly corrode in a chlorine containing environment and leak after being installed and used for less than six months; Water leakage during hydrostatic testing indicates that there are invisible defects inside the pipe, and welding can only repair the surface. The internal hidden danger still exists, and it will leak again next time; Unqualified intergranular corrosion means that the pipe will crack from the inside in high temperature or corrosive environments, which are safety red lines that cannot be compromised. Even if this batch of pipes cost a lot of money, they have to grit their teeth and be scrapped, otherwise they will be installed in the heat exchanger, and the cost of later shutdown maintenance and losses caused by leakage will be much more expensive than scrapped pipes.
There is another key step in the processing: isolation and labeling. Once non-conforming pipes are found, they should be immediately separated from qualified ones, such as by making a “non-conforming” mark with red paint or placing them in a dedicated isolation area to prevent workers from accidentally mixing into qualified batches and installing them. Many accidents are caused by the lack of isolation, where broken pipes are used as good ones. The repaired pipes cannot be used directly, they need to be placed separately and undergo a complete set of testing again (such as measuring wall thickness and water pressure on polished pipes). If they are confirmed to be qualified, they will be returned to the qualified batch. If they fail the re inspection, they will still be scrapped.
Finally, the disposal of scrapped pipes should not be casually thrown into construction sites or garbage dumps. Stainless steel is a recyclable metal, and professional waste recycling companies should be contacted to remove it for melting and refining, which is both environmentally friendly and resource efficient; At the same time, it is necessary to keep records, such as the manufacturer, model, non-conforming items, and scrap quantity of this batch of non-conforming pipes, and provide feedback to the procurement department. Next time, the manufacturer should be consulted or a reliable supplier should be replaced to avoid receiving non-conforming products again.
In general, problematic stainless steel heat exchange tubes should be investigated first and then classified according to their situation. If they can be repaired, they should be repaired (re inspection is necessary after repair), and if they cannot be repaired, they should be scrapped (scrapping must comply with regulations). The bottom line of “no need for unqualified products” should be upheld, after all, the safe operation of heat exchangers depends on every tube meeting the standards.