【JST-009】Preventive Maintenance, Corrosion Protection Technology – One-on-One Consultation Office

This post was last edited by Benbenxuan on 2012-4-2 at 23:41. This field is widely used in foreign companies, but it’s quite rare in state-owned enterprises. I am currently working in this field, and I hope to have more exchanges and receive guidance from others in the future. Let’s start with a brief introduction: The inspection of heat exchange tubes in fixed-tube-sheet heat exchangers is a quite challenging task. Currently, ECT is commonly used for detecting defects in these tubes as well as measuring their thickness. However, since the signal generated by ECT is an analog signal, the accuracy of the inspections depends on the precision of the reference tubes used, the sensitivity of the testing equipment, and the judgment skills of the inspectors – experience being particularly important in this regard. All these factors result in less than ideal detection outcomes and low efficiency in the inspection process. I wonder if there are any more advanced and efficient detection techniques for inspecting heat exchange tubes? (Publisher’s note: Please refer to post #3 for replies.) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- The issue is further explained on post #12 as follows: Very professional! There are no issues with the specifications of ECT probes; there are many manufacturers in China specialized in producing such probes, and there are no major technical problems. The main issue lies in the high requirements for the personnel responsible for testing, as well as the significant influence of human factors in the interpretation of test results, which reduces the reliability of this technology. IRIS uses ultrasonic technology to enable real-time imaging, allowing for the quantitative determination of the actual wall thickness of tubes as well as the shape of defects. It is not limited by the size of the heat exchange tubes, and it does not require high qualifications from the inspectors. Its greatest advantage over ECT is the ability to quantify defects; it has been in use abroad for a considerable length of time. However, IRIS has a significant drawback: its detection speed is too slow. We have also used it, and the typical detection rate is around 100 heat exchange tubes per 8 hours. For heat exchangers with a large number of tubes, or those having thousands of heat exchange tubes, this efficiency is far too low; it is usually used only for a small number of rechecks on the results obtained from ECT testing. Additionally, the ECT testing of non-ferromagnetic pipes you mentioned should rely on the far-field eddy current principle; we have no experience with that, as we use metal pipes only. :( The issue is further explained on the 13th floor as follows: I have recently learned about acoustic imaging technology, which is said to be an effective tool for inspecting the tubes in heat exchangers. Its inspection results are excellent and its efficiency is high. I was wondering if anyone could provide more information or relevant details about this acoustic imaging technology? Are there any companies in China that offer such technical testing? (Publisher’s note: Please refer to post 16 for replies.)

I am also constantly exploring ways for preventive inspection and maintenance. I’m glad to see that there are peers in this field, and I hope we will have the opportunity to exchange ideas and cooperate, either on this forum or in real life. Returning to your question: In China, there are very few companies that are capable of inspecting the heat exchange tubes in shell-and-tube heat exchangers. The most common approach is to clean them during maintenance and then check for air leaks; if no leaks are detected, it is considered that the heat exchanger is in good condition. Moreover, due to reasons such as management philosophy and testing costs, many companies prefer to replace the damaged items with new ones rather than spending an extra penny on testing them. In fact, they only focused on the cost of replacing the new equipment (which might be low), while ignoring or being unwilling to consider the parking losses that would result from this. If everything is taken into consideration, such examinations are highly necessary. The traditional ECT you mentioned is not very suitable for inspecting heat exchanger tubes; in addition to the issues you cited, there is also the problem of matching the probe size. There are currently instruments designed specifically for testing heat exchanger tubes, which also utilize the eddy current detection principle. However, they are mainly used for non-ferromagnetic tubes (although some manufacturers claim that they can be used with ferromagnetic tubes as well, the results are not satisfactory). Interpreting the test results from such instruments requires highly skilled professionals. Their advantage is that once mastered properly, the testing speed is very fast – usually just a few seconds per tube. In my opinion, the best instrument for inspecting the tubes of all metal heat exchangers at present is IRIS (Internal Rotation Inspection System). It is suitable for both ferromagnetic and non-ferromagnetic metals; using water as a medium and based on ultrasonic principles, it enables the quantitative determination of the actual wall thickness of the tubes as well as the shape of any defects. This technology has over 20 years of proven experience abroad, and it is now gradually being recognized in China as well. Disadvantage: The cleanliness of the inner and outer walls of the heat exchange tubes affects the accuracy of the testing, so they need to be cleaned before testing ； The testing speed is not fast; usually, around 100 tubes can be tested per day. I have many years of experience using this method; if you would like to discuss it in more detail, feel free to reply. There are also endoscopes, which can provide a preliminary assessment of the condition inside the tubes.

The inspection of the heat exchange tubes in air coolers and shell-and-tube heat exchangers is not being carried out properly at present. Many companies simply grind the tube sheet, conduct a PT inspection of the tube ends, and then carry out a hydrostatic test. The wall thickness of the pipeline and its corrosion status are unknown. . . Technologies such as IRIS and acoustic eyes still have a long way to go in terms of adoption in China, while abroad this field has been developed for many years and the technologies are already quite mature. . .

This post was last edited by Benben Xuan on 2012-4-2 23:28. Are there any monitoring points or records for the corrosive areas in hydrocracking units? Thank you! (Publisher’s note: For replies to this question, please refer to posts on floors 6, 7, and 15.)

I have no experience with the device you mentioned, but all chemical processing equipment shares common characteristics; we just need to take into account the specifics of each process when making plans. Regarding your question, I first recommend that you take a look at API 571; on the appendix to this standard there is a diagram showing the corrosion patterns associated with petrochemical cracking processes, which you can use as a reference. Secondly, since the operating pressure of your process is very high, in addition to routine inspections, the following additional suggestions are provided for your reference during daily inspections and shutdown inspections. 1. Welding quality control. For welding operations, it is of course essential to strictly adhere to the requirements for non-destructive testing. As for the welds on the equipment itself, it is recommended that when checking the wall thickness on a regular basis, the inspection area should include the heat-affected zone of the welds. The advantage of this approach is that this area has a higher tendency to corrode due to the effects of welding, and visual inspection can also be carried out to detect any defects during the wall thickness checks. 2. Conduct regular inspections of safety accessories, and it is recommended that you carry out periodic online tests on them. 3. For the equipment associated with heating furnaces, that is, high-temperature equipment, its welds must be subject to regular non-destructive testing, as such materials generally tend to develop delayed cracks. 4. The tank roof is insulated; it is recommended to increase inspections of the wall thickness of the tank roof in winter and summer. It primarily prevents the insulation from being damaged or from poor sealing allowing rainwater to enter and cause corrosion. 5. For equipment equipped with reinforcement rings, or those with different materials on the upper and lower parts (usually tower types), regular inspections of the wall thickness at the junction areas are necessary. 6. Regularly inspect or replace the fixing bolts and other components of major equipment. The above are the things that come to mind for now; they may not provide a specific solution to the issue you raised. If you could describe a certain type of device in more detail, we can discuss it further. The attachment is the “process corrosion diagram” mentioned above; I hope it will be helpful to you.

This post was last edited by yuchenchf on 2012-4-4 09:06. First of all, thank you for your answers and information. In hydrocracking units, measuring the wall thickness is not very effective; generally, the hardness of welds and heat-affected zones is checked, as well as the hydrogen flux – these are the aspects that need to be examined in high-pressure hydrogen systems. (Forum moderator’s note: Refer to answer from floor 15.)

This post was last edited by Benbenxuan on 2012-4-2 23:36. Hello, moderator! It’s great to see such dedicated professionals at HaiChuan. I would like to ask for advice regarding the preventive maintenance of shielded pumps. We are a warehousing terminal, and the operation of the shielded pumps is carried out on an irregular basis, mainly in connection with shipments. Many of these pumps have been in use for over 5 years; they worked properly at first, but in the past year there have been several instances of wear on their bearings. It is likely that this wear has accumulated over time. We hope to be able to detect such issues promptly and would appreciate any suggestions from everyone (Publisher’s note: Please refer to floor 11 for replies.) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- The issue is further explained on floor 18 as follows: Thank you! The analysis is very professional. The basic situation is consistent with our approach; we use horizontal shielded pumps that do not have a backwash mechanism. No preventive maintenance has been carried out over these 5 years, including disassembly. We are establishing norms in this area. I would like to learn more in depth: 1. The backwash structure of shielded pumps? Our operating conditions: The products are frequently changed, and they mainly consist of chemical raw materials such as toluene, propylene, methanol, and isopropyl alcohol – chemical solvents of this kind. There are basically no problems with the pumps used for handling these materials. The issues that have arisen recently have involved monomeric chemicals such as butyl acrylate and isooctyl acrylate; these substances have the tendency to polymerize on their own, which likely results in the formation of small polymer particles. 2. We have over 100 pumps and machinery; as our company is in the logistics sector, disassembling and repairing them takes a long time, and it also affects shipments. This is also why we have been seeking preventive maintenance. (Publisher’s note: Please refer to post 19 for your reply.) ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ The issue is explained in more detail in post 20: Thank you! Our situation is exactly as you said – quite complex: 1. Public warehouse, with a large number of products that are constantly changing. 2. Our pumps are mainly used for train departure, and automation has been implemented for this process – the pumps are started directly from the side of the train platform. There is no one monitoring the pump area, with only occasional inspections carried out. I would like to learn more about: 1. Automatic monitoring devices for shielded pumps, such as current, pressure, temperature, or other parameters. It facilitates the monitoring and early warning of pumps in the control room. 2. What are the key points for preventive maintenance of pumps under such operating conditions? For example, if I disassemble them once every two years, what should I focus on? We now conduct monitoring once a day: bearing monitors, insulation tests for pumps twice a year, and explosion-proof performance checks for pumps ； The pump should be disassembled every 2 years to check the wear of the graphite bearings. Thank you for your continued attention! (Publisher’s note: Please refer to floor 21 for replies.)

This post was last edited by Benben Xuan on 2012-4-2 23:37. I would like to ask how preventive maintenance can be carried out in the field of instrumentation; could you provide some general guidelines? (Publisher’s note: For a response to this question, please refer to floor 14.) ）

This post was last edited by Benbenxuan on 2012-4-2 23:37. Some instruments exhibit periodic failure patterns; it is advisable to carry out appropriate cleaning and maintenance before such periods arrive.

Shielded pumps are currently considered to be one of the most easy-to-maintain pumps, and their leak-free nature is widely utilized. Compared to other pumps, shielded pumps have a very simple structure, and they are also easy to operate and maintain. Your question isn’t clear enough; I’ll ask you a few more questions: 1. What is the type of this shielded pump – vertical or horizontal? 2. What are the properties of the medium used? Such as viscosity, high temperature, particles, and so on. 3. Regarding the frequent repairs you mentioned recently, apart from bearing wear, what’s the condition of other components? During maintenance, are the sleeve, thrust disc, and bearings replaced all at once? Here are our suggestions regarding the current situation: 1. The device has been in operation for many years and has functioned properly up to now, which indicates that there were no issues with its selection or other aspects. 2. Since the gap between the rotor and stator of a canned motor is very small, operating with such a narrow gap requires regular cleaning of the interior ； Or the pump should have a backwash mechanism. 3. The bearings wear out continuously; as mentioned above, during maintenance, the shaft sleeves and thrust discs should be replaced at the same time (if possible, replace both the front and rear bearings as well). After maintenance, check whether the pump vibrates during operation. 4. If Article 3 has been implemented, then please check the dynamic balance of the rotor as well as the wear condition of the impeller. 5. Even if there are no problems, it is recommended to disassemble and inspect the pump every 2 years or so, as unevenness may occur due to the wear of the graphite bearings, which can eventually affect the rotor. Since shielded pumps have a simple structure, their daily maintenance and repair do not require much complexity. I hope the information provided above can be helpful to you.