Cracks have appeared on the outside of the bolt holes in the fixed tube sheet – what is the cause? Tube sheet thickness: 120; material: 16MnⅡ; steel gasket: around 360 degrees
Thread Content
Take a look; just grind the surface, otherwise it will need to be replaced.
It shouldn’t be; more than 10 identical heat exchangers were made by several companies, and this issue occurred only in one of them. I asked the user unit; it started occurring after more than 10 start-up and shutdown cycles. The gasket has not been replaced, and when leakage occurs during start-up and shutdown, a tension rod is used to tighten the studs. 2 studs of 35CrMoA were pulled apart. I wonder what other problems could arise? I haven’t heard either that tight studs can crush the tube sheet. The bolt holes with cracks account for 2/3 of the total, and they are all located on the outside or inside of the bolt holes
According to you, it must be fatigue; it’s normal for cracks to appear in that case.
For a tube sheet 120 mm thick, is using grade II forgings not a bit too lenient of a requirement? Using grade III forgings seems better, as it allows for additional ultrasonic testing. The design temperature of this device is also high, and it may be caused by temperature fluctuations.
It is very likely a quality issue with the material itself.
Grade 3 forgings must be used, as specified by the standards.
I don’t think it’s fatigue, because even when the intervals between starting and stopping are short, they still amount to several days. Moreover, cracks only appeared at the studs, and some of the studs were also broken using force application rods
By fatigue, I don’t just mean driving and stopping the vehicle; leaks can cause stress changes, just as tightening nuts does, and temperature changes also lead to fatigue. Also, your secondary forged parts are definitely used incorrectly; it should be grade three.
May I ask which standard and at which position?
First, it seems that there is a problem with the design of this equipment itself; the grade of the tube sheet forgings is inadequate. Secondly, if problems arise after long-term use, for example after one cycle, it may be related to fatigue; but if the issues occur during the manufacturing process, it is most likely due to a problem with the material of the fasteners themselves. It is recommended to conduct physical and chemical tests on the damaged threads.
The design meets the standard requirements; both the tube sheet and stud materials have been tested and retested, with no issues found. And over the past few years, many such devices have been produced, with only one unit having problems. It’s just that the cause can’t be found.
It must be caused by fatigue; the heat treatment wasn’t thorough enough.
It’s probably not fatigue; it’s likely a manufacturing defect
Was the bolt that developed a problem inspected?
All bolt forgings have passed inspection. Including chemical composition, mechanical properties, heat treatment
It is very likely a quality issue with the material itself.