Manhole deformation issue
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Hello, experts. There is an engineering case that needs to be solved. It is a stainless steel storage tank for pharmaceutical use (a type 1 pressure vessel), with an oval-shaped manhole measuring 300X400. Its structure is as shown in the diagram below. No abnormalities were detected during manufacturing at the factory, and it was sent to our project site for installation. During testing, it was found that the manhole did not seal properly; upon removing the manhole flange, it was observed that the flange of the manhole cover was uneven, as shown in the diagram below. More than a month had passed since the tank was shipped from the factory to the site for testing when this issue was discovered. The factory’s technical staff identified two possible causes: (1) delayed release of welding stress, resulting in deformation of the flange (the main cause) ； (2) Improper tightening during use. Therefore, the factory advised us to adjust the design of the manhole structure. (Note: We are the general contractor for the project; we are responsible for the design, installation, and commissioning of the equipment, while the processing of the equipment is outsourced.) I have doubts regarding my two analyses of the factory technicians; I would appreciate it if experts could give me some guidance. Thank you.
Is this a standard manhole? I’ve never encountered a situation where welding stress is released delayed. The tower I installed recently also has several manholes, and there were no problems with those either. Could it be that the tower was damaged during transportation?
If the stress is released slowly, that would be a big problem; what level of pressure does this device have?
For such non-circular manholes, there is definitely a sequence to follow when tightening them, mainly for the sake of ensuring proper sealing. However, the fasteners generally will not damage the flange cover; if the bolts could damage it, then there would be an issue with the material used. I need to check the forging grade of the flange cover by looking at the steel stamp data.
It’s not a standard manhole; it was designed by us. The equipment operates at low pressure, with a working pressure of 0.3 MPa. Transport should not pose any issues regarding the manhole, as it is welded to the tank and is not transported with its cover removed.
It could be that flange; for such non-standard components, there should be a calculation sheet to check whether the thickness calculation is correct I think it’s still a design issue.
To be honest, this is the first time I’ve seen a flange deform.
Isn’t this size of manhole too small? Additionally, the deformation of the lower flange is likely caused by welding processes. On-site treatment will likely require manual grinding to level it out; dealing with that sealing groove is a problem. If it’s a manhole cover, it’s easy to handle – just machine it again. It is recommended to focus on solving problems; arguing only harms both parties. The pressure isn’t high; a little processing and adjustment should be sufficient. There are so many power tools these days; I guess just having one of them will suffice. If there’s argumentation,,,
This post was last edited by jiu*ao123 on 2019-9-15 09:09. Pharmaceutical machinery and equipment generally operate at low pressures; therefore, the arch covers and flanges are not very thick. It is possible for deformation to occur due to improper fastening. However, the most common cause of deformation is found at the corner welds between the arch covers and flanges, where a large amount of welding is required. If proper welding sequence and techniques are not followed during welding, deformation is likely to occur. Solutions include: 1. If the deformation is minor and does not affect the insertion of the O-ring, the pressure test, or normal operation, the equipment can be reused, provided the user agrees； 2. Secondary processing of sealing surfaces and other joint surfaces, via grinding or turning ； 3. In the future, effective deformation control methods should be developed, and process inspections should be strengthened; if deformation gets out of control, secondary processing should be considered ； 4. The last option is to change the structure, which is only applicable when the above methods fail; after all, it will increase costs.