MechD
Mechanical
- Jul 5, 2010
- 12
Hi all,
I am currently struggling to agree with our Third Party Inspector on dimple jacket hydrotest requirements per ASME VIII, Div 1. I have always treated dimple jacket welded onto a pressure vessel as it's own unit, rather than another vessel chamber and / or combination unit. (For example, vessel -1 to +3 Bar, dimple jacket -1 to +8 Bar)
For this reason, we would test the vessel to 1.5 times vessel design pressure (4.5 Bar), and the dimple jacket to 1.5 times jacket design pressure (12 Bar). We would not usually allow for the max. differential pressure between vessel and dimple jacket, as would be the case with a conventional jacket (1.5 x 9 = 13.5 Bar). In essence, a pressure vessel put into atm conditions is not designed for FV just because the dimple is rated for FV. Instead, a proof test is completed on a sample piece of dimple as per Appendix 17 to verify backing plate thickness, dimple design & fabrication.
Third Party believes that the vessel shell i.e. any interface which could be subjected to max. differential pressure has to be designed & therefore hydrotested to 1.5 times this potential pressure. I would agree if we were using conventional jacket or if this were separate chambers of a pressure vessel. However I was under the impression that dimple / embossed assemblies are considered a stayed surface, thereby acting as a stiffener to the vessel walls, rather than applying additional pressure to it. Even referring to ASME VIII, Div. 1, I seem to find more grey areas & more questions rather than solid definitions. The bone of contention is UG-21 which states that the vessel must be designed for worst case of temperature & pressure & loadings.
So referring to UG-99 for hydrostatic tests, combination units are explained in UG-19 (a). This section advises the user to refer to Appendix 9 for jacketed vessels, which is the case in my vessel. However App. 9 explicitly states it DOES NOT apply to dimpled / embossed assemblies. Instead you are directed to UW-19 Stayed Surface Construction. This section advises dimensional requirements & proof test requirements but no specific hydrotest for the duplicate parts built to the same dimensions as the proof test. Instead the user is advised to refer to UG-99 for any duplicate parts... which brings me back to the original question - Is dimple jacket regarded as a separate chamber i.e. combination unit? If so we have been under hydrotesting for many years. My third party inspector keeps bringing it back to the basics of a flat plate with FV one side and 8 bar on the other side, so logically the flat plate should be designed & tested for 9 Bar. I can understand this train of thought in theory, but I was under the impression that a stayed surface was not regarded as another vessel chamber in this way. I guess I would like to know if other people are hydrotesting their dimple jackets to 1.5 times Design pressure, or 1.5 times Max. Differential pressure. I would appreciate some insight or feedback from someone with more experience in the field, not only to resolve the current issue but also to ensure adequate design & testing of future vessels.
Thanks,
MechD
I am currently struggling to agree with our Third Party Inspector on dimple jacket hydrotest requirements per ASME VIII, Div 1. I have always treated dimple jacket welded onto a pressure vessel as it's own unit, rather than another vessel chamber and / or combination unit. (For example, vessel -1 to +3 Bar, dimple jacket -1 to +8 Bar)
For this reason, we would test the vessel to 1.5 times vessel design pressure (4.5 Bar), and the dimple jacket to 1.5 times jacket design pressure (12 Bar). We would not usually allow for the max. differential pressure between vessel and dimple jacket, as would be the case with a conventional jacket (1.5 x 9 = 13.5 Bar). In essence, a pressure vessel put into atm conditions is not designed for FV just because the dimple is rated for FV. Instead, a proof test is completed on a sample piece of dimple as per Appendix 17 to verify backing plate thickness, dimple design & fabrication.
Third Party believes that the vessel shell i.e. any interface which could be subjected to max. differential pressure has to be designed & therefore hydrotested to 1.5 times this potential pressure. I would agree if we were using conventional jacket or if this were separate chambers of a pressure vessel. However I was under the impression that dimple / embossed assemblies are considered a stayed surface, thereby acting as a stiffener to the vessel walls, rather than applying additional pressure to it. Even referring to ASME VIII, Div. 1, I seem to find more grey areas & more questions rather than solid definitions. The bone of contention is UG-21 which states that the vessel must be designed for worst case of temperature & pressure & loadings.
So referring to UG-99 for hydrostatic tests, combination units are explained in UG-19 (a). This section advises the user to refer to Appendix 9 for jacketed vessels, which is the case in my vessel. However App. 9 explicitly states it DOES NOT apply to dimpled / embossed assemblies. Instead you are directed to UW-19 Stayed Surface Construction. This section advises dimensional requirements & proof test requirements but no specific hydrotest for the duplicate parts built to the same dimensions as the proof test. Instead the user is advised to refer to UG-99 for any duplicate parts... which brings me back to the original question - Is dimple jacket regarded as a separate chamber i.e. combination unit? If so we have been under hydrotesting for many years. My third party inspector keeps bringing it back to the basics of a flat plate with FV one side and 8 bar on the other side, so logically the flat plate should be designed & tested for 9 Bar. I can understand this train of thought in theory, but I was under the impression that a stayed surface was not regarded as another vessel chamber in this way. I guess I would like to know if other people are hydrotesting their dimple jackets to 1.5 times Design pressure, or 1.5 times Max. Differential pressure. I would appreciate some insight or feedback from someone with more experience in the field, not only to resolve the current issue but also to ensure adequate design & testing of future vessels.
Thanks,
MechD
