johnnymist2003
Mechanical
- Apr 16, 2003
- 100
Hi all,
bearing in mind that vessels are not big, huge pipe supports, we vessel engineers are increasingly required to consider and "design in" allowances for piping loads.
I would appreciate some advice / guidance on the following.
For a vessel designed to ASME VIII Div.1, for internal pressure, with nozzles which have applied piping loads, it is accepted that the WRC-107/297 results (or FEM as applicable) are compared to certain allowables. These allowables are not specified in ASME VIII, Div.1. Typically I would use the allowable stresses of the Div.1 materials and substitute these values for Sm in Appendix 4 & Fig. 4-130.1 of ASME VIII, Div.2. Using this method has been accepted in the past with no problems. The upper limits of the stress intensity being 3 Sd, with the general membrane stress limit at Sd, and the local membrane limit at 1.5 Sd (Sd = material allowable stress at temperature). However, what happens when the vessel in question is a vacuum vessel ? In vacuum vessels, we are being forced by end users to utilise FEM (the argument being that WRC-107/297 does not take into consideration the effect of external pressure correctly - the software typically takes the vacuum pressure stresses and changes the sign convention to "minus", which is not correct). Obviously FEM cost can become very high when you have a column with lots of nozzles / attachments. Does anyone know of any investigations / published papers on this issue ? How do you limit the stresses in order to overcome the effect of local buckling where typically the combination of vacuum pressure and nozzle loads would result in stresses which are quite high (say up to the 3 Sd limit) ? Do you need to worry about this high localised "compressive stress" ? Can you compare the local deflection at the junction to the out-of-roundness tolerances specified in ASME VIII, Div.1, clause UG-80 ? I'd appreciate any relevant assistance.
Thank you.
bearing in mind that vessels are not big, huge pipe supports, we vessel engineers are increasingly required to consider and "design in" allowances for piping loads.
I would appreciate some advice / guidance on the following.
For a vessel designed to ASME VIII Div.1, for internal pressure, with nozzles which have applied piping loads, it is accepted that the WRC-107/297 results (or FEM as applicable) are compared to certain allowables. These allowables are not specified in ASME VIII, Div.1. Typically I would use the allowable stresses of the Div.1 materials and substitute these values for Sm in Appendix 4 & Fig. 4-130.1 of ASME VIII, Div.2. Using this method has been accepted in the past with no problems. The upper limits of the stress intensity being 3 Sd, with the general membrane stress limit at Sd, and the local membrane limit at 1.5 Sd (Sd = material allowable stress at temperature). However, what happens when the vessel in question is a vacuum vessel ? In vacuum vessels, we are being forced by end users to utilise FEM (the argument being that WRC-107/297 does not take into consideration the effect of external pressure correctly - the software typically takes the vacuum pressure stresses and changes the sign convention to "minus", which is not correct). Obviously FEM cost can become very high when you have a column with lots of nozzles / attachments. Does anyone know of any investigations / published papers on this issue ? How do you limit the stresses in order to overcome the effect of local buckling where typically the combination of vacuum pressure and nozzle loads would result in stresses which are quite high (say up to the 3 Sd limit) ? Do you need to worry about this high localised "compressive stress" ? Can you compare the local deflection at the junction to the out-of-roundness tolerances specified in ASME VIII, Div.1, clause UG-80 ? I'd appreciate any relevant assistance.
Thank you.
]. I actually never considered your suggestion to convert the inward radial load to an equivalent pressure. Good idea. Any others / further info wrt papers etc ?