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Allowable Limits for Nozzle Loads with Vacuum Pressure

Allowable Limits for Nozzle Loads with Vacuum Pressure

Allowable Limits for Nozzle Loads with Vacuum Pressure

(OP)
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.

RE: Allowable Limits for Nozzle Loads with Vacuum Pressure

An interesting problem.
My first thoughts:
-when you satisfy opening reinforcement rules for vacuum, you should consider that this protects you from local buckling effects under the effect of pressure including the end effect of pressure acting through the pipe connected to the nozzle and the nozzle neck: it has never been clear to me whether specified nozzle loads include or not the pressure end effect, but I suspect they normally do
-of course if there are axial (compressive) loads acting on the nozzle beyond the pressure end effect, there is a risk of local buckling; a simple way to deal with this would be to transform the load into an equivalent pressure (force divided by opening area) and to check the opening reinforcement with this increased pressure
-bending moments acting on nozzle shouldn't be at risk of local buckling, so for them I would stay onto the normal procedure, where of course the membrane stress due to pressure would be added as PeD/2t(circumferential) or PeD/t(longitudinal) as relevant.

prex

http://www.xcalcs.com
Online tools for structural design

RE: Allowable Limits for Nozzle Loads with Vacuum Pressure

(OP)
Hi Prex,

thank you for the prompt response.  As far as the end pressure from the piping acting at the nozzle-shell junction goes, this has never really been an issue for us, so I would tend to agree that the tabulated loads which the end users have include some allowance for this [although I won't be asking them :)].  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 ?

Regards
John

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