Engineering                            Pressure Vessels
     ASME Sect VIII, Divs 1 & 2 expertise
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Nozzle Loads

Do nozzle loads drive you crazy?  Some of the specifications I see defining their magnitudes or methods and criteria certainly put me close to that edge.  Nozzles do need to be evaluated, it's just a matter of balancing conservatism with the effort to conduct an appropriate analysis.  And this is sometimes not trivial with the potential of load reversals and loads from different sources that produce varying stress limits.

How does one address nozzle loads?

In ASME Sect VIII, Div 1 nozzle loads fall into U-2(g) where 'proper engineering methods' are required.  Guidance can be found In Div. 2, Appendix 4 where load sources and stress classifications are made to identify what stresses to examine for differing load combinations.

The most common pitfalls?

• Nozzle loads from piping analyses with overly stiff boundary conditions at the nozzles.
• Overly conservative pressure load stress results.  (Table AD-560 is for peak stress, not membrane!)
• Inappropriate stress limits for load combinations.

What to look for to identify that a pitfall exists.

• Nozzle loads exceed loads from vessel's own mass.
• Suddenly unacceptable stresses for an opening that satisfies area replacement rules when nozzle loads produce little stress.

What can you do?

The first step would be a thorough review of ASME Sect VIII, Div 2, Appendix 4 with focus on Table 4-120.1 and Figure 4.130.  What these show is that membrane stresses are primary local while bending stresses are secondary.  Further, secondary stresses are determined using operating pressure, not design pressure.  Primary stress limits may include a 'k' factor while secondary stresses may not.  If you're wondering how to address bending stresses with the design pressure, check out AD-160 where the fatigue exemption rules are located.

What can I do for you?

My endeavors with nozzle load evaluations has lead me to develop a number of tools for examining stresses at nozzles, including parametric FEA models for both thin (shell element) and thick (solid element) shells.  Since pressure is typically the dominating load, just getting an accurate stress result for the pressure load is often 90% of the important work.  Below is a chart from a thick shell evaluation of pressure stresses.

The largest values represent a factor of 1.5 on the Pr/t stress which would imply that this particular opening has very little strength left for nozzle loads if the shell is set to it's minimum thickness for pressure.

While simply combining the stress intensities (SI) of pressure and nozzle loads is conservative, it often is sufficient.  If greater scrutiny is required, my Excel spreadsheet 'tools' can be used to combine stress components, as appropriate, for proper determination of SIs.

These same parametric models can also be used to define accurate nozzle stiffnesses for piping analyses by applying unit loads and examining the displacement response.

contact email address - CBoyakPE@sonic.net

"An engineer is someone that can do for a nickel what any damn fool can do for a dollar."