ASME Section VIII Division 2-Elastic Nozzle Comprehensive Analysis (pre-2007) - CAESAR II - Help

To address the local allowable stress problem, you should have the endurance curve for the material of construction and complete design pressure/temperature loading information. Carefully consult the code before performing the local stress analysis if:

• any elastic limit is approached

• there is anything unusual in the nozzle/vessel connection design

The material Sm table and the endurance curve for carbon steels used in this section are for illustration purposes. You should only use values taken directly from the code in your design.

There are three criteria you must satisfy before considering stresses in the vessel wall due to nozzle loads within the allowables. The three criteria are summarized as:

Pm < kSmh

Pm + Pl + Pb< 1.5kSmh

Pm + Pl + Pb + Q < 3Smavg

Where Pm, Pl, Pb, and Q are the general primary membrane stress, the local primary membrane stress, the local primary bending stress, and the total secondary stresses (membrane plus bending), respectively; and k, Smh, and Smavg are the occasional stress factor, the hot material allowable stress intensity, and the average material stress intensity (Smh + Smc) / 2.

The stress classification defined by the Section VIII Division 2 code in the vicinity of nozzles, classifies the bending stress terms caused by any external load moments or internal pressure in the vessel wall near a nozzle or other opening, as secondary stress Q, regardless of whether they were caused by sustained or expansion loads. This definition causes Pb to disappear and leads to a more detailed classification:

• Pm - General primary membrane stress (primarily due to internal pressure)

• Pl - Local primary membrane stress, which may include the following:

  • Membrane stress due to internal pressure

  • Local membrane stress due to applied sustained forces and moments

• Q - Secondary stresses, which may include the following:

  • Bending stress due to internal pressure

  • Bending stress due to applied sustained loads

  • Membrane stress due to applied expansion loads

  • Bending stress due to applied expansion loads

Each of the stress terms defined in the above classifications contains three parts: two stress components in normal directions and one shear stress component. To combine these stresses, the following rules apply:

1. Compute the normal and shear components for each of the three stress types, that is, Pm, Pl, and Q.

2. Compute the stress intensity due to the Pm and compare it against kSmh.

3. Add the individual normal and shear stress components due to Pm and Pl; compute the resultant stress intensity and compare its value against 1.5kSmh.

4. Add the individual normal and shear stress components due to Pm, Pl, and Q, compute the resultant stress intensity, and compare its value to against 3Smavg.

5. Determine if there is an occasional load as well as a sustained load, these types can be repeated using a value of 1.2 for k.

These criteria can be readily found from Figure 4-130.1 of Appendix 4 of ASME Section VIII, Division 2 2004 and the surrounding text. Note that the primary bending stress term, Pb, is not applicable to the shell stress evaluation, and therefore disappears from the Section VIII, Division 2 requirements. Using the same analogy, write the peak stress limit as:

Pl + Pb + Q + F < Sa

The preceding equation need not be satisfied, provided the elastic limit criteria of AD-160 is met based on the statement explicitly given in Section 5-100, which is cited below:

"If the specified operation of the vessel meets all of the conditions of AD-160, no analysis for cyclic operation is required and it can be assumed that the peak stress limit discussed in 4-135 has been satisfied by compliance with the applicable requirements for materials, design, fabrication, testing and inspection of this division."