Page 225 - Pressure Vessel Design Manual

4

Special Designs

PROCEDURE 4-1
DESIGN OF LARGE-DIAMETER NOZZLE OPENINGS 111

There are three methods for calculating the strength of it was still superior to what we used before this paper was
reinforcement required for openings in pressure vessels: published. The ASME has now revised the applicability of
the procedure to the cases where it has been deemed safe.
1. Area replacement rules per UG-36(b). Large openings calculated by this procedure are limited to
2. Analysis per Appendix 1-7. openings less than 70% of the vessel diameter. There are
a. 2/3 area replacement rule. four cases that can be solved for, depending on your
11. Membrane-bending stress analysis. nozzle geometry.
3. FEA.

The Code defines when and where these methods apply. Reinforcement for Large-Diameter
Reinforcement for large-diameter openings has been in the Openings
Code for a long time. The previous rule was simply to move
the majority of the area replacement closer to the nozzle Per ASME, Section VIII, Appendix 1-7(b)l(b), the rules
neck, also called the 2/3 rule. Unfortunately, there were a for “radial nozzles,” not oblique or tangential, must meet
few cases of flange leakage where the flange was located strength requirements in addition to area replacement
close to the shell. It was discovered that as the opening rules. The following lists the parameters for which these
opened up, the flange was distorted. It was actually bending. additional calculations shall be performed:
In addition, the 2/3 rule did not allow for an accurate way to
determine MAWP for the vessel without proof testing. a. Exceed the limits of UG-36(b).
This issue was addressed in 1979 by McBride and Jacobs. b. Vessel diameter > 60 in.
Jacobs was from Fluor in Houston. The principle was to c. Nozzle diameter > 40 in.
calculate stresses in two distinct areas, membrane and bend- d. Nozzle diameter > 3.4&.
ing. Membrane stresses are based on pressure area times e. The ratio R,,/R < 0.7 (that is, the nozzle does not
metal area. Bending is based on AISC beam formulas. The exceed 70% of the vessel diameter).
neck-and-shell section (and sometimes the flange as well) is
assumed as bent on the hard axis. This is not a beam-on-
elastic-foundation calculation. It is more of a brute-force Table 4-1 shows the ratio of vessel diameter, D, and shell
approach. thickness, t, where the values of 3.4& are greater than 40.
This procedure was eventually adopted by the Code and The heavy line indicates the limits for which 40 is exceeded.
incorporated. Unfortunately, it turned out that the proce- For nozzles that exceed these parameters, a finite element
dure, while good for most cases, was not good for all. Yet analysis (FEA) should be performed.

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