Volume XLIV.II: Openings in Code Pressure Vessels - Part II

  • Volume XLIV.II: Openings in Code Pressure Vessels - Part II

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

Openings in ASME Code Pressure Vessels - Part 2

A Matter of Inclination

An opening whose Z axis is not perpendicular to the shell (or head) plane, or parallel to any vessel axes, is said to be inclined. The inclination angle is designated β. Keep in mind that in many cases one axis plane of an opening will be radial while the other is inclined. The value of d' for an inclined nozzle is defined by

Like the circular hill-side and tangential, a circular inclined opening produces an elliptical shape.

 

Example 3 – Normal and Inclined Openings

Problem: Two 24 inch diameter openings are required in the conical head of a pressure vessel whose one-half apex angle is 45°. One opening must have normal entry and the other near horizontal entry. What is the difference in the governing axis dimension of the two openings?

 

 

Example 4 – Hill-side verses Inclined Opening

Problem: For comparison purposes only, treat the hill-side opening of Example 2 as if it were an inclined opening. Compare the value of d' determined through the hill-side and inclined analytical methods.

Solution: Re-inspection of the Example 2 diagram will show that an acute central angle which will be called θ, which radially locates the center of the opening, is equal to:

and that the pseudo comparative inclination angle β is equal to:

β = 90° – θ

Conclusion: While both hill-side and inclined circular openings produce elliptical shapes, they cannot be considered identical. Inclined openings should not be confused with the so called hillside orientation. Use of the inclined opening analytical method for the hill-side opening of Example 2 produced an error of -2.3% in the determination of the Code defined governing opening size. The margin of error may be more pronounced depending on the ratio of d/D.

RULES OF ELLIPTICAL OPENINGS

  1.  If the opening is not radial for the plane under consideration, but the opening’s Z axis is parallel with the vessel axis, use the hill-side analytical method. Otherwise, use the inclined method.
  2.  If uncertainty exists as to the opening’s governing orientation, calculate the major axis (X) dimension by both methods to determine the largest value of d'.

Size Does Matter

Avoiding the Grand Opening

The main Code rule paragraphs are for normally proportioned openings. In order to be normally proportioned and avoid being classified by Code definition as a large opening, the following relative size parameters must be met:

If the vessel is 60 inches or less in diameter, all penetrations are limited to ½ the diameter, up to a maximum of 20 inches. If the vessel diameter is larger than 60 inches, penetrations are limited to 1/3 the diameter, up to a maximum of 40 inches. If these boundaries are exceeded, the opening must receive special reinforcement which is calculated through the supplementary design formulas of Code Appendix 1.

Call Me Insignificant

Certain openings are considered insignificant with regards to vessel strength. In order to qualify, strict size and relational dimensional parameters must be satisfied. Additionally, the vessel cannot be subjected to rapid fluctuations in pressure. It has been suggested that this should include any vessel which would be subjected to greater than 1,000 cycles of pressure variation exceeding 20% of the design pressure.

Openings of 3½ inches or smaller are considered insignificant if they are made in plate of d inch or less in thickness and their finishing connections are welded. The opening must not exceed 2d inches if the plate is greater than d inch thick. Threaded, studded, or expanded connections may not exceed 2d inches regardless of plate thickness. To qualify, the openings must be normal orientation.

Two adjacent openings not exceeding the sizes listed above must have a minimum center to center distance equal to the sum of their diameters in order to remain insignificant.

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