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Am I understanding the following PG.29.3 section of the code correctly.

"When a head, dished to a segment of a sphere, has a flanged-in manhole or access opening that exceeds 150 mm in any dimension, the thickness shall be increased by not less than 15% of the required thickness for a blank head computed by the above formula, but in no case less than 3 mm additional thickness over a blank head. Where such a dished head has a flanged opening supported by an attached flue, an increase in thickness over that for a blank head is not required. If more than one manhole is inserted in a head, the thickness of which is calculated by this rule, the minimum distance between the openings shall be not less than one-fourth of the outside diameter of the head."

I am only concerned with the bold section.

Are they saying that only access opening must exceed 150mm? Flanged-in manhole is exempt from this dimension?

The reason why I believe they are separate is because of the following paragraph.

Paragraph PG-44 specifies that boilers have suitable openings, such as manholes, handholes, inspection openings, and washout openings. Elliptical manholes shall not be less than 300 × 400 mm in size. Circular manholes shall not be less than 380 mm in diameter. Handholes shall not be less than 70 mm × 89 mm. There are no specific dimensions for other inspection or washout openings.

Inherently the flanged in man-hole will indeed be bigger than 100mm according to the above.

Thank you for your help.

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  • $\begingroup$ I'm not understanding why it matters. What would you do differently in a design if the 150 mm dimension does or doesn't apply in the case of a manhole? $\endgroup$
    – Mark
    Apr 2, 2022 at 16:01
  • $\begingroup$ boilers need openings, not all pressure vessels do, and other openings then accesses exist. $\endgroup$
    – Tiger Guy
    Apr 3, 2022 at 1:15

2 Answers 2

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No they are not.

Why do you think it says "only"?

Do you think it covers holes less than 150mm?

Basically it says " a flanged manhole greater than 150mm must have an increased thickness of 3mm minimum or 15% of the required thickness if that is greater.

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  • $\begingroup$ Hi Mike. Thank you for your response. Mike, I provided more info as to why I think they are referred to as separate. $\endgroup$
    – Amon
    Apr 2, 2022 at 15:37
  • $\begingroup$ So where do you get 100mm from? Have you misread the first para you mentioned? $\endgroup$
    – Solar Mike
    Apr 2, 2022 at 15:50
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"Are they saying that only access opening must exceed 150mm?"

No, the provision has not set 150 mm as the minimum for either type of opening. It says "when ....has a flange-in manhole or access opening that exceeds 150 mm...the thickness shall be....", which means "when an opening in exceeds 150 mm, it requires...".

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