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.

  • $\begingroup$ 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


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.

  • $\begingroup$ 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

"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...".


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.