Jim, Talking about cubic inches is a mistake. You
must talk about square inches of cross section to comply with ABS or ASME (
which are the same). You cannot use the volume of the disc removed.
With volumes, as you have used, in an example in my design I am 133% over
designed whereas in using the areas I am 103% over designed (i.e. 3% safety
factor on ASME requirement) I am not suggesting that you are under designed
but you will find your safety margin is significantly less. If you can
give me your thicknesses, hole diameter, and corrosion allowance I can do
a quick calc. This is a common fallacy and it should be highlighted for
P-subbers somehow. Any suggestions? Maybe we can do a spreadsheet
similar to the shell one. I have one but it is typically hard to
follow. Regarding 100% being insufficient you must remember that ASME
generally works on a 400% safety factor or 4:1 so if you design to 100% of
their suggestion then the safety is already built in. Rgds Hugh
From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of kocpnt
tds.net
Sent: Sunday, 4 April 2010 8:36 a.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hull Penetration and Reinforcement
While I am certainly not qualified to suggest specific
engineering standards, this is what I used as a guideline for my sub.
By seat of the pants I felt that 100% replacement was
inadequate!
The hull is 48 inches and 1/2 inch grade 70 steel.
I was shooting for 250%. I have a 1 inch thick lower portion
of the conning tower which is 24 inches in diameter and translates to 527 cubic
inches at the minimum point.
I removed about 226 cubic inches from the hull and ended up
with 233% replacement.
On Sat, Apr 3, 2010 at 12:40 PM, Jon Wallace <jonw@psubs.org> wrote:
I'm looking for help understanding some specific elements of engineered
reinforcement requirements for hull penetrations as per ABS, and also opening
up a discussion of best practices. For reference, I'm looking at the ABS Steel
Vessel Rules document:
Part 4 - Vessel Systems and Machinery
Chapter 4 - Boilers, Pressure Vessels and Fired Equipment
Section 1 - Appendix 1 – Rules for Design
7 Openings and Reinforcements
First, I have a specific question related to Figure 8 on page 311, elements Tn
and Trn at the top left of the nozzle. Did they mark the illustration
incorrectly? The variable Tn is defined as the nozzle wall thickness exclusive
of corrosion allowance. The variable Trn is defined as the required nozzle wall
thickness exclusive of corrosion allowance for a maximum working pressure. The
equations using these elements look correct (Tn-Trn) which would complement the
idea of calculating the amount of reinforcement provided by the thickness of
the nozzle above that required for the working pressure. However, the
illustration for Trn appears to show a sliver of metal as the actual difference
between Tn and Trn, rather than how Trn is defined in the text. Looking at the
illustration, I would have expected Trn to be shown as the material with the
"\" lines through it (or the "/" lines in the right
segment) rather than the sliver of metal it is currently pointing to.
2010 ABS rule 4-4-1A1/7.1 restricts the calculations for penetration
reinforcement of shells 60in diameter or less, to 1/2 the shell diameter but
not exceeding 20 inches. For a typical K-sub (36 inch diameter) this means the
calculations are valid for a conning tower or viewport no wider than 18 inches
(457mm). The rule states "Reinforcement of larger openings is to be
submitted for specific approval".
I don't think I've ever seen a conning tower on any PSUB that is 18 inches or
smaller, and most appear to be 22 to 24 inches in diameter. For those of you
not working off of K-sub plans, what are you using as a guide to calculate the
thickness of the conning tower and/or reinforcement metal? Busby (http://busby.psubs.org)
stated in Chapter 5, Page 255 (Hull Penetrations) that an ASME standard is to
replace 100% of the material taken from the hull. If I calculated correctly
however, Kittredge replaced the K-sub conning tower penetration with about 400%
of the material taken from the hull (can anyone check my numbers? Hull
thickness 1/4inch, conning tower ring is 24.5 x .75 x 9 inches).
Have I missed another ABS document that deals with this issue? Notwithstanding
submission of a design to an ABS surveyor, what is the best practice PSUBS be
promoting for this issue?
Jon
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