Inside dia. of pipe in Internal Design Pressure Thickness Calculation
Inside dia. of pipe in Internal Design Pressure Thickness Calculation
(OP)
Folks,
Reference ASME B31.3 Para. 304.1.2 (Straight Pipe under Internal Pressure" equation (3b), where "d" is stated as inside dia. of pipe.
1) In order to calculate value for "d", Do we need to subtract 12.5% mill tolerance from the nominal wall thickness, for example
4" Sch. 120 pipe
O.D of pipe = 4.5"
Nominal wall thickness = 0.438 (120 Sch. pipe)
d = O.D - 2(Nominal wall thickness x .875)
d = 3.7335"
Where 0.875 is to subtract 12.5% mill tolerance
NOTE: This will ultimately gives you higher "pressure deisgn wall thicnkess"
OR
d = O.D - 2(Nominal wall thickness)
d = 3.624"
Which value should be inlcuded in equation (3b).
Regards,
Reference ASME B31.3 Para. 304.1.2 (Straight Pipe under Internal Pressure" equation (3b), where "d" is stated as inside dia. of pipe.
1) In order to calculate value for "d", Do we need to subtract 12.5% mill tolerance from the nominal wall thickness, for example
4" Sch. 120 pipe
O.D of pipe = 4.5"
Nominal wall thickness = 0.438 (120 Sch. pipe)
d = O.D - 2(Nominal wall thickness x .875)
d = 3.7335"
Where 0.875 is to subtract 12.5% mill tolerance
NOTE: This will ultimately gives you higher "pressure deisgn wall thicnkess"
OR
d = O.D - 2(Nominal wall thickness)
d = 3.624"
Which value should be inlcuded in equation (3b).
Regards,





RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
tm = t + c
where t is equal to the "minimum required thickness, including mechanical, corrosion, and erosion allowances"
d = "For pressure design calculation, the inside diameter of the pipe is the maximum value allowable under the purchase specification.", and that's consistant with wall thickness t being a minimum.
The minimum thickness must be what you have left after applying manufacturing allowances, so the minimum thickness would be t and the manufacturer would have to add his tolerance and set his fabrication marks accordingly. Here we assume he adds 12.5%, but we're really not sure, as we know nothing about the mill and we really don't care anyway, because we specified the required minimum wall thickness in the Purchase Order to be "t" and that's what the mfgr must produce regardless of what his manufacturing tolerance is.
You calculate you need 0.875" t.
The mfgr sets his equipment at 1" so his minimum will be 1.000 +0 and 1.000 -0.125 => 1.000" max wall and 0.875" min wall, which is what you calculated you need for pressure containment and what you will write to be specified as the minimum required wall thickness in the PO.
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RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
For most B31.3 - plant - applications, people tend to order based on standard pipe schedules. The mill provides pipe which meets the specification for the schedule. Thus, it is incumbent on the piping designer to account for the undertolerance. For most of my experience, that is 12.5%.
Using SA-106 pipe as a pipe material commonly found in process plants...
3.1 Orders for materials under this specification should
include the following, as required, to describe the desired
material adequately:
...
3.1.5 Size (either NPS and weight class or schedule
number, or both, or outside diameter and nominal wall
thickness, ANSI B36.10),
Note the use of nominal wall in the ordering data.
16.3 Thickness — The minimum wall thickness at any
point shall not be more than 12.5% under the nominal wall
thickness specified.
The temptation for a mill to make the entire pipe 12% under the nominal thickness is reduced by the weight tolerance:
16.1 Weight — The weight of any length of pipe shall
not vary more than 10% over and 3.5% under that specified.
So... for process plant piping designed to B31.3, the pipe specified in the PO must allow for the 12.5% under tolerance permitted for SA-106.
jt
RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com
RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
Perhaps an important distinction in my version of B31.3 and that which you quoted is that the last words in the definition of "d" are "purchase specification". Not P.O. I've very rarely seen a P.O. for a pipe list a minimum thickness. I commonly see a particular Schedule listed on the P.O. Maybe its a regional thing, but if I were to order a 12" NPS x 1.00" min wall SA106 pipe, I think it is all too likely that I would not receive Sch 160 pipe. I think that I would most likely receive Sch 120 pipe. I just don't have faith in the pipe warehouse folks to catch that issue.
It is my opinion that the words "purchase specificaiton" refer not to a P.O. but to the pipe specification such as SA-106. This includes the tolerance in the definition of "d". I could be wrong...
Jumping back to Meck91's post, perhaps the answer to his question lies in Interpretation 21-13:
At the end of the day, the thickness calculated using both the ID and OD should come out the same. See also Int. 1-54, attached.
Finally... S301.2 provides a bit of clarity on the definition of "specified minimum pipe wall thickness, T" as well.
jt
RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
I can still see it.
1) 100 Ft, 12.75" OD x 0.375" minimum WT, xxx weight per foot, Seamless, ASTM A106 Gr. B, see attached specification.
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RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
With your example above, would you expect to receive XS pipe or some nonstandard nominal thickness?
Surely you did not expect to receive Standard wall pipe?
jt
RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.live.com