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Inside dia. of pipe in Internal Design Pressure Thickness Calculation

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,




  

RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation

Finish reading the definitions of all variables.  

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

I think BigInch is coming at this from a pipeline perspective, where shaving a few thousandths of an inch can make a large difference for a mill providing a custom thickness pipe hundreds of miles long.

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 came at it from the B31.3 code's point of view (which us pipeline types <sometimes> use in pump, compressor, terminals stations) quoted from 2008 edition, which says you should "specify the minimum required wall thickness in the PO".  If you want to turn your calculated wt into into a schedule number of course you are free to do so, but then your minimum calculated thickness should be lower than 12.5% thinner than that schedule's wall thickness, or something just might come out wrong.

"I am sure it can be done. I've seen it on the internet."  BigInch's favorite client.

"Being GREEN isn't easy." Kermitfrog http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation

BigInch-

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:

Quote:

Question (1): ASME B31.3-2004, para. 304.1.2  rovides two equations to calculate pressure design thickness for straight pipe, eq. (3a) and eq. (3b), the former using the outside diameter and the latter using the inside diameter. Do these two equations yield the same results?

Reply (1): Yes, when the corrosion allowance in para. 304.1.1 is applied to the inside diameter of the pipe.

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

Don't you ask for a wall thickness with the PO???  I worked two years contracted to the purchasing dept of Northern Natural Gas and that info I can tell you was on a whole lot of POs I issued, specified as minimum wall thickness.
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.

"I am sure it can be done. I've seen it on the internet."  BigInch's favorite client.

"Being GREEN isn't easy." Kermitfrog http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

RE: Inside dia. of pipe in Internal Design Pressure Thickness Calculation

No, the way I've seen pipe purchased is on a Schedule basis, not a minimum wall basis.

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'd expect to see the next thickest standard wt pipe the supplier had in his immediate stock for a length that short, because it is definitely not qualifying for a mill order.  If there were another 4 zeros, I would expect to see the exact minimum wall thickness quoted.  If it was some intermediate length and the supplier was wanting the order bad enough, he would call the mill on his own initiative and ask if they could do a special run on a smaller intermediate quantity, rather than quote some schedule number with a much greater  wall thickness that I specified and lose the chance to make a sale.

"I am sure it can be done. I've seen it on the internet."  BigInch's favorite client.

"Being GREEN isn't easy." Kermitfrog http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

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