Pipe Thickness to ASME B31.1-2007 1

[DSB123]

The ASME B31.1 Code provides equations to determine the required thickness tm for straight pipe (i.e. Eqn 3 Para 102)which includes any allowances for corrosion etc.

Table 102.4.5 states that the minimum thickness of straight pipe prior to bending is some factor mutiplied by tm (the multiplier dependant upon the bend radius). This implies that the minimum required thickness prior to bending is dependant upon the pressure (since tm is the required pressure wall thickness plus allowances). This does not seem logical since during bending the thinning is solely dependant upon its own physical properties and the bending process and not the pressure to which it is subject to in operation.
From my point of view the original nominal thickness should be taken and then a thinning allowance applied to it and then any other allowances taken off. The resulting thickness must be greater than tm to be Code compliant.

Any comments?

 

[kacarrol]

I think that factor takes into account the thinning over several processes. With a factor of safety on top.

If you read this article:

http://www.tubefirst.com/PinesTechnology.pdf

You can approximate a thinning factor of 0.16 for a 90 degree bend at 1.5 diameters radius. This is almost equivalent to the correction factor listed in table 102.4.5 for a 3D bend (1.25tm) which would thin less since the bend radius is not as tight.

Another good read is here:

http://www.arpnjournals.com/jeas/research_papers/rp_2008/jeas_0208_82.pdf

Although, this deals more with ovality than thinning, they do talk about thinning.

Hope that helps,
K

 

[DSB123]

kcarrol,
Thanks for the reply but you seem to be not getting the point of the question. I am not questioning the value of the factors just the application. Read my post again.

 

[BigInch]

I've never read the B31.1 code, nor do I think I understand your question, so I hope you don't mind my commenting.

The code's method doesn't seem particularly illogical to me. The additional thickness has been related both to the bend angle and to pressure. The relation to pressure effectively is additionally relating the expected thinning to the wall thickness to diameter ratio, wt/D. I would expect that the wt/D ratio has some effect on thinning.


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[DSB123]

BigInch,
I agree the wt/D ratio has some effect on thinning but it is the actual wt/D ratio not the pressure thickness/D ratio. The Code suggests that you account for pipe thinning by increasing the "pressure thickness" of the pipe by a factor given in the Code. Wall thinning cannot be associated with pressure because it is a manufacturing process dependant upon the process and the physical pipe being bent, and is irrelevant of pressure. If your thickness to resist pressure is small then the Code suggests that the thinning is small. This is more important where you have allowances (corrosion etc) to account for and your pressure wall thickness is close to the selected pipe schedule. By multiplying the pressure wall thickness by the the factor and checking against the nominal wall thickness less mill tol less corrosion could yield acceptability whereas taking the nominal thickness less mill tol less wall thinning less corrosion allowance (as I beleive should be done) could yield un-acceptability.

Hope this is not too confusing.

 

[BigInch]

I almost included this part, but kacarrol had already mentioned it; safety factor is included too. Otherwise it would be as you say; tm being only related to pressure. Indirectly the tm relation goes all the way back to a factor based not only on wt required for pressure alone, but exactly as kacarrol has already said, to wall thickness required for (pressure+safety). That extra safety margin wouldn't get included, if it was based on wt/D alone.



"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

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[kacarrol]

DSB123,

I think I see what you are trying to ask. But I think you may be missing the point of the code. If you read it over again, particularly 102.4.5 (A) and (A.1):

102.4.5 Bending. The minimum wall thickness at any point on the bend shall conform to (A) or (B) below.

(A) The minimum wall thickness at any point in a completed bend shall not be less than required by eq. (3) or (3A) of para. 104.1.2(A).

(A.1) Table 102.4.5 is a guide to the designer who must specify wall thickness for ordering pipe. In general, it has been the experience that when good shop practices
are employed, the minimum thicknesses of straight pipe shown in Table 102.4.5 should be sufficient for bending and still meet the minimum thickness requirements of para. 104.1.2(A).

The code clearly states the minimum thickness of the bend is based on the required wall thickness to withstand the pressure but then goes on to say that empirical data has proven you can safely use a pipe wall thickness given by using the multiplier in the table with the straight pipe wall thickness (calculated in equation 3 or 3A) to ensure that the thinning of the pipe wall during bending will not let the thickness drop below code requirements. This is simply a rule of thumb put in there to make the job of the designer easier.

It is not saying that the thinning is dependant on the pressure to which it is subjected to. The code is doing what you think it should be doing. It is taking the original nominal thickness and then applying a thinning allowance. It’s just that the thinning allowance is expressed as a percentage based on what radius you are bending the pipe to instead of giving a set thickness to add on, which makes sense. The tighter the bend is, the thinner the pipe will get and the higher the percentage added. The pipe doesn't thin by a set amount, but by percentage as is shown in the articles I reference previously. They preferred to express thinning as a ratio or percent because it makes sense.

To break it down mathematically:

Tb = Pipe wall thickness for the pipe to be bent
Tm = Pipe wall thickness required by pressure calc
Ta = Thinning allowance

What you are thinking about would look like:
Tb = Tm + Ta
Where Ta was some set thickness.

But what the code is giving is a percentage. So, to add on an allowance, you multiply the original thickness by the percentage and add this onto the original thickness:

Tb = Tm + Ta*Tm

Simplify the equation:

Tb = (1 + Ta)*Tm

Now let’s say that we know the pipe will thin by 25% when we bend it, so Ta = 25% = .25

Tb = 1.25*Tm

Which is the information given in Table 102.4.5.

Hopefully it's clear now.

K

 

[BigInch]

That's how I understand it, having finally seen the thickenss formulas. Thanks kacarrol.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[DSB123]

Hi,
Perhaps I am not explaining my concern clearly. Please refer to attached example calc.

 

[prex]

My calculation gives:
0.594*0.875/1.25=0.416 > 0.401 OK
To solve your 2nd problem consider that the maximum thinning is at the extrados, the intrados could even thicken, so the actual local thicknesses should also be OK when checked on the elbow.

prex

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[DSB123]

prex,
The Code 1.25tm basically says that the thinning is 25% so using you approach the calculation is :-
0.594*0.875*(1-0.25)=0.390<0.401 No Good.
Also extrodos thickness is 0.377 <0.401 No Good

 

[kacarrol]

DBS123,

Things are much easier with a spreadsheet! Thanks for attaching that so I can see your logic, unfortunately this is a common mistake in calculations that use percentages.

You can't subtract 25% off the final value and get the same results, the math doesn't work, just take a look at your tax bill next time. Try to calculate the amount you are paying in tax by multiplying the after tax bill by the percentage of tax you are paying. It won't work. Prex has the correct equation. Here is the math:

tallow = 0.25*tm
rearrange: tm = tallow/0.25

tbend = 1.25*tm
rearrange: tm = tbend/1.25

set the two above equations equal
tallow/0.25 = tbend/1.25

solve for the allowable:
tallow = (0.25/1.25)*tbend
tallow = 0.2*tbend

substituting the values into the equation:
tallow = 0.2*0.52 = 0.10395 (not 0.13)
and tm = 0.416 which is well above the pressure minimum of 0.401

To check that the calculation is done correctly you can substitute tm back into the second equation:
tbend = tm*1.25
tbend = 0.416 * 1.25
tbend = 0.52

Where as if we take the tm off your spreadsheet it does not give the correct backwards check answer:
tbend = 0.39 * 1.25
tbend = 0.49

Hopefully that makes things a little more clear.
K

 

[kacarrol]

Okay, before you come back again, you’re right I did say: “Now let's say that we know the pipe will thin by 25% when we bend it, so Ta = 25% = .25” in the 28 Jul 10 9:42 post. I probably should have worded that differently. My bad. Perhaps I should have said the pipe is allowed to thin by 20%.

To prove that the values in the code are very very conservative I would like to take the worst case out of table 1 in the first link I provided (

http://www.tubefirst.com/PinesTechnology.pdf)

From the document:
Bend Radius: 1.5D
Nominal Starting Pipe Wall Thickness: 0.23”
Bend Angle: 90
Worst Case Wall Thinning (Sample 9): 11%

From B31.1 Table 102.4.5
Bend Radius: 3D
Thinning Allowance: 1.25*tm

Now I realize that we are considering apples and oranges here since I’m using a 1.5D example with a 3D thinning allowance. Since B31.1 does not supply the values for a 1.5D bend I considered extrapolating from the values in the chart, but realized that taking the 3D thinning allowance was still enough to cover this example therefore further proving just how conservative these values are. (For those of you with out copies of B31.1 the value of the thinning allowance increases as the bending radius decreases)

Calculated thinning allowance = 0.23” * (1.25 – 1)/1.25 = 0.046”
Actual amount that pipe thinned = 0.23” * 11% = 0.0253”
Percentage difference between allowed and actual = 0.0253”/0.046” (*100) = 55%

I'd like to re-iterate that the table 102.4.5 values are rules of thumb that have stood through the test of time. They aren't meant to be a hard and fast calculation that will tell you exactly how much the wall of the pipe will thin. The aren't meant to be used to back calculate the amount of thinning. They are meant to be conservative enough that they can be used with a well controlled, quality bending process to ensure that the end result will more than meet code.

Let me know if I am still missing the question. It's hard to understand what someone is trying to convey through a typed message sometimes.

K