Question from piping thickness calculation B31.3.

[mariolucas75]

Dear Forum members please help with one easy question from B31.3 pipe thickness calculation.
I am preparing myself for an exam to take API570.
This is one of the questions I encountered and got stuck:

An NPS 4(4.5” o.d.) seamless pipe made from ASTM A-106 Grade A material operates at 300 psi and 400 degrees F. The pipe must cross a small ditch and it must be capable of supporting itself without any visible sag. A piping Engineer states that the pipe must be at least 0.25” thick just to support itself and the liquid product. He also states that a 0.10” corrosion allowance must be included. Calculate the thickness required for the pipe.
a. 0.292”
b. 0.392”
c. 0.350”
d. 0.142”
The answer sheets says that the correct answer is b 0.392 referring to B31.3,304.1.2 (a).

But when I use that formula to calculate the thickness I get 0.0418” for pressure design thickness.
If we add 0.1” for corrosion allowance we get for minimum required thickness 0.1418”.

Since the exam question above says that min required thickness is 0.25” that is thicker then calculated value so we can pick the nearest value to 0.25 from the given variants and which logically is 0.292”.

However the answer sheet says 0.392”.

Could you please help me why is that?

Thanks so much.

 

[LittleInch]

I don't have the wording to hand but the basic is this.

You're told the min wt required is .25" based on structural issues not pressure containment. Then you add corrosion allowance then add manufacturer tolerance, prob 10%. So .25 + .10 =.35 + tol. Nearest will then be .392"

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Cwsmith35]

nietrino,

Correct me if I'm wrong, but I think you've already done the heavy lifting here. Since you need to both withstand the pressure AND the weight, the wall thicknesses requirements are additive. So, taking the pressure design thickness (0.0418) + minimum wall thickness required for support (0.25) + corrosion allowance (0.1)=0.3918 Rounding to the nearest 1/1000 yields b) 0.392. I hope this is helpful!

MSI Supply, Inc.

http://www.msisupply.com

 

[mariolucas75]

Thanks Gents for your reply but if i already have that much thick pipe (0.25") to withstand pressure requirement with corr allowance (0.014"+0.1) i wonder why i cant simply use 0.25 pipe or nearest to it.

May be i need to re-read codes.....

 

[BigInch]

[highlight #3465A4]"pipe must be at least 0.25” thick just to support itself and the liquid product"[/highlight]

That means you need the 0.25" for structural support only.
Now add the pressure requirement.
Add the corrosion allowance.
Now round up.


"He's declaring war on the planet itself."- Vicente Fox

 

[LittleInch]

The question is badly worded and I suspect the question/answer may be referring to 304.1.1 a, not 1.2 a

tm = t+c

I believe the question allows that t = 0.25" includes for mechanical and internal pressure issues. Then add the corrosion allowance, then make allowance for the manufacturing tolerance.

However BI's answer is also feasible if you ignore the manufacturing tolerance required by B 31.3, though I've never seen it done that way. normally you work out the pressure containment and then see if it thick enough for everything else like static loads, stress etc.

the t in this case is 0.25" so the CA needs to be added to it so that even in a "corroded" condition there is still enough metal to maintain the required t of 0.25".

For an engineering question to state something as vague as "without visible sag" is really odd - one mans visible sag is another mans good enough straight line.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Thnks LI. I forgot to include the corrosion allowance. I edited my original response to include it.

Possible that "without visible sag" was added to emphisize that the 0.25" thickness was for structural requirements.

"He's declaring war on the planet itself."- Vicente Fox

 

[KevinNZ]

The required thickness for pressure does not need to be added to the thickness required for sag.
Pressure is a hoop stress. You need to add longitudinal pressure stress (1/2 hoop stress) to the longitudinal 'sag' stress.

 

[BigInch]

Yes, it is true that longitudinal stresses need to be added together. It is also true that the longitudinal stress must be combined with hoop stress and checked again against the allowable combined stress. So, sag stress adds to longitudinal stress, which is a part of combined stress as is hoop stress, so in effect sag and hoop stresses are taken together, just not added together directly.

"He's declaring war on the planet itself."- Vicente Fox

 

[LittleInch]

Hence the point is that the 0.25" covers hoop, longitudinal and bending stresses so can be taken as t min. Then add CA, then add manufacturing tolerance.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

IMO, yes.

"He's declaring war on the planet itself."- Vicente Fox

 

[KevinNZ]

Piping codes (B31.1 31.3) do not have 'allowable combined stress' that includes hoop stress

They
1, Calculate min thickness with hoop stress, corrosion allowance and manufacturer's tolerance
2, Sustained stress from gravity moments and longitudinal pressure stress
3, thermal expansion stress range (Which does include 'left over' Sustained stress)
4, occasional stress from occasional and sustained moments and longitudinal pressure stress

You can see hoop stress is not included in 2,3,4. These cases also do not include manufacturer's tolerance (See B31.3 302.3.5). (In B31.1 you do not have to include CA, but you should!)

Assuming the 'piping engineer' has included longitudinal pressure. I would answer the OP as .25 + .1 = 0.350.
MA is only applied to the hoop stress calculation and not in the longitudinal calculations.

B31.3,304.1.2 (a) is only the min thickness Calculation and does not add to the 0.25" required for longitudinal stress.

The whole exercise is missing a step. When the pipe thickness is selected the 'piping engineer' has to redo their sag calculation as the pipe weight is now known.

But the answer could be use the thickest pipe because that sags the least.

 

[BigInch]

31.3 ... Right!

"He's declaring war on the planet itself."- Vicente Fox

 

[KevinNZ]

Real world problem

 

[BigInch]

With combined stress.


"He's declaring war on the planet itself."- Vicente Fox