Piping Minimum Thickness 2

[ohman10]

When calculating minimum thickness for a pipeline using ASME 31.3 equations and values I come up with a value that is too small for wall thickness (~0.002") because of small internal pressure (150psig). Is there an absolute minimum thickness requirement, which says if you calculate a value smaller than this - use this value?

I am using ASTM A106 Grade B Sch 80 carbon steel seamless with a diameter of 1/2".

Thanks in advance.

 

[BigInch]

Use the smallest standard size wall thickness. Probably pipe schedule "5" or "10".

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[PAN]

ohman10

We should not consider only internal pressure for the design especially for this small bore piping...

Your selection of carbon steel pipe diameter 1/2" Sch. 80 may come from additional stiffness to withstand external loads plus corrosion allowance...

 

[ohman10]

@BigInch- Thank you for the response. Is this an industry standard? It seems like common sense information but is there a reference that I can use for justification of this method?

@PAN- Thank you too for your response. I will make sure to take into account external load and corrosion before I go about using the smallest standard wall thickness as BigInch suggested.

 

[BigInch]

Common sense. Its cost effective. You will have to pay an extreme amount of money to get special order pipe. For something that small, you will never be able to get a mill order, 500 tons or so, so you will never, never, never get a good price on a small quantity of special order. Go with the smallest in stock, sch 5 or sch 10, maybe even sch 20 on some days.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[osoloco]

Ohman: are you going to butt-weld, socket weld, thread the pipe?
welding the little stuff is difficult... threading the thin stuff is impossible. Generally go with the cheapest to fabricate, labor is usually more expensive than materials. Unless you get into the stainless or exotic materials you will most likely be limited to Sch40 or heavier for pipe less than 8"nps.

 

[ohman10]

We have had the schedule 80 pipe installed for more than 10 years, we do an ultrasonic NDT every year on it. This year, the inspection revealed that the wall thickness of the pipe is below the min thickness value on a drawing, indicating that it needs to be replaced. I have been unable to get an answer as to which standards were used to get this minimum thickness/pipe replacement number. As I mentioned before, I tried to calculate minimum wall thickness via the equation in ASME 31.3 but got an unreasonable number.

I need to be able to reference a standard in order to justify the replacement of the pipe, a reference to a drawing that nobody can tell me anything about does not seem right. Thank you again for your help!

 

[BigInch]

Nope. You've crossed the line now. Pressure is obviously not the only load if you got almost no required wall thickness and the previous design resulted in sch 80. You don't go looking around for standards to confirm you're wrong. You find out what the original design loads were and redo the calculatioins correctly, or you replace it with the same kind of pipe and the same original wall thickness of the pipe that you are removing.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[PAN]

ohman10,

You should consider the detail of API 570...

 

[chicopee]

Have you checked out regulatory requirements? Eventho let's say schedule 20 may be excessive for the internal pressure, if regulation require that, then you have no choice.

 

[racookpe1978]

Something else is "odd" based on the original design selection of Sch 80 for a 150 psig fluid when a "current" pressure analysis calls for tissue paper thicknesses.

150 psig would "normally" be a Sch40, so the heavier original wall had to be something else: corrosion (outside and inside?), long span distance needed between pipe supports? Some unusual bending requirement where the heavier wall made minimum thickness after bending easier to meet?

 

[BigInch]

Or it was the thinnest wall pipe they had in stock!

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[racookpe1978]

Awe man! Now you're usin' just that ole "pick-it-up-off-the-shelf" solution, and here the original poster wanted a unique and special one calculated up real special .... 8<)

 

[osoloco]

3/4" nps sch 40: od = 0.840", wall = 0.109, 150psig

B31.3 para 304.1.2 gives D/6 = 0.140 therefore equation 3b applies. (Sch 80 would not as the wall thickness is greater that D/6, I am not going there).

t = (P*D)/(2*S*E) = (150psi*0.84")/(2*16ksi*1) = 0.004"
I personally have not encountered 3/4" A106 in lighter than sch 40.

This is only looking into internal stress....
probably sch 80 was selected for environmental reasons, i.e. corrosion, external loads, etc. BTW hat is in the pipe? Is it hazardous?

any further analysis into whether sch 40 or 80 should be used is most likely not economically justified.

 

[Jmoore1]

As A side note....the cost increase from sched 40-80 is much likely far less expensive than the cost of re-installing pipe at an earlier date.

 

[BigInch]

They don't make this small diameter stuff in thin wall, because there is a simple gross lack of mechanical strength even for the lightest loads. Why have a pipe if you have to build a strong box to put the pipe inside. It would then just be easier just to let the fluid flow in the box.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[ohman10]

Schedule 80 pipe because the chlorine institue specifies it. I found that in 31.3 the abs min thickness for a pipe is 0.0625". Thanks for all your help!

 

[XL83NL]

@ ohman10: could you specify where in B31.3 this is mentioned? Schedule 10 1/8" pipe has a wall thcikness of 0.049". Furthermore, this would mean that for 1/8", 1/4" and 3/8", certain schedules (mostly Sch. 10 and Sch. 40) arent allowed to be used.

 

[BigInch]

The remaining hole in the pipe would be hard for an ant to turn around.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[racookpe1978]

Well, yes, an ant could not turn around in the pipe. But at least you would not need an ant-check valve....

All ant-flow could be in only one direction.