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Piping Minimum Thickness
2

Piping Minimum Thickness

Piping Minimum Thickness

(OP)
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.
 

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

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...

RE: Piping Minimum Thickness

(OP)
@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.
 

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

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.

RE: Piping Minimum Thickness

(OP)
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!
 

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

ohman10,

You should consider the detail of API 570...  

RE: Piping Minimum Thickness

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.  

RE: Piping Minimum Thickness

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?  

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

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<)  

RE: Piping Minimum Thickness

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.
   

RE: Piping Minimum Thickness

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.

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

(OP)
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!

RE: Piping Minimum Thickness

@ 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.  

RE: Piping Minimum Thickness

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

RE: Piping Minimum Thickness

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.

RE: Piping Minimum Thickness

In 1/2"nps Sch 40 is the lightest pipe available.  Sch 10S is available.  Are you confusing carbon steel pipe with stainless pipe?   

RE: Piping Minimum Thickness


NO! Stainless is the shiney one.

If it was from a mill order, it might be any WT.  There are standard wall thicknesses and an infinite number of others.  You can special fab any WT you want.

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

RE: Piping Minimum Thickness

Minimum wall thickness is always 87 1/2 % of your nominal pipe schedule. You cannot deviate from that by any code.  

RE: Piping Minimum Thickness

So it also applies to e.g. the EN 13480 piping code?

RE: Piping Minimum Thickness

All ASME, ANSI, and ASTM Codes. He said his calcs were from ASME B31.3 Not familiar with the specifics of the European Codes.  

RE: Piping Minimum Thickness

NStamp,

You're 87.5% SMWT isn't the right limit for this discussion.  You're talking about the manufacturing tolerance for a specific requested wall thickness pipe (SMWT) on some purchase order, which is SMWT - 12.5% SMWT.  It has nothing to do with the wall thickness required for guaranteeing adequate wall thickness for pressure or handling needs.

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

RE: Piping Minimum Thickness

@ NStamp,( not to be childish, but) I thought you said any code.
@BigInch; if NStamp was referring to fittings has right, isnt he?

RE: Piping Minimum Thickness

Can't see how he'd be right.

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

RE: Piping Minimum Thickness

For fittings one should take 87.5% of the nominal wall thcikness of seamless pipe, minus 12,5% mill tolerance, right? (B31.3 para 302.2.2)

RE: Piping Minimum Thickness

@xl83nl: That's fine if you are calculating a the required wall thickness for a new installation.  However, manufacturing tolerance wouldn't come into play for UT testing of an existing line to determine if it has corroded to the point of needing replacement.

@ohman10: I don't think there are any ASTM/ANSI/ASME specs that dictate a replacement thickness (beside the thickness required for pressure, which, as you noted from the beginning, is extremely light).

For small bore lines like you have (which I would consider anything under 2", for certain), the wall required for pressure is often a non-issue.  As BigInch noted above, structural integrity will generally limit the minimum wall thickness.  You don't want some operator walking down the line, tripping and tearing a hole in the process because he grabbed a pressure gauge to steady himself and the 1/2" pipe that connects it to the header rips off due to a .002" wall thickness.

In most specs I run across in refinery service, particularly for cheap carbon steel, using a wall lighter than Schedule 80 is rare and Schedule 160 is not uncommon.

Now, regarding your original question about where the replacement wall thickness came from - likely some experienced guys 50 years ago figured out that when your 1/2" line corrodes to a specific thickness it is in much greater danger of failure due to accident and therefore should be replaced at the next turnaround.  Odds are good that the guys are long since retired, or probably got laid off in the 80's and the company never bothered to save the knowledge that went into developing the spec you are now trying to live with.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

RE: Piping Minimum Thickness

The original poster hasn't checked back in two weeks, so I guess he got whatever answer he was looking for.

Another reason for using thicker pipes is the "Bubba climbing factor" -- where a plant operator has to reach a valve (or a guage or whatever) and decides to use an instrument line to rest his foot."  While they probably would avoid a 1/2" tube, I've seen where 1" on up is fair game for climbing.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: Piping Minimum Thickness

Or to keep from ripping the pipe out when you're just trying to open a stuck valve using a wrench.  1" or less, I've always used sch 80 for everything I've done for the last 30 years.

vpl,  Right.  This is one of those topics like "how much straight pipe in front of a pump" that everyone has some kind of an opinion on, right or wrong, and that is apparently very difficult to keep to one's self. smile

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

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