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.

 

[osoloco]

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

 

[BigInch]

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

 

[NStamp]

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

 

[XL83NL]

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

 

[NStamp]

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

 

[BigInch]

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

 

[XL83NL]

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

 

[BigInch]

Can't see how he'd be right.

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

 

[XL83NL]

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)

 

[StressGuy]

@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.

 

[vpl]

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 for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

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.

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