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Pipe Stress Analysis Criteria (What to analyze) 3

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famm

Mechanical
Jan 17, 2007
2
Hello Guys,

I've got to design a cold water system for a refinery as per ASME B31.3 (fluid is sea water @ 85ºF, and pipe is cement lined), that has pipes from 8" to 60", and I can´t find a table that says what I have to analyze.
I read that as per "Design of Piping Systems" (Kellogg), with this conditions (temperature and sizes) I might have to analyze with a software only pipes over 48", and the rest of the pipes by manual method. Is that right?
 
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You can use a program for any line size and should analyze all pipes that may have stresses close to the B31.3 limits, as long as you recognize the limits of the program's capability. Since economically chosen line sizes and wall thicknesses should have values slightly below allowables, its often difficult to tell exactly which lines may produce the highest stress/limit_stress ratios when under certain loading conditions and Kellogg doesn't know your configuration, so the highest stresses in your piping just might be in the 2". It is best to be conservative and check each and every line until you have developed enough personal experience to make "eyeball" judgements on which pipe segments must be rigorously checked and which can be passed "by inspection".

BigInch[worm]-born in the trenches.
 
You will get lots of opinions on this one.
Here is what I use and teach.

1. The 1500 rule (from David Diehl, COADE).
a. If the line size (nominal pipe size) times (x) the temperature (degrees F) is below 1500 then the line "may" not need formal stress analysis.
Example 3' (x) 400 degrees (F) = 1200

b. If the line size (nominal pipe size) times (x) the temperature (degrees F) is above1500 then the line "may" need formal stress analysis.
Example 4" (x) 400 degrees (F) = 1600

2. If the line (regardless of temperature) connects to a pump, a compressor or other alignment sensitive equipment then it should be submitted for formal sterss analysis.

3. The "May" noted above relates to the application of good judgement when considering other factors such as metalurgy and the wall thicknesses above schedule 80.

 
Each company normally has their own criteria about what lines they what reviewed by a Piping Engineer and which lines from the first group the want to see a formal computer analysis.

For those lines that aren't required to have a computer analysis, it's normally up to the Piping Engineer to perform the level of analysis (inspection, hand calcs or computer analysis)he/she thinks is required to insure a safe piping design that meets the code requirements and also to provide support load information as required to the structural folks.

Most criteria that I have seen requires Piping Engineering review for liquid lines above 12"-14" or so. This is when support loads start becoming significant. Consider the following.
1) Cement lining will add weight to and increase the stiffness of your pipe
2) Check with pipe lining vendor for any design requirements he may impose and the physical properties of the lining system.
3) Evaluate load bearing and local stresses on pipe wall at support location for the larger lines.
4) Control pipe as required for any seismic or wind criteria
5) Question whether the 85 deg. F. is adequate for flexibility and thermal growth requirements. While 85 deg. F. may be a maximum for the sea water source, a non-flowing line may heat up to 150 deg. F. or more sitting in the sun.
6) Provide all loads as require to your Support Designer/ Structural Engineer. (Friction, wind, seismic as required in addition to dead weight)


NozzleTwister
Houston, Texas
 
pennpiper,
The pipe size unit depictions of the examples you gave may confuse the situation a little (I suspect though you probably meant inches diameter in both cases)
 
rconner and others,

The 3" is for 3" NPS (nominal pipe size)
I think that is clear enough.
 
pennpiper

I think what rconnor is referring to is the example # 1 - you use nominal pipe diameter with a designation in feet, i.e. 3'. We (or at least I) wasn't sure if you were referring to 3-inch or 36-inch pipe. But your clarification cleared it up, it is in inches.

Greg Lamberson
Consultant - Upstream Energy
Website:
 
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