Flex analysis for piping 2

[cardin]

I tried to find a clearer answer on when it is required to perform a Flex. analysis of piping? EN13480 sets some basic requirements,but it doesn't give a clear guideline for me. It is more or less open for interpretation i think. Any ideas on this?

 

[XL83NL]

EN 13480 has the same standpioint as B31.3. Refer to 12.2.10.2:

12.2.10.2 Formal analysis required
Any piping system which does not meet the criteria in 12.2.10.1 shall be analysed by a method of
analysis which is either simplified or approximate or comprehensive.

Similarly, I could have posted 12.2.10.1, which lists the criteria that dont require you to do an analysis.

Please note that the 3rd indent of 12.2.10.1 is a useful tool, but hardly used in real (piping life). There's sufficient discussion on that (and the other 2 indents) here @ ET;
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https://www.eng-tips.com/viewthread.cfm?qid=352391

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https://www.eng-tips.com/viewthread.cfm?qid=418267

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https://www.eng-tips.com/viewthread.cfm?qid=246231

(this one is important - JohnBreen's post are considered very valuable. Take his word, and scavenge his older/other posts to learn from this reputable ex-B31.3 committee member)
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https://www.eng-tips.com/viewthread.cfm?qid=102044

Whats mostly found in industrial plants is that either the client has specifications which outline (in much more detail) which lines need analysis, or the contractor has its own set of requirement.

I have always found PIP PNC00004 very useful - we follow it's outlines in general to tell our client when we do a flex. (pls note the hyperlink is the old revision - the current one is from 2019)

 

[pennpiper]

Cardin/XL83NL,
Is "Flex analysis for piping" the same "as Stress analysis for piping"?
Or, is "Flex" the new term?
When did it change?
Who changed it?
And, why did it change?

Sometimes its possible to do all the right things and still get bad results

 

[RVAmeche]

"flexibility analysis" is actually the old school term generally referring to a thermal expansion/contraction evaluation. Stress analysis is the newer, more inclusive term for analyses that include occasional loads and other inputs aside from temperature analyzed via software.

 

[XL83NL]

To my knowledge it didnt change; some people say flex analysis rather than stress analysis - stress analysis of course being the correct term. But I understood what he meant.

 

[cardin]

@XL83NL, that is what i know too, but then again i come to the same comment as you do.

"Whats mostly found in industrial plants is that either the client has specifications which outline (in much more detail) which lines need analysis, or the contractor has its own set of requirement."

This results in a lot of different interpretations of the Standards requirements. So you can't always say "this needs a flex. analysis" since the code leaves room for interpretation.

I use the word "flex" because of the fact that EN13480 uses this, but for me it's the same thing.

 

[XL83NL]

cardin, the standards are quite clear to my interpretation. Using B31.3 for example (EN 13480 has been derived from B31.3 to a very large extent, so can be considered the same):

319.4.2 Formal Analysis Requirements
(a) Any piping system that does not meet the criteria in
para. 319.4.1 shall be analyzed by a simplified, approximate,
or comprehensive method of analysis, as appropriate.
(b) A simplified or approximate method may be applied
only if used within the range of configurations for which its
adequacy has been demonstrated.
(c) Acceptable comprehensive methods of analysis
include analytical and chart methods that provide an
evaluation of the forces, moments, and stresses caused
by displacement strains (see para. 319.2.1).
(d) Comprehensive analysis shall take into account
stress intensification factors for any component other
than straight pipe. Credit may be taken for the extra flexibility
of such a component.

The only room for debate I see is what someone calls a simplified or approximate method. Compreshensive is quite clear; from a practical point of view, this is the analysis performed in pipe stress analysis programs like CII or AutoPIPE. EN 13480 has chart or simplified methods in annex Q; I doubt it will be used a lot. B31.3 doesnt even have such methods in the Code. Given todays capabilities of pipe stress analysis programs, engineers (especially the younger ones who arent acknowledged with chart methods) can much easier do a quick and simple analysis in the aforementioned programs.

Cardin/XL83NL,
Is "Flex analysis for piping" the same "as Stress analysis for piping"?
Or, is "Flex" the new term?
When did it change?
Who changed it?
And, why did it change?

B31.3 actually calls it Flexibility Analysis in 319.4:

319.4 Flexibility Analysis
319.4.1 Formal Analysis Not Required. No formal
analysis of adequate flexibility is required for a piping
system that
(a) duplicates, or replaces without significant change, a
system operating with a successful service record
(b) can readily be judged adequate by comparison with
previously analyzed systems
(c) is of uniform size, has no more than two points of
fixation, no intermediate restraints, and falls within the
limitations of empirical eq. (16)⁸

 

[RVAmeche]

I'm not sure why B31.3 still calls it flexibility but I guess it's easier to keep it the same instead of changing it.

There are definitely rules of thumb and various sources that provide guidance on when to perform a full analysis based on pipe size, material, pressure/temperature but I don't think any of them are definitive.

Technically B31.3 implies a full analysis is almost always required unless you're exactly copying an existing system. Engineering judgement comes into play where some small/borderline systems such as a 1" steam pipe may not be analyzed. Additionally, in the ASME B31.3 piping course our instructor recommended we essentially ignore Equation 16 since it's not necessarily conservative and no proof of it's utility exists.

 

[cardin]

Thank you I aggree with you, but I was a bit unsure.

319.4.1 Formal Analysis Not Required. No formal
analysis of adequate flexibility is required for a piping
system that
(a) duplicates, or replaces without significant change,
a system operating with a successful service record
(b) can readily be judged adequate by comparison
with previously analyzed systems
(c) is of uniform size, has no more than two points
of fixation, no intermediate restraints, and falls within
the limitations of empirical eq. (16)8

Point (a) implies that a system that is replaced by a system of the same design and materials doesn't need a analysis since it has shown it is OK.
Point (b) Means that is you made a system with more or less the same specs (materials, Pressure, temperature) you are good to go without further review.
Point (c) A small line (ex. spools)do not need a extra analysis since these are not "complicated" enough to need special consideration by analysis.

I understand that most of the designers would choose to analyse, but now i actualy have a plant owner who says that they are replacing a 500m line with another line.

They don't need a new verification since the line will only be changed at some locations. They are now placinge the line besides another line so they have an example. They changed from 316 to Duplex so "better material" and they compared the new material to the older material. In short they just point to the standards point A and B.

 

[XL83NL]

Better in which way? Thats such a bull^%&% argument.
- Duplex isnt good for anything > 250°C.
- The coeffiicent for thermal expansion is different to 316.
- ...

 

[RVAmeche]

Unless the Duplex has the same expansion rates/weight/allowable stress I don't think that's a valid argument.

 

[GD2]

Cardin,
Problem: The Owner wants to replace an existing 316 line with Duplex St. Steel ( I would guess for better corrosion resistance) and donot want to go for a formal stress analysis.

I don't see any issue with it (no stress analysis required) for following reasons:

[ul]
[li]Duplex steel is stiffer that 316 - higher Yield and tensile strengths. Hence basic allowable stresses are higher than 316. Allowable Stress Range value S_A (B31.3) for Displacement /Expansion Stress Range will be higher than 316.[/li]
[li]Thermal expansion / Coefficient of linear expansion less than 316.[/li]
[li]Weight of the pipe material very comparable to 316.[/li]
[/ul]

I also don't see any issue with the sustaining and supports loads with the difference in pipe weights. The supports also have a factor of safety usually from 2.5 -3. In fact,you might even see a thinner wall duplex pipes due to higher allowable stresses.

GDD
Canada

 

[KevinNZ]

•Duplex steel is stiffer ? you mean stronger?

If is stiffer then thermal stress will be higher

 

[curtis2004]

Hi Cardin,

I think, the owner wants to save money, which is OK. However, before moving forward they need to consider the following questions:

1. What is overall cost of installation of 500 meter duplex steel at that plant?
2. What is overall cost of engineering and project management portion allocated for the project?
3. What is cost of failure this new line, including rework, loss of production, etc.?
4. Was original line properly designed and installed? Do they have all engineering documentation of existing line at hand. Sometimes, but not always, looking at existing supports can tell you a lot about quality of job done previously.

Thanks,
Curtis