Manual Calculation in Pipe Flexibility Analysis 2

[EngrPaper]

Hello all,

After reading through a lot of the responses to this thread about structural calculations (

https://www.eng-tips.com/viewthread.cfm?qid=451680)

it got my thinking more about manual calculations for pipe flexibility analysis.

On my end, flexibility analysis is not something that I do, but it is a direction that I want to learn and eventually work in. I know most firms use something like Caesar II or CAEPipe for their flexibility/stress analyses. But for those of you who do these calculations in your regular work, what place do hand calculations serve? Do you use them on the front end of routing pipe to minimize rework within Caesar or perhaps when in the field to locate critical locations in a piping system? Or it could be possible that no one does manual calculations for this anymore given the ease of use for computers.

Inquiring minds want to know.

 

[KevinNZ]

We use manual (spreadsheet) calculations for pipe span, but never do the manual calculations for other stress. We have a good feel for how big expansions loops need to be and go straight to the computer for stress analysis.

There are rules for low risk piping where computer for stress analysis is not required. But even then is often easier to use the computer anyway.

 

[XL83NL]

I dont do them. I know EN 13480 has an appendix detailing how to do a simplified (manual) pipe stress analysis. Ive always wanted to read and review it, but never did so. I guess from experience, I can tell if a first result is good or no go. Making a simple calculation in AutoPIPE is time effective that hand work cant beat that, timewise. Nevertheless, theory should be known at hand, I think.

 

[EngrPaper]

Thank you both for the responses. I've been reading through Design of Piping Systems by MW Kellogg, which lays out a lot of these manual calculations. So far, they seem very spreadsheet friendly.

KevinNZ, would those rules for low risk piping be primarily around design temp and pressure (for creep or bursting)?

 

[RVAmeche]

Oldschool flexibility calculations are sufficient for basic thermal growth calculations. If you're dealing with new ASME piping at high temperature, you'll likely need to do pipe stress analysis with the appropriate occassional loads (ice, wind, seismic, etc) as required by code.

 

[Riley15]

See attached document.
Attached are guidelines I use for ASME B31.3 piping (lines up with most of our client's standards as well) for a "visual analysis" meaning old school expansion hand calculations are acceptable for a combination of diameters & design temperatures. Once you get into a diameter & design temperature that is outside the shown boundaries then a formal analysis or comprehensive analysis is required, in other words, needs to be an actual pipe stress program like Caesar II. Hand calculations have their place for spot checking but as an avid Caesar II user I trust the software as I have been using it for a while now.

 

[XL83NL]

Ah, the got 'ol PIP chart. We use it as well, added with some further limitations we learned over the years. The graph is useful if you know it's limitations.

 

[EngrPaper]

Riley15, thanks for the chart. I'll keep it handy.

 

[KevinNZ]

EngrPaper

See B31.3 section 319.4.1 Formal Analysis Not Required.

 

[EngrPaper]

KevinNZ, got it. Thanks for the reference. So essentially, if the design is a replacement of a successful design, can be validated by a similar design, or falls within the confines of the provided equation (small pipe, short anchor distance, etc.) then flexibility analysis is not required.

 

[RVAmeche]

Note that the equation has a massive amount of fine print at the bottom essentially saying the results may not be conservative.

Also, even if you're copying an existing piping layout, some clients will want the stress analysis report for their documentation.

 

[XL83NL]

You might want to look into EN 13480-3 annex Q, which seems be a copy of AD2000 HP 100 R. See this set of 2016 MHD questions (page 23) from the EN 13480 committee.

 

[NovaStark]

Hello all,

But for those of you who do these calculations in your regular work, what place do hand calculations serve? Do you use them on the front end of routing pipe to minimize rework within Caesar or perhaps when in the field to locate critical locations in a piping system? Or it could be possible that no one does manual calculations for this anymore given the ease of use for computers.

Generally at my facility, we don't usually install new piping systems without some form of analysis. For small bore piping, we have set standards for support spacing based on overhang which we know to work well based on 20+ years of experience.

For alternations to piping, depending on the situation, hand calculation by breaking down a system via approximate methods are sometimes more cost effective than to model an entire piping system. For example, if installing a check valve on a water line in a pipe rack, we may not consider a C2 analysis and just opt for a hand calculation to show supports are okay. Now if we have a large change on a system with many springs+ high temperatures/long runs/complex layout, then a full stress analysis for sure. Hand calculations here are easy to make mistakes - especially when someone uses a spreadsheet and enters "10" instead of "1" or similar.