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#74223 - 11/21/19 10:14 PM Manual Calculation
sandystress Offline
Member

Registered: 03/02/15
Posts: 2
Loc: Singapore
Hello,
I would like to clarify the following fundamentals. All these are basic and i have no hesitation that sometimes I stuck up even after working in CII for few years.

1. Is there any reference where I can find on how to arrive at the forces at different supports (of course depends on the pipe routing, process conditions etc) without the help of computer analysis. Any paper or examples from books would help. I often see some very senior guys who can appropriately (almost 90% of the time) finalize the supports or suggest the pipe routing by calculating the movements (which of course I can do as well) and force values.
2. How the support stiffness affect the calculation? And how to arrive at the stiffness value; F=Kx?
3. How does the seismic 'g' affect the stress calc? We often see providing additional restraint (guides, line stops) help in reducing the seismic stress values, but I would like to know how does those function? Same as wind...

Apologize for the long post.The intent is like many of us who started the career with software sometimes stuck with the first principles. Any help in this matter will be appreciated

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#74232 - 11/22/19 06:39 AM Re: Manual Calculation [Re: sandystress]
anubis512 Offline
Member

Registered: 08/24/18
Posts: 90
Loc: USA
There's a lot of information out there for manual sizing. See Kelloggs Design of Piping Systems and Peng Pipe Stress Engineering. Coade/Integraph/etc have released numerous white papers on various topics, usually written by Dave Diehl and they typically have examples. See the link below.

With regards to seismic/wind, one of the issues is that the force results in significant lateral deflection of the pipe. Think of vertical supports - we put supports every so often to ensure the pipe doesn't sag. Lateral supports can be overlooked, so if you have a strong lateral event the pipe will have severe deflection.

http://www.coade.com/Mechanical%20Engineering%20News%20Index.shtml
(Apparently this link is dead...does anyone know the new link thru hexagon?)


Edited by anubis512 (11/22/19 06:40 AM)

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#74235 - 11/24/19 03:52 AM Re: Manual Calculation [Re: sandystress]
Sigma Offline
Member

Registered: 10/22/06
Posts: 38
Loc:
The newsletters mentioned by anubis512 can be found at

https://cas.hexagonppm.com/resources/mechanical-engineering-news-index

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#74238 - 11/25/19 03:37 AM Re: Manual Calculation [Re: sandystress]
sandystress Offline
Member

Registered: 03/02/15
Posts: 2
Loc: Singapore
Thanks anubis512 and Sigma for your support. there are quiet a few useful documents in the link

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#74246 - 12/02/19 03:01 PM Re: Manual Calculation [Re: sandystress]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 829
Loc: Louisiana, US
1. I have no response to question 1.
2.a.
How does stiffness affect the calculation?

The result is typically both conservative and non-conservative, depending on your pipe routing, support layout, and estimated versus real world stiffnesses.

What do I mean by "both?" Imagine you have an anchor, "short" pipe, a bend, "short" pipe, and anchor. On both "pipe" elements, you place 0" gap guides of standard flexibility, and anchors of standard flexibility.

Because your pipe is quasi-infinitely flexible compared to the supports, your simulation shows the elbow collapsing more than reality.

Because your supports are quasi-infinitely rigid compared to the pipe, loads on guides and anchors will be higher than reality.

However, because your guides are infinitely rigid, they act as a mechanical fulcrum, and instead of a load in the positive direction, it shows load in the negative direction. It may be higher in magnitude. It may be lower in magnitude. Given enough iterations, you might even find a case where the location of the guide can fictitiously zero out the load in a given direction.

2.b.
How do I arrive at stiffness value F=kx?

If you modeled an anchored pipe and applied a load, F, laterally, to the end, it will displace x. You just calculated the spring factor for pipe.

A structural member isn't very different from pipe, in that way.

3. Lateral loads, whether from earthquake or wind, will result in large scale deflection in long, unguided pipe. In the absence of guides, friction will have a dampening effect on how far the large scale deflection will be, but CAESAR will easily outpace what you want your pipe to travel (and run into other things, fall off supports, fail, etc).

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#74249 - 12/03/19 04:57 AM Re: Manual Calculation [Re: sandystress]
mariog Offline
Member

Registered: 09/29/07
Posts: 692
Loc: Romania
About point 1, I would recommend you EN 13480‐3:2017, Annex Q (informative) with title Simplified pipe stress analysis.
It is not my favorite one (because is complicated in the good tradition of the European Codes), but at least is a documented way to understand how your "very senior guys" work. In fact, appart of the calculation made, they have a very good understanding of how works a particular piping system, or better said, how the system- flexible enough and well sustained- will work after "convenient" placing the restraints. Of course their experience and knowledge is hidden under the word "convenient". In brackets, when look to a formal stress analysis results, the first alarm bell would be when the results are against your intuitive expectations.

Annex Q includes a version of the "guided cantilever method". Needs practice to understand why is conservative for particular situations. Being conservative, it's a good tool for design but must be prudent when evaluate a system in terms of "pass"/ "no pass".

About placing restrictions for seismic case, a reference would be ASME B31Ea-2010 (Addenda to ASME B31E-2008). However for complex systems the "art" is to have proper restrictions for seismic (occasional) case that not restrict significantly the thermal displacements.

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