Are stress isos obsolete 1

[boilermaker1018]

This is in reference to the 2006 discussion under thread378-173268

I have searched old threads and I believe that father time may have changed some opinions on this matter.

My question first and foremost is how do you define stress iso? How is it different from a piping/fabrication iso? Secondly, does anybody use a stress iso as it was once defined? I know that Caesar and Autopipe generate "stress isos" but these are typically nothing more than piping isometrics with notes next to the supports. Does a true stress iso need to convey the thermal movement of the pipe and the stresses in the system?

The reason I ask is because the term stress iso gets thrown around from time to time, but I feel that it has lost its meaning. In my experience, it makes far more sense to share fabrication isos and the piping analysis files. Why would I want a stress iso, when someone could just give me a file with every load case built in? I can review and run all the system checks I desire.

I just wanted to gather the opinions of those on this forum for the best way to share stress information with customers, fellow engineers, record keeping, etc.

Thanks in advance

 

[BigInch]

Stress Iso: nothing more than piping isometrics with notes next to the supports.
From that you will develop the load cases and do the stress analysis.
Not the reverse.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[XL83NL]

BI's definition is one, mines is different.
Ask 10 people, and get 10 definitions.

 

[boilermaker1018]

I expected different opinions, but that's perfectly OK. Gathering opinions and then combining those into a final decision is often a good way to go about doing something.

 

[pennpiper]

I see properly defined Pipe Stress Isometrics not only as a valuable tool in the engineering and design of a facility but in the historical record of what was done. I have been involved in the investigation of suspected "bad design" (investigation proved it to be serious operational error) and three cases of refinery fire rebuild.

By "properly defined" I mean it has all the information (manually or electronically applied) to tell the whole story.

More and more, the "Bean-counters" with the engineering companies and in Client organizations have been cutting out things like this. Their justification is "its a waste of money". The real reason is it takes funds away from the executive bonus pool.

They do not understand the value of having documentation of the plant when and if there is a need to rebuild or repair after a serious accident.




prognosis: Lead or Lag

 

[kacarrol]

Also, a file is only good provided your software will read it. If you need to analyze a stress file from many years back, maybe the new version of the program won't read the old file. Or maybe your company has changed software providers. If you have a hard copy (stress iso) it's usually more flexible.

 

[StressGuy]

For ourselves, the "stress iso" is pretty much just an internal document now that we use to communicate to our piping design and civil/structural groups. It's a rare thing that we actually turn them over to a client. We retain them on our project server for our own history along with the other files that form our "calc file" for a particular review.

Now with everything being 3D modeled, the ISO's we start with are just the unchecked version of the file that gets extracted from the piping model.

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.

 

[BigInch]

Way back when, WHEN ENGINEERING COMPANIES ACTUALLY DID USEFUL THINGS, isos were really piping fabrication drawings that the contractors would later use to measure, cut and weld up the pipe. When we started actually doing stress analysis, isos were convenient for us pipe stress guys to use, as they already had pipe lengths, offset dimensions and wall thicknesses noted down on them, which were easily copied to the code sheets that the typists used to make the punch cards, after which we then collected and dropped off in the basement so the PDP-11 computer system operators could pick up, take into their vaulted computer chambers and drop into their card readers. We just had to add support type notes to the isos and give them to the structural engineers and the pipe drafters.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[XL83NL]

Apart from the "code sheets that the typists used to make the punch cards, after which ... drop into their card readers" we still do the other part. A welder and workshopforeman still need to have the drawing to make the actual spool pieces.
Stress iso's, regardless of how theyre defined and what should (not) be on it; we dont make them, except when clients ask for it, or the notified body requires it for PED compliance and design appraisal checks.

 

[DSB123]

pennpiper,
Totally agree that there are benefits in having proper documentation. I have, without success so far, tried to get the Company I work for to incorporate the correct and valid information into their stress reports which at the moment consist of:
1. Brief introduction
2. Plot of system
3. Reactions for load cases at anchors/supports.
4. Displacements at supports for different load cases.
5. Maximum "Code" stresses in system.

For me this is not sufficient. The branch types are not indicated. The maximum displacements are not provided such that other disciplines can account for the displacements. There is no information on where external loading has been applied (e.g. RV discharge loads). For me it is a document which provides insufficient information for a "third party" to review and as such is not worthy of production. I am from the "old School" where you have a stress isometric or even final fabrication isometrics which indicate the Node numbers and the types of restraints required. Also the maximum displacements are provided in the system for others to use. External loads are clearly indicated along with the load case to which they have been included. The "stress engineer" in charge does not grasp the full information transfer which need to be documented to ensure the "final" design is satisfactory. And as you say it should be such that if anything were to go wrong it would provide good evidence of the "Design".

 

[StressGuy]

DSB123,

While I personally don't see the value is cluttering the IFC ISO with stress comments (unless is is something that construction specifically needs to do, like implementing a cold spring or a specific guide gap), the stress group should retain sufficient documentation internally to defend itself if questions come up later.

Here, I add a stress approval stamp (and it looks like here in Texas I'll be PE stamping them now) to the IFC for the lines that we have flagged for stress review. It is insurance for the piping design group that I agree they have incorporated all our comments and the layout is substantially the same as what we analyzed. Whether the analysis was done by myself or one of my subordinates, I review the stress comments ISO that we sent to piping and compare it to the final.

How else would I be able to approve the final iso?

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.

 

[RoboCop16]

I am used to the stress analysis being documented by a stress report issued to the client. In addition to the stress isometrics serving internal communication purposes during the design, they are an essential part of the stress report - without them it would be nearly impossible to read the report.

The stress isometric should have node numbers and support configuration (rest, guide, stop, spring). It doesn't necessarily have support forces or piping displacements; these can be in reports generated from the stress program. The important thing is that the design office has an efficient and reliable method for informing support forces, large displacements and other outcomes of the stress analysis to the relevant disciplines - by mark-up on the stress isometric or otherwise.

For audibility and transparency, it is in my view preferable that the final stress isometrics are mark-ups of the approved and signed fabrication isometrics. That way it is easy to verify the stress model against the the actual information used in the piping fabrication and installation. A so called 'stress isometric' generated from the stress model doesn't allow that verification and is hence of less value in terms of audibility and quality control.

 

[DSB123]

Stressguy,
I thought my comments/thoughts were in line with yours!!! I agree with full documentation - it is the "nugget" at the Company I work for that does not and will not listen to sense. I also agree that all critical lines should bear a stress approval signature on the fabrication isometrics and have always insisted on this when I have been in control. At the moment and also in the last job for the offshore this was not done. Also in that Pipe Stress Engineer position we were instructed not to review the pipe support details - so long as we had specified the type of restraint and given the loads and deflections to be accommodated then that was the end of our responsibility. I argued against this but to no avail. Near the end of the job with pipe systems in place we discovered that many of the supports were not as defined by the stress engineer as the pipe support designers basically put in what they wanted. We had to reanalyse and re-issued loads and displacements based on the actual pipe supports. What a mess it was.