Piping Stress Analysis 12

[ChrisProcess]

Hello, first time poster with a query regarding piping stress analysis.

At the moment I work in plant design Process Engineer. Before issuing drawings we send selected lines out for Stress Analysis (our somewhat simplified criteria for Stress Analysis is D>2", dT>100 Deg C). It is now desired to do this analysis in-house.

I've taken it upon myself to research this.
I've assembled numerous guides (including the CASTI guidebook to ASME B31.3 & Process Piping the Complete Guide by Charles Becht).

I know that Caesar, Autopipe and others are used, but for various reasons they don't want to go this route. Instead they want to establish either a guideline or program that will cover the stress analysis.

We can characterise out piping networks quite easily, in terms of fittings, equipments dimensions, operating conditions etc. via our database system and 3d model.

So what I'm really looking for is somebody that works at this day to day, to give some pointers. I've spoken to my former mechanical engineering lecturer who thinks developing it from the ground up is a bad idea (i.e. go the Caesar route). I would like to get some other opinions on this. If it really is a bad idea, its best to find out at this early stage.

From reading the guides, a lot of it seems pretty vague, or at least up to the designers dicretion.
I would like to know how those working at it proceed and if to develop our own properitary procedures/software is feasible.

I'm assuming for all this that ASME B31.3 is the main guideline to consider regarding Stress Analysis for Process Piping.

Thanks for getting this far!

 

[ChrisProcess]

Hi KernOily, well as I say at the moment I am purely researching the whole topic, so if I'm underestimating the scope then now is a good time to find out.

The overall impression I'm getting is that software is not the catch all solution.

When we get the returned stress reports from the Contractor, done using Caesar II, we get a list of stresses, moments and strains. These are then compared against values which the report says are defined by ASME B31.3.

If we were to consider the job of stress analyst and piping designer as 2 seperate roles (is this a dangerous assumption?). Can the stress analyst not input the designed model and operating conditions and have the program workout the various forces etc.
I am assuming that when Caesar says it has various codes built in, it is referring to acceptable limits and such as defined by the codes.
I assumed perhaps that it would then state if the system stresses were within these acceptable limits.

If anyone could post a sample Caesatr output i'd be very interested to see what kind of things need to be considered.


I don't have the stress report to hand, but I can post the details from it next monday if anybody is interested.

 

[JohnBreen]

..............another good book that addresses the "design" (used to be call drafting) aspect:

http://www.amazon.com/Pipe-Drafting.../ref=pd_sim_b_njs_title_1/002-9394735-4414412

Some of the problem in deciding who does what is caused by the more recent "titles" that are in use (as opposed to "job descriptions"). What is a "designer"?

The people who do the piping layout work and figure out where the supports will be placed (based on where the supporting structure will be) and do the material take-offs etc. are NOW called "designers". Not many years ago they were called "draftsmen" and then (unisex) "drafters". These same people were charged with doing table look-ups for sizing hanger rod thicknesses and since hangers are "load rated" that lead to determining all the rest of the hanger assembly component "sizing". The graduate engineer was charged with picking the spring hanger units (cans) based upon weight and thermal expansion movements and for doing any required calculations. Of course these lines were blurred when very experienced "draftsmen" were involved.

It MUST be recognized that the design of piping is based upon the Codes and Standards required by the jurisdiction in which the piping system will be installed. The ASME B31.3 Code has been mentioned here. The ASME B31.3 Code is largely based upon beam theory and the person responsible for the "design" of piping (the person responsible for the calculations be they done by hand or by computer) MUST TRULY UNDERSTAND the Codes and must equally understand the associated beam theory calculations. Without that background, the "design engineer" cannot truly be responsible and it would not be prudent to assign such responsibility to a person who did not have the appropriate background. Of course the Code allows the responsible engineer to apply more rigorous methodologies (more rigorous that beam theory, which in some cases, for some components will not predict the highest stresses due to loadings) when it is appropriate. I personally believe that if it is beam theory software (typical for piping analysis) or if it is more general finite element analysis software there MUST be a mechanical engineer in the line of responsibility. I have seen some excellent "designers" who are very capable of developing computer drawings and are equally capable of developing accurate beam element computer piping models. The software will routinely run through the calculations but then there must be an engineer to interpret the calculated results and to back-check the model and the loadings. This engineer SHOULD be charged with the responsibility of REVIEWING every aspect of the design calculations and signing off on its credibility. Anything less than that would be cheating the client.

Over at the client's office, there SHOULD be someone who is charged with the responsibility of REVIEWING every aspect of the design calculations and signing off on the credibility of the resulting design.

Some of the "more routine" decisions regarding when must a piping system undergo "formal" analysis then made by the simplistic "rules of thumb" that were written into company standards. These standards cannot be blindly applied and the limits of these "rules of thumb" should also be part of the company standards.

Regards, John

 

[richay]

ChrisProcess - CAESAR II pricing can be found here:

[URL unfurl="true"]http://www.coade.com/PricingPages/CAESARII_2008_PriceList.pdf[/url]

Richard Ay
COADE, Inc.

 

[DSB123]

ChrisProcess,
I know PED and ASME B31.3 have different implications BUT if the PED is applicable then the analysis must be consistent with the requirements of the PED. ASME B31.3 does not implicitly address the PED requirements.
Also you say that the lines you intend to analyse will be installed on site and are exempt from the PED. I think you need to confirm this with your appointed NoBo. Generally the entire plant needs to be given a certificate of conformity by the NoBo. Individual Vendor packages are/or should be supplied with a Certificate of Conformity from the Vendor stating that the unit meets the PED. However the pipework between individual vendor equipment (even if installed on site)needs to meet the requirements of the PED unless the pipework is Designed/procured/fabricated and installed by the user. If the user sub-contracts the fabrication then the fabrication is not supervised by the user and the PED applies. I have argued with a NoBo over this point and stated that the PSSR regulations apply but their stance was that if the "user" does not "supervise" then the PED applies to on-site installation of pipework.

 

[ChrisProcess]

hi DSB123,

Thanks for that pointer about the PED. To be honest, i didn't even consider PED for stress analysis. We always considered it a vendor responsibility. But, as you say, if we are in effect the Vendor for the piping, then we may be responsible. I'll have a look over this point.
I'm guessing it's a grey area - if you were arguing with your NoBo about it.

 

[DSB123]

ChrisProcess,
Yes it is a grey area but for piece of mind I would talk to your designated NoBo. Also the materials must be PED compliant so if you are intending to use ASME materials then you need to get PMA's prepared and approved by the NoBo. Risk assessments for the systems will be required to show how the piping system meets the PED. If you are doing the piping design and getting a sub-Contractor to install then you need to get a Module B Cerificate for the design and give that to the Sub-Contractor so he can show the NoBo. Also the Sub-Contractor should be certified under the PED by a Nobo to perform the work.
I have gone through all this for what should have been user installed pipework on a large industrial site. The NoBo came in to review the Stress Analysis and the design documents including pipe specs etc and we had to show how we had addressed all parts of the PED. When he was comfortable with the design we received a Module B Certificate to give to the Fabricator who was also certified to Module H for the construction.

 

[JohnBreen]

Hello DSB123,

May I ask you to do all of us North American piping engineers a favor?

Would you, for our edification, explain the PED requirements that you guys work with. It would be very useful for us to understand Module B and Module H and NoBo and some of the rest of the terminologythat you used above. We thank you very much for the education.

Best regards, John.

 

[ChrisProcess]

DSB123,

I'm looking over the PED implications for Stress Analysis this morning. When you say that the user "Supervises" the installation, what would be your definition of this? Are recorded inspections etc. required as part of this?

 

[europipe]

John, here is the info site PED

 

[europipe]

John, For calculation and category
Click on the grey rectangular picture left of:
"Klik op de afbeelding hiernaast om het programma te downloaden."
When You run the program, You can choose the english version

 

[DSB123]

ChrisProcess,
What our NoBo said that for a "User" installation (therefore exempt for PED) it must be seen that the "User" supervises all activities Design, Fabrication,Installation, Testing and NDT and must ensure these activities are in accordance with the relevant Codes.

 

[ChrisProcess]

DSB123,

We would be operating on behalf of the user. In essence we would taking the place of the user for this supervision. All those items you mentioned would be covered by our piping team during the installation. I assume then that the Client Project manager would have final sign off on all documents generated.

Essentially we carry out all the work and the client ("User") approves it. I would assume that this would exempt the Site Installed piping from PED.

It's an interesting point though, and one that can probably be interpreted a few ways.

 

[ChrisProcess]

I should have mentioned above that the Mechanical Contractor would carry out the actual work (NDT etc..) and that we would inspect the work and approve the documentation, which the client would then have final approval of.

 

[UtilityLouie]

Just a quick note on transferring data via pcf file:

We've tried it and it has never worked really well. It is not a seamless transition to say the least. We found that we needed to at least cross check our iso's with the model that was imported. This too can be time consuming. If there are any strange fittings, they can throw off the import.

HOWEVER, we do not use AutoPLANT and thus have never tried an AutoPLANT to AutoPIPE transfer. When you speak with Bentley, they are pretty confident that it is pretty easy to do. I wouldn't swallow that one whole, but it may be partially true.

Some of the issues that I've seen throw of the import process is any customization that you've done to your piping database in your model program. The coversion process doesn't seem to hold up well with customization.

Getting an engineering estimate time to enter/analyze pipe models is a tough nut to crack. My thoughts are that it depends on engineering experience with stress analysis. On a large job --- 100,000' of pipe --- we spent an average of .1 hours per ft of pipe, but a lot of this pipe was rack pipe with similar supporting techniques. I wouldn't even use this .1 hours per foot as a starting number. There were complex pipe arrangements that required well over an hour per foot of pipe. I'd guess .5 hours per foot as a starting point.

The biggest thing for management to realize --- because they hear the "sales pitch" of perfect importing of lines into any program --- is that it is not perfect. We tried importing pcf files into Caesar II, Triflex AND AutoPIPE. None of the imports were satisfactory and we decided to hand enter all iso's to make sure they were correct. The way we lessened cost was to have a pipe designer enter the pipe into the program and the engineer would analyze the pipe. This way we felt we were getting our designers started on the "road" to learning stress analysis. This also helped because then the designers would take the hand marked iso's and make the changes designated by the engineer. The designer was on the front and back end of the stress analysis.

 

[ChrisProcess]

Thanks for that UtilityLouie, its good to have ballpark figures that can translate into ballpark costs (& potential savings)

The .pcf import did sound too good to be true, I imagine hand checking the model is the only way to be certain.

I finally got the B31.3 guide today, so I'll be absorbing that over the next few weeks. In the meantime if anyone has anymore thoughts/suggestions please let me know. Thanks.

 

[jistre]

You're right on the money regarding the pcf/pxf import features.

It certainly imported whatever was in the file quickly and accurately, but that just means you can get garbage into your analysis model that much more quickly. When an incorrect analysis puts my career and reputation on the line, there is no way I'm trusting an analysis model built off of an import file generated from the designer's 3D model.

The designer has to model in a specific way to be able to trust that correct and complete information will be imported into the file, and he just may not know how to do that or be set in his ways and refuse to do it. The databases on both ends that first generate the pcf/pxf file from the main model and then translate the pcf/pxf file into an analysis model might be flawed and errors in translation may occur.

There are just too many things between there and here that can go wrong, and I'd end up hand checking the models against the isos (which are controlled documents here). Building the model from the isos was quicker and easier and gave me an extra layer of sanity checking as I could often spot mistakes on the isos that would have gone unnoticed had I just imported the model.

If a foul-up in analysis means a steam line ruptures and kills someone, I'll be darned if that foul-up is because I blindly trusted a multi-stage process to work correctly. Your process might be set up differently, but the moment you can't trust the input to the analysis, you've lost any benefit the import process has gotten you.

That's my two cents.

 

[KernOily]

We've had good success importing Cadworx isos into CAESAR At first we had some pretty bad teething problems. During the import, CAESAR would create nodes everywhere in the iso, which wreaked havoc during the stress analysis. But we worked through it and now we do all our isos in Cadworx for direct import into CAESAR. Saves a lot of time.

 

[DSB123]

ChrisProcess,
"Essentially we carry out all the work and the client ("User") approves it. I would assume that this would exempt the Site Installed piping from PED".

From this response you are acting as a Contractor not the "User". Therefore PED applies to the site installed piping.

We had the situation that the "User" was another branch of the Company I worked for but still the NoBo said we were acting similar to a Contractor as we were not the "user" in the sense of the PED. Therefore we had to furnish a CE mark to the actual "user" for the plant. (Ie global CE mark for the site installed piping)

 

[ChrisProcess]

DSB123,

It's an interesting one alright.
I assumed that we are checking the mechanical contractor's(not us) work, on behalf of the User. The user would then approve and sign off our documentation (i.e. approving our supervision of the contractor). I would have thought that this may constitute "User Supervision".

I'll be interested to see the responses I get when I bring that point up here, I must read up the PED guides first though.

 

[DSB123]

ChrisProcess,
Also remember that plant modifications(unless under the supervision of the "user") could fall under the PED if they are "significant". However to get a NoBo to agree on what a "significant" modification constitutes is difficult. Best thing is to discuss with the NoBo before you assume that it is an insignificant modification and then find out you needed NoBo involvement. Difficult to do this retrospectively (especially fabication requirements)