×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Nozzle loadings exceeding exchanger inlet nozzle
6

Nozzle loadings exceeding exchanger inlet nozzle

Nozzle loadings exceeding exchanger inlet nozzle

(OP)
Dear

My nozzle loadings are exceeding for exchanger inlet nozzle (drawings attached)

My piping parameters are as follows:

1. Piping material is SA 335 Gr 11 Cl 2 & dia 18 inches
2. Operating Temperature = 350 deg.C
3. Design Pressure = 30 kg/cm2g4.
4. Flange rating is 300 lb (Weld Neck)

Overall flexibility/stress ratio is 0.90 as reported in the software output file.

Can any piping expert or an experienced person tell me how to reduce the stresses on a piping system whose stress ratio are still within the limits but nozzle loading are exceeding.

My idea if i go for higher strength material or high rating flange.

Thanks
  

Osama Nusrat Ali

RE: Nozzle loadings exceeding exchanger inlet nozzle

You have two options,

1) additional piping flexibility through loops aditional lenght and the judicious use of supports.

2) Have the vessel nozzle handle more load, this is a simple request to the vessle manufacturer and will cost more money.

It's hard for me to tell from you attached Iso so I need to ask how are you modeling the vessle nozzle the piping is attached to? Are you taking into acount the vessles growths? Are you modeling the nozzle to vessle connection as a rigid anchor at the nozzle flange (with thermal ancor displacements acounted for)? Are you modeling it like a pipe tee (this will gove some flexibility and acount for vessle thermal growths)? Are you using the vessles flexibility to your advantage, using the blijaard, ASME flexibilty method, or vessle vendor provided flexibilities at the vessle shell to nozzle interface?

Just my two cents worth

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!  

http://www.ap-dynamics.ab.ca/

RE: Nozzle loadings exceeding exchanger inlet nozzle

Its no wonder that its exceeding allowables.  How do you expect to handle any expansion at all with what is a configuration composed of nothing but fitting to fitting make up?

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Nozzle loadings exceeding exchanger inlet nozzle

MQM90,
       18in Dia pipe is relatively stiff and you would need to generate significant forces to overstress the pipework. With a stress ratio of 0.9 the the internal loadings within the piping must be significant. Generally with larger sized piping it's not the pipe stress that is the limiting factor for system acceptability it is the loads generated onto equipment as you have discovered. Not having looked at your configuration I think you need to consider the recommendations given above. The best way forward is to increase the inherent flexibility of your "stiff" system.

RE: Nozzle loadings exceeding exchanger inlet nozzle

2
Build your model through the connected equipment, including modeling the exchanger shell down to its supports and incorporating whether they are fixed or sliding.  The exchanger nozzle (E-311) will want to move upwards and that should help offset the nozzle loads.  The nozzle oriented horizontally (R-311) will want to move towards E-311, which will create some issues, but if the support nearest E-311 is sliding that might help quite a bit.  Specify both nozzles as such in CAESAR II (or whatever software) and incorporate the nozzle stiffnesses generated.

I high-rating flange may not be the answer, as much as maybe a repad or gusseted nozzle might be.  It might also help if you can move the reducer to the E-311 nozzle to take advantage of more smaller diameter piping in the bends.

I agree that the piping looks pretty stiff, being fitting to fitting, but I have seen tight piping in exchanger banks before.  On first inspection, this didn't look too bad to me.

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Snorgy,  You have inadvertantly redefined the term, "It didn't look too bad".   

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Nozzle loadings exceeding exchanger inlet nozzle

It's like anything else, I guess.  You see if it works; then if it doesn't, you see how close it is to working; then you fiddle with what you can fiddle with until it works.  What I saw in the sketch - if the sketch shows the extent of what has been modeled - is an incomplete model that may be painting a worse picture than what might be the case in reality if one expands the analysis a bit.

If after all that there is still a problem, then one can go to the next level towards fixing it.

One would have a better understanding of space and clearance constraints if one was sitting at the CAD station looking at the 3-D model.  Presumably, the piping was laid out as shown with a view towards available constraints.
 

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

MQM90 (Mechanical)


You may want to run an additonal check using
PCL Gold

Level 3 FEA: PCL-Gold The First Pipe Sttess Tool to Automaticall Run FEA   

RE: Nozzle loadings exceeding exchanger inlet nozzle

Then again, maybe he should just address the lack of flexibility.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Nozzle loadings exceeding exchanger inlet nozzle

I'm with BigInch - just address the lack of inherent flexibility of the system - no need to go to PCL Gold.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Fair enough.

But, the way to address the lack of flexibility is to completely and accurately assess what you have, quantify how far away that is from what you need, and take the required steps to fix it.

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Agreed, but its for sure you can solve the problem faster, if you don't start with 350C and F/F.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Nozzle loadings exceeding exchanger inlet nozzle

(OP)
ColonelSanders83.

"Additional piping flexibility through loops or additional length" is not possible as per field constraints (ohh). However, i have taken equipment nozzle as an anchor with its thermal displacements.

Model is attached.

BigInch,
 
You are right with a configuration composed of nothing but fitting to fitting, the nozzle load may exceeds its allowables.

DSB123,

Inherent flexibility cannot be increased due to the field constraints Sir.

SNORGY,

If i assume complete rigid anchor at R-311 and move the reducer to the E-311 nozzle to take advantage of more smaller diameter piping in the bends, the piping system stress ratio reduces to 0.89

WONDERFUL APPROACH BUT PLEASE CONFIRM TAKING R-311 anchor as complete rigid anchor i.e. without incorporating thermal growths and shifting reducer to E-311 side. SNORGY is this a correct approach or not?

 

Osama Nusrat Ali

RE: Nozzle loadings exceeding exchanger inlet nozzle

Personally, I would avoid modeling the endpoints as rigid anchors for a variety of reasons that could be justified even though it might not constitute good practice within the boundaries of comfort for some (whose opinions and judgement should be paid due respect).

(1) There will be some flexibility at the nozzles at both ends.  I would model the system through the equipment as I mentioned earlier to account for these flexibilities, assuming that the software has the capability to generate the nozzle flexibilities (or stiffnesses) for you.
(2) There will be small lateral and axial displacements between flange faces, as well as some gasket crush or gasket compression.  Again, not much, but enough to avoid having to model perfectly rigid anchors.
(3) You could try to see if you could find some data for axial, torsional and lateral nozzle stiffnesses - or calculate them (WRC-107, WRC-297)? Then you could model the endpoints using suitable restraint stiffnesses.

Be aware that if you try any of these things, you are really taking away anywhere between "some" and "all" conservatism in your design, and that your results will only be as accurate as your assumptions.  I would only take these approaches if I was "close to passing", i.e., within about 25% or so of allowables.  Even at that, you are obliged to clearly specify the assumptions that you are forced to make - given your layout constraints - in order to arrive at a model that "passes".  You wouldn't normally want to go about things in this way.

Is there a process justification for not moving the reducer to take advantage of smaller pipe?  If that is a deal-breaker from the point of view of hydraulics (increased pressure drop), for example, then my opinion is - given the sizes involved - you have other design issues besides stress and nozzle loads.

There are some very smart people here who have expressed their concern that the flexibility appears inadequate to them, based on their first inspection and drawing upon their experience.  Even if you do proceed with cutting things a bit finer in your modeling, pay due regard to the advice rendered by those folks.  The *best* answer might be just as they suggest: increase flexibility by re-routing the pipe.  That might give some bosses and Project Managers / Engineers some heartburn, but detailed design people and stress engineers often have to be the bearers of bad news and, while they can make an honest effort to achieve compromises in their analyses, they ultimately cannot cave to project-driven pressure if something just isn't right.   

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Myself, I usually require that we start by modeling nozzles as fixed and then if we can't reasonably meet nozzle load requirements with this conservative assumption, I relent and allow nozzle flexibility to be considered.

I would caution that you have a solid sound technical basis for determining the degree of flexibility assigned to the nozzle.  I had a very bad situation on one project where an EPC firm arbitrarily assigned a very high degree of nozzle flexibility to a injection compressor discharge nozzle in order to get themselves out of a bind in the stress study.  It was not until the system was built that I became aware of this - at that point, I noted that there was in fact no discharge nozzle - the mating flange facing was machined into the compressor body (designed and built for 7200 psi) - and in fact the assumption of nozzle flexibility was completely invalid ...

RE: Nozzle loadings exceeding exchanger inlet nozzle

(OP)
Hi All,

Actually inherent piping flexibility or additional length is not possible as per our field constraints.

I am finally going with the reducer shifted to E-311 side and converting the remaining pipe to 16 inches. And incorporating thermal growths (as per above recommendations) at both ends can atleast brings stress  low i.e. 0.85 than the original 0.9.

This way i understand i am not taking care of nozzle flexibilities but in actual i think the system will be more flexibile and will not have any adverse effect on flange or nozzles.

Special thanks to SNORGY , BigInch and ColonelSanders83.

If i am still not very correct, please advice.

 

Osama Nusrat Ali

RE: Nozzle loadings exceeding exchanger inlet nozzle

I would have to see how "over-loaded" your nozzles were to make a judgement on the appropriateness of your path forward.  But, if you were "over - but close..." then your approach is one I would probably try.

Make sure your calculation assumptions are clearly documented.  You might want an internal independent check or peer review for QC purposes.  I often request one of my colleagues to cast an independent set of eyes on my results or use a different methodology to see if they come up with something close to what I came up with.

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

It reminds me of a modification associated with similar size nozzles on an HE.  Why do you want to change the pipe layout - we have 10 identical systems which have all worked for 30 years?  On a site visit, I noticed 10-15 fire extinguishers near each system.  I asked an operator why.  Oh, we have a flange fire every 3 months.  No problem - we just replace the gasket. We have done so for the last 30 years!
A realistic full system calc is a must but you also need to consider how realistic the theory is for short legs even if you did manage to get an acceptable answer.  In this case, I don't believe very much.  I am with BigInch on this one.

RE: Nozzle loadings exceeding exchanger inlet nozzle

4Pipes:

I don't disagree with either of you.

However...if a properly and accurately modeled piping / nozzle / equipment system gave rise to computed nozzle loads that met "pass / fail" acceptance criteria, would you ordinarily seek to further increase the piping flexibility in order to additionally reduce the loads on the flanges and gaskets and, if so, by what percentage of the loads imposed on the nozzle?
 

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

2
SNORGY, I think that performing "flange" calculations should be an intergral part of any decent pipe stress analysis.  (I favour the equivalent pressure method myself).  In many equipment, the flange loads may govern rather than the stresses in the equipment.

And I'm willing to bet a malt beverage that the leaking flange in the previous example was an 8NPS Class 150 B16.5 flange.

To the OP, if the stress doesn't work, it doesn't matter how much the project folks of the designer, etc don't like it.  It either gets fixed or it doesn't go in.  I've gone toe-to-toe with some of the most "important" project managers around.  As much as they yell and scream, they can't argue with the "math".  Only weak and impotent engineers back down in the face of baseless impositions by (often non-engineering) project managers.

RE: Nozzle loadings exceeding exchanger inlet nozzle

TGS4:

Thank you for that response.  I would give you two stars if I could.

In the industry and region where I work, there is always enormous pressure put on the "engineers" (the guys and gals who actually do the math and design stuff) by the "non-engineers" (the MBAs and - worse - "Project Engineers / Managers" and - even still worse - "Clients") to do as little engineering as possible and "finish" things as quickly and cheaply as possible.  More than occasionally, the "engineers" are threatened with their jobs or with being excluded from subsequent project involvement because they are perceived to be irritants rather than valued professionals if they are to take the time to do everything right.

That said, if flange calculations, as you suggest, were to be made part of all routine stress analyses, the sad truth is that most "clients" and absolutely no "managers" would be on board with the concept and they would most likely not pay for it.  With the prevailing budget, schedule, cost and "minimization of engineering" mentality, pretty much nothing would ever be done if it was to be done correctly.  It's sad, but it it's an honest encapsulation of the truth.

Honestly, I don't routinely check for flange opening / closing forces if the service otherwise has a low risk assessment rating due to the consequences of loss of containment.  One would hope that an engineering team's collective judgement would identify where such extra design effort is mandated, but, sadly, such is seldom the case.  We exist in a business where the stakeholders want fast, cheap answers at little to no engineering cost.

Hitherto, after 27 years, my judgement has served me well and I have not produced anything that has "failed" or "given rise to an incident".  I now check the things that I feel are important to check, and do the things that I feel I need to do, irrespective of whatever else I am told.  The "superiors" sometimes get anxious and impatient, but so far they haven't fired me.  Sometimes just handing them my calculator, pencil, and a pad of paper is enough to make them go away until I am "done".

I wouldn't go so far as to say "weak and impotent", although I am compassionate with respect to that sentiment.  I would, rather, suggest that engineers get beaten up to the point to where they begin to believe that maybe all of these things that they thought they were supposed to be doing really don't matter.  They are not impotent.  They are victims of dangerous propaganda from those who refuse to accept the value - or from their perspective, "cost" - of sound engineering.

I agree with you.  However, don't underestimate the power of "The Project Team" or "The Client".

Still...star.

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

SNORGY - well said.  I too have encountered the exact same thing.  The intimidation factor can be enormous.  (The way you describe it, we may have worked on the same project teams smile ).

Reminds me of the project where the client wanted a "leak-tight commissioning and start-up".  We determined the cost of controlled-bolting for every flange, with a specific procedure for each flange - and there were >6000 flanges.  The cost and schedule made him physically gag (good thing I was there to see the reaction!).  He then said something to the project manager about wanting a "leak tight start-up, but without all that extra cost those engineers dreamed up".  At least our project manager was a decent engineer and told him to basically "bugger off".  Pay me now or pay me later, but you WILL pay.

RE: Nozzle loadings exceeding exchanger inlet nozzle

"Please write down what part you don't want me to do." also works quite well.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

RE: Nozzle loadings exceeding exchanger inlet nozzle

MQM90 (Mechanical)

Question
What SIF are you using for the Elbows??

RE: Nozzle loadings exceeding exchanger inlet nozzle

Bravo...!!! SNORGY...!!!!

Describing engineering as an "irritant" is spot on...

Things are getting worse...not better.

Managers perceive engineers as only a necessary "sponge" for responsibility...and never as the only real key for project success.

Your phrase

"to do as little engineering as possible and "finish" things as quickly and cheaply as possible."

is probably written on the back of all MBA certificates awarded today.

-MJC

 

   

RE: Nozzle loadings exceeding exchanger inlet nozzle

(OP)
Thanks

To All

Nice discussions.

Take Care

Osama Nusrat Ali

RE: Nozzle loadings exceeding exchanger inlet nozzle

Snorgy,

..by what percentage.. If the system analysed was a reasonably practical application of the theoretical basis of the software then the answer is none.
However, if the theoretical basis of the software is not a practical application for the sytem analysed then the answer could be quite a lot.  In this OP, I would suspect that the theory is being stretched.  Maybe not to its limit but at least to suggest caution.

A factor with Caesar and every other software application is that the writers assume that the users are qualified. They do not include warnings about limitations of scope.  
Many years ago, I suggested to Coade that warnings about limitations might be useful - I was very quickly put back in my place.  Coade told me in no uncertain terms that the software is intended for users who know the limitations of the theory.  For example, you could apply a force to a simple cantilever that has a small bending stiffness at its fixed end.  Caesar will (or used to) not object even if the rotation of the cantilever goes past 90 deg. No over stress, no problem. You can just as easily come up with less silly examples that are equally beyond the validity of the software.     

RE: Nozzle loadings exceeding exchanger inlet nozzle

4pipes:

I don't disagree with you.  I just work in an environment with a different culture or approach towards "when to do" something and "when not to do" something.

The obviously correct answer from the pure engineer's perspective is to calculate everything in exact accordance with the theory.

The reality is that if you have run a CII analysis - for example - and determined that nozzle loads are within acceptable limits, then in most instances, you will not be afforded the man hours or the liberty to extend that analysis into more detailed calculations related to flange opening / closing / misalignment - or at least such activity will be strongly discouraged.  What people seek are quick guidelines: "...if you are within (x)% of *this*, then you don't do *that*...", or so appears to be the culture.

I never said it was right, I just said that that's how it is.

I would like nothing more than to force everyone to accept that EVERYTHING SHALL BE CALCULATED.  To accomplish this, though, will require nothing short of a rebellion of monumental proportions.  A good place to start would be to ban all MBA's from positions of authority in engineering companies.

Not that I am in any way bitter.

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Further to the above...

Another very sad truth is that, if an engineer undertakes to perform detailed calculations for everything, no matter how correct or elegantly performed, most "employers" that I know will get rid of that engineer at the very first available opportunity, using justification somewhat like:

"Great guy (girl), but made a big technical deal out of everything, slowed us down, hurt our projects, frustrated our clients, and really wasn't any good to us in the overall scheme of things...".

I've seen it too often.

I am not saying it's right...but that it is what it is.

The key is to somehow know when something needs to be flogged rigorously versus when it can be readily judged one way or the other.  I am not sure what teaches that other than experience.  And, when experience is used towards that end, it is usually supported by a guideline founded upon a rule of thumb or approximate premise similar to the one I described.  

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

Snorgy,
It's a funny old world.  I agree on all your comments but cannot agree that everything must be calculated.  It is becoming very much the norm to do lots of calcs.  However, oftentimes a comparison, common sense, a fag packet or a chart would be just as good and equally as accurate when appropriate.  How accurate is your input? How accurate is wind and friction?  A guess to 3 decimal places is still a guess.
I do sometimes have sympathy with project managers complaining about time taken by stress.  Doing multiple runs to get a "pass" by moving a support a couple of inches at a time is missing the point. Pipe stress is not watch making. This creates the situation with your great guy.  He/she does not do anybody any favours by crying wolf.  Real problems then get totally ingored.

  





   

RE: Nozzle loadings exceeding exchanger inlet nozzle

I agree.

I guess it depends on the definition of "calculation".

Referring to a chart or a nomograph (as is frequently done) becomes a calculation when it produces a quantifiable output as a function of a quantifiable input.  For example, visual inspection of a Moody diagram for "f" is just as good as a Newton Rhapson iteration of the Colebrook equation.  Also, a person with enough experience saying, "...f will be about 0.015..." might be just as good as either.  I have done either or all of the above, depending on how accurate an answer the task warranted.

What is *not* a calculation is s statement like, "I dunno...that's what they did on the last job...", or "..We just do it that way because that's what others do...", or "...That's the size that the Client wants...", or other such similar technical justification.

So, I guess it would be more accurate to state, "Everything must be given at least some semblance of coherent engineering thought."

As an aside...

We prefer to call them "cigarette boxes" in Canada.

 

Regards,

SNORGY.

RE: Nozzle loadings exceeding exchanger inlet nozzle

(OP)
Special Thanks to SNORGY & BigInch & 4Pipes

Osama Nusrat Ali

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources