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#26825 - 04/21/09 09:39 PM Re: WNC load case [Re: Aslam Mohammed]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi Aslam
I guess you are not pointing me as inefficient successful engineer.
Just kidding……..

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#26826 - 04/21/09 09:52 PM Re: WNC load case [Re: shr]
RK Offline
Member

Registered: 02/24/09
Posts: 173
Loc: India
Hi Habib,
Agreed with the point what Habib whats to say in his privious reply.

Dear Aslam,
Yes we (stress Engineer) needs to know the mannual calcualtion before we are actually performing the stress analysis. it will always helpful. At the same time we must know what happens at site after we perform the analysis.
In spring hanger case I had seen on site that lot of maintainance engineer are facing problmes dealing with the springs.

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#26827 - 04/21/09 10:04 PM Re: WNC load case [Re: RK]
Aslam Mohammed Offline
Member

Registered: 02/10/09
Posts: 47
Loc: Japan
Dear Habib,

Surely,it was not directed on you or to any of the forum members.

Regards,
Aslam


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#26831 - 04/21/09 11:10 PM Re: WNC load case [Re: Aslam Mohammed]
Shiny Mathew Offline
Member

Registered: 01/12/09
Posts: 55
Loc: UAE
Original question is whether WNC case is required for nozzle checking or not. In my previous post where I was comparing the results of the two attached files, what I was pointing to was: With Sustained result and operating result the trend is already clear. In sustained case full weight of the content is present. At that condition already upward force is acting. Now it is obvious what will happen when all weight is removed or weight is partially removed. It will cause even more upward force. That means the WNC case for nozzle checking was not required if with proper routing and locating the spring, a suitable spring was selected. Why select a wrong spring which can overturn the vessel with higher spring load . Then check with WNC case to find out the load is more!!! That is, it could have forces downward in both sustained and operating cases in this example. So that the difference could have been reduced. So that the upward force could have reduced when the content is removed. So that when content is not there no unwanted upward force occure. Otherwise any number of load case can be created and checked as one wish. Any multiplier can be applied on load cases. But I prefer to reduce the load cases to the minimum and try to understand the result by comparing the basic simple load cases.

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#26832 - 04/21/09 11:16 PM Re: WNC load case [Re: Shiny Mathew]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi Shiny
Don't term "right spring" "wrong spring"
Spring we select as per our requirement to pass stress & pass sensitive nozzle load.

Regards
Habib

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#26833 - 04/21/09 11:20 PM Re: WNC load case [Re: shr]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
WNC a case may effect/may not effect your system having spring.
If you are working with gas or steam no issue.
If you are handling liquid situation may demand for additional WNC check.

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#26845 - 04/22/09 05:52 AM Re: WNC load case [Re: shr]
muthappan Offline
Member

Registered: 10/09/08
Posts: 12
Loc: Haryana ,India
Shr

How much percentage variation we can allow between the WNC case and the Sustain Case?
As per my knowledge , once the rod removed , stiffness of the Whole system will get disturbed and there is some increase in load . that new load is called actual cold load. there is some degree of variation between the two( this because of 12.5% of material increase per feet). By value , it should be between theoretical cold load and the operating load....

correct me if i wrong


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#26848 - 04/22/09 06:16 AM Re: WNC load case [Re: muthappan]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi muth
"once the rod removed , stiffness of the Whole system will get disturbed and there is some increase in load"

---There could be some increase in load or decrease in load when spring settle down accordingly spring position become slightly up of down.

Check sustain load & displacement in caesar, it will be slightly different than theoretical installed load.

Spring load variation maximum allowable as usual 25%.
But spring at WNC may affect surrounding support & nozzle seriously.
Nozzle load need to maintain as per normal standard allowable.

Regards

Habib


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#26863 - 04/22/09 09:03 AM Re: WNC load case [Re: shr]
Aslam Mohammed Offline
Member

Registered: 02/10/09
Posts: 47
Loc: Japan
Dear Muth

Could you please explain the below quoted statment.

"As per my knowledge , once the rod removed , stiffness of the Whole system will get disturbed"

I would expect some form of mathematical formulation and explanation involving a simple system say cantilever one end fixed (Nozzle) with vertical load at free end and spring support somewhere in middle of nozzle and free end.
I would like to see disturbance in the stiffness for this simple system first.

Dear habib your input are also required as you have also understood this point clearly.

The below point was also beyond my imagination....could you explain it further.
"( this because of 12.5% of material increase per feet)".

Regards,
Aslam

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#26872 - 04/22/09 08:12 PM Re: WNC load case [Re: Aslam Mohammed]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi Aslam
I can not explain the subject with differential equation form. I was never present at site when spring was fixed.
Still I can try to explain in layman style & the way I understand the fact.
1) How spring is fixed with pipe. We hold the pipe in pipe centerline elevation ( zero movement) then we attach the spring with it, gradually set spring for specified load. Then we remove the holding, release the spring, allow it to work.

2) Now say we have straight line node 10,node 20, node30. Let’s say node 10 & node 30 we put resting support, but middle one node 20 spring. If say when we put all resting support each one node take 5000N load.

3) Spring at piping act as intentional upward force which try to balance downward load (Pipe+fluid weight) acting at that point.

4) If we do not put support at node 20 at all, then there will be sagging & piping will form a bow shape. If say we set the spring with load 4800N at zero level then release. Then spring will not take all load still there will be small sagging. When we release spring to work it will slightly push down ( to settle down) Spring load at sustain will change , same time it will slightly change weight taken care at node 10 & node 30 , resting support.

5) If say we set spring with load 5200 N. Again similar things happen. Piping will try to settle down this time piping will form bow shape in upward direction. This time also spring load at sustain & sustain load at support node 10 & node 30 will disturb & re –distributed. That’s what we call as spring settle down.

6) If you check my test file since I overdesign spring it move up 0.575 mm in sustain to settle down. ( In that particular case I don’t like to call spring overdesign , it’s my conscious decision to set load at spring like this to achieve better nozzle load in sustain & operating case).

7) More I differ to set spring load (then pipe+fluid) there will be more settle down problem.

8) It is basic requirement to put pipe weight, fluid, valve weight accurately while designing a spring. I mention before over design of spring & under design of spring is equally bad. Some times wrong selection of spring may create more harm than good in sensitive piping system.

9) There is also a hidden accuracy load calculation problem. Piping allow 12.5% milling tolerance. That additional or deducted pipe weight could also effect our proper spring setting.

10) Once I use some spring in large bore line where valve was there close to spring but valve weight was not confirmed. We give tentative installed load with clear note that site people to set spring to suit at site for those particular springs.


Regards
Habib

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#26873 - 04/22/09 08:46 PM Re: WNC load case [Re: shr]
Aslam Mohammed Offline
Member

Registered: 02/10/09
Posts: 47
Loc: Japan
Dear Habib,

You took lot of pain for explanation but in vain.I have taken a simple mechanics problem for explanation of the statement quoted by MR. Muth and endorsed by you.

I am sure both of you can develope a relationship between different parameters to explain this point theoritically.

I am asking explanation from qualified engineers not from a laymen.So the explanation has to be in that way only.


Best Regards,
Aslam

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#26874 - 04/22/09 09:15 PM Re: WNC load case [Re: Aslam Mohammed]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi aslam

Yes I agree, sometime as a day to day working engineer my preference is to understand the practical problem well & learn the remedy for that, rather than breaking my head doing research on the issue.

I am sure there are people who is working on that & definitely like to learn in so called engineer's way.

If I remember correctly there were similar type of discussion before in this forum & some gentleman expert explain the things in similar fashion.
may be my explanation was not so good & accurate.

Regards

Habib

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#26876 - 04/22/09 10:11 PM Re: WNC load case [Re: shr]
Aslam Mohammed Offline
Member

Registered: 02/10/09
Posts: 47
Loc: Japan
Hi habib,

The most sought after excuse to kill the sprit of learning.

Regards,
Aslam

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#26879 - 04/22/09 10:47 PM Re: WNC load case [Re: Aslam Mohammed]
machoguy Offline
Member

Registered: 11/16/07
Posts: 25
Loc: Nigeria
Hi Aslam and Shr,

I have doubt with these theoretical installed load in hanger report and the sustained load in restraint summary. We are making data sheet for spring that we need to give to client for evaluation. There is one data for dead load that we must to fill in data sheet. Some said get these values in sustained load in restraint summary and some said it is the theoretical installed load in hanger report. I compare these two values, it is very close. May I know which one is better?

Another question is in hanger spread sheet. Is it necessary to remain check the allow short range springs? By pressing F1, there is explanation of these option so my understanding I dont need to check this option so that caesar will choose more standard and already available. I compare the results in hanger report then I found out these may obtain different results. Which is true?



Regards,

Machoguy

-----------=------------------------------------------
"Do not postpone until tomorrow what you can do today"


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#26880 - 04/22/09 11:13 PM Re: WNC load case [Re: machoguy]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi Machoguy

I think you can use theoretical installed load to fill data sheet.
Yes if you do not do the manual selection theoretical installed load & sustain load should be almost same.
How this two load become different, I tried my best to explain in my previous post. You can have a relook again.

For particular spring vendor generally supply four different stiffness spring.
Hard stiffness allow less travel, design also quite compact, required less space to place.
Softer spring have less stiffness value, allow more travel, required more space to accommodate it, sometimes more useful to manage nozzle load, But more problematic for dynamic analysis.

If you manually select, you can choose as per your requirement. If ask caesar to select better don't activate the box.


Regards

Habib

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#26883 - 04/23/09 02:32 AM Re: WNC load case [Re: shr]
Aslam Mohammed Offline
Member

Registered: 02/10/09
Posts: 47
Loc: Japan
Hi Machoguy,

Habib is correct.

Regards,
Aslam

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#26884 - 04/23/09 04:07 AM Re: WNC load case [Re: Aslam Mohammed]
Shiny Mathew Offline
Member

Registered: 01/12/09
Posts: 55
Loc: UAE
We avoid springs if possible in our projects. We can find in many cases that there are locations which will satisfy the requirements if rigid supports are placed at these locations instead of using springs. We need to find out the appropriate locations. These are only my own views. May be mistakes!!! If a spring is placed and which gives even higher loads then what is the use of that spring. This is what I meant in my previous response. You can call it wrong selection of spring rather!!! Or placing at the wrong location. In the file attached place the spring at node 40. Let Caesar2 design the spring. The result is different. Now the question may be of space availability etc. Or try with location node 55. In this case add a (+LIM) with 0 gap at node 60. Here also let Caesar2 design the spring. You get another result. So it is not a must to place the spring at a location where it can produce adverse effect.
Regarding the theoretical installed load and actual installed load refer User Guide page 7-18 (Hanger table with text). Refer my previous response above.

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#26887 - 04/23/09 06:09 AM Re: WNC load case [Re: Shiny Mathew]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi shiny
Yes you are right we should place spring in unavoidable cases only.
We can always ask caesar default to select spring but problem is that it can not solve all our problem related to sensitive nozzle load. Some time proper manual selection can produce much better effective result.
You are absolutely right to say we should not use/select spring that produce adverse effect.

Test file I made was to indicate that spring can produce adverse effect in one more case called WNC other than producing adverse effect in operating or sustain case.
I think that clarify the point.

Regards

Habib

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#26933 - 04/25/09 05:46 AM Re: WNC load case [Re: shr]
Shiny Mathew Offline
Member

Registered: 01/12/09
Posts: 55
Loc: UAE
This means, even before the line is in operation the nozzle was overloaded. How this pipe will be fitted to the nozzle to start with, with such high initial force existing. For erection as well as in case after maintenance. How the spring will be set for two loads namely operating load and as installed or wnc load


Edited by Shiny Mathew (04/25/09 05:56 AM)
Edit Reason: word wnc added

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#26935 - 04/25/09 07:21 AM Re: WNC load case [Re: Shiny Mathew]
shr Offline
Member

Registered: 02/16/07
Posts: 508
Loc: Singapore
Hi Shiny
At the time of erection/maintenance spring is kept under locked position.
So do the produce upward load.
In running plant when particular equipment is in standby position can take slightly more load. So nozzle load at wnc could be slightly more than allowable.

We should try to design the system in such a way so that it is safe in operating/sustain condition as well do not produce very serious effect in the system at WNC condition.


Regards

Habib

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