#26825 - 04/21/09 09:39 PM
Re: WNC load case
[Re: Aslam Mohammed]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/24/09
Posts: 173
Loc: India
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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]
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Member
Registered: 02/10/09
Posts: 47
Loc: Japan
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Dear Habib,
Surely,it was not directed on you or to any of the forum members.
Regards, Aslam
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#26832 - 04/21/09 11:16 PM
Re: WNC load case
[Re: Shiny Mathew]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 10/09/08
Posts: 12
Loc: Haryana ,India
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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]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/10/09
Posts: 47
Loc: Japan
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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]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/10/09
Posts: 47
Loc: Japan
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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]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/10/09
Posts: 47
Loc: Japan
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Hi habib,
The most sought after excuse to kill the sprit of learning.
Regards, Aslam
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#26880 - 04/22/09 11:13 PM
Re: WNC load case
[Re: machoguy]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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]
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Member
Registered: 02/10/09
Posts: 47
Loc: Japan
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Hi Machoguy,
Habib is correct.
Regards, Aslam
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#26887 - 04/23/09 06:09 AM
Re: WNC load case
[Re: Shiny Mathew]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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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#26935 - 04/25/09 07:21 AM
Re: WNC load case
[Re: Shiny Mathew]
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Member
Registered: 02/16/07
Posts: 508
Loc: Singapore
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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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