Dear all

When we have to consider the WNC load case. Is there any specific considerations to use this load case.
If we consider the WNC case do we need to activate the cold loads for spring hangers.

Hi.
The WNC is only a item, with this and others items you can construct a load case, for example WNC+T1, this case simulates the pipe without fluid content and temperature T1.

In mine and metals generally we do not have high operational temperatures for the pipes, but, the pipe diameters are bigger than others piping process, for this situation we evaluate the WNC+ T1 (the case that generate the higher expansion), where T1 is the max. temp. for the pipe considerating max. solar radiation and max. environmental temp., this load case generate high loads on restraints.

Also the pipe is evaluated in the full range of temp., this is:

L1:WNC+T1 (OPE) [max. temp. of the pipe without content]
L2:WNC+T2 (OPE) [min. temp. of the pipe without content]
L3:L1-L2 (EXP) [full range expansion code compliance check]

Regards.
JP.

I think WNC case is for to address spring upward load issue. Sometime spring support is used closer to equipment nozzle to minimize operating load at nozzle. When plant is shut down or line is not in operation with no fluid inside spring may produce unacceptable upward load on nozzle.

Yes, the "WNC" item is so that you can evaluate "empty pipe", without duplicating the model and zeroing the fluid density.

Dear shr,

I have never evaluated nozzel for WNC load case.Have you done so in any specific case?If so, please share your experience.

Dear Nikhil,

The most important use of WNC load case is for Nozzle Alingnment Check in case of Pump/Compressor/Turbine/Vessels

Following WNC cases are generally used in process and power piping.Moreover they are project specific and varies with projects and clients.

1)WNC+H (HANGER ACTIVE)This gives pipe and insulation weight considering hangers active on provided locations.

2)WNC+H (HANGER AS RIGID)This gives pipe and insulation weight considering rigid support at all hanger locations.

Pick highest values from above two cases.

3)W-WNC This gives process fluid weight

4)WW-WNC this gives water wight for hydrotesting

Informations from these load cases are used by Civil and Equipment(If pipe supported from equipment)for structure design.

I hope this will be of some use.

Regards,
Aslam

Hi Aslam
WNC may have significant effect if there is spring in model otherwise I guess will not have much effect on piping stress.

For example if there is top pump discharge line ( long vertical length) & we introduce spring to reduce vertical operating/sustain weight on pump nozzle.
Now when that particular pump is in standby condition or plant is in shutdown condition (no fluid in vertical leg of piping) may produce unacceptable upward load at nozzle because spring may still be in active condition.

To accommodate tank settlement if spring is used WNC may again have some serious effect That's why sometime bellow is considered as preferred solution for settlement issue.

Regards

Habib

Dear Habib,

I agree with your general explanation but I asked you.....

1)Have you ever evaluated nozzle for WNC load case.If yes then please through some light.

2)It is senseless to evaluate nozzle for WNC load case.

3)It does make a sense to evaluate nozzle in sustained load case.

4)WNC load cases as mensioned earliear are used for civil structure design.

5)WNC+H(Hanger stiffness set to rigid) used for Nozzle Alignment Check.

6)In case of top-top pump of large sizes (suction or dischage)the spring sizing is done by considering zero density in the vertical leg.The reasoning is that the weight of the liquid in vertical leg will be supported by pump impellar and not by the pump nozzle flange.Therefor it all depends on the good engineering judgement employed to size and design the system.

Regards,
Aslam

Hi Aslam
What you are telling about top pump discharge line, there is a quite good discussion on the issue in these forum.You can not exclude total fluid from vertical leg of discharge line you can only exclude Nozzle ID area times vertical leg volume of fluid.
Generally reducer is placed in pump nozzle. sometimes reduction of size is quite high.
The project I am doing now have line size 10 inch whereas pump nozzle size 3 inch.
Even some expert advice not to exclude any amout of fluid for spring design.
For me fluid exclusion at annular area quite logical.
Remaining fluid that affect nozzle load sometimes could be quite high.


I do not know where from you draw conclusion no need for sustain load check

***3)It does make a sense to evaluate nozzle in sustained load case?????.***

If you wish I can send you file where I can pass nozzle operating load by excess manually selected spring load input but that exceed moment in suatain case.

Example first support from nozzle take sustain load 1000N I take out resting support & put spring with cold load 4000N.

I work with some good company & excellant stress engineer who suggest to minimizse sustain load at nozzle whenever spring support is placed closed to nozzle since sustain load is a primary load.

If you do not like to consider WNC case it's your choice But if I am the person to take decision on project I must ask my people to include that case, My guide have teach me like this.
obviously I am open to new idea & ready to take out WNC case if there is solid technical justification for that, till then I like to consider it even if it look overdesign to someone.


Regards

Habib

Dear Habib,

I respact your views and understanding.

I think you have not read and understood my previous reply before writing.Is anything wrong with point number 3?


[[[["I do not know where from you draw conclusion no need for sustain load check"-I confirm, I have not said that and I can not say it.

***3)It does make a sense to evaluate nozzle in sustained load case?????.***]]]]


You have not answered first point.
Have ever evaluated nozzle for WNC load case?I am not asking for sustained load case.There is difference between WNC and sustained load case.


Regards,
Aslam

Hi Aslam
In case of WNC there also will be some load at nozzle. For me yes we need to check it.
Is your question whether that should have to qualify exactly with vendor/code allowable ?

That is quiye tricky one. Like say some people/company allow 50% more allowable load for pump when pump is in standby condition.

For WNC case also if calculated load exceed allowable with some small percentage I see no serious problem But it should not be very high.

I think it's better to know the exact situation. Since it,s not a operating/sustain case we may be quite flexible with the calculated figure.

Regards

Habib

Dear Habib,

Could you please suggest any singal instance where the system has designed such a way that nozzle loads exeeds vendor or code allowable in WNC load case and passes in Design/Normal Operation/Sustanied load cases.If I come across such a design I have to simply scrap it.

We consider following load cases for nozzle evaluation.

1)Primary loads(Sustained load)-This load is caused by the internal pressure,pipe weight,inside fluid and insulation weight.

2)Primary load + Secondary load(operation load)-This load ia combination of primary load and secondary load caused by thermal expansion.

Now make appropriate combine load case for nozzle evaluation.
We make various load cases to analyse a piping system and we are not supposed to evaluate nozzle for each and every load case.
Does we do it?The answer is no.

The general practice in all engineering companies I worked for is to evaluate nozzle in Design,Normal operation and Sustained load cases.

If your piping routed and supported in a way that nozzle loads are within allowable in design/normal operation and sustained load cases then loads on nozzle in WNC load case can never exeeds allowable loads.

As I mensioned earlier one of the most important use of WNC load case is to check nozzle alignment.

I think I need to put some light on it.In addition to the nozzle loads check,it is required to perform displacement check at mating flange face of nozzle when pipe is empty and disconnected with the equipment to ensure the balance of piping and supports.Especially piping connecting to rotating equipment shall be check carefully.This is simmilar to the field allignment check.There is procedure to do that in caesar II and it varies with equipment to equipments.The general creiteria is that displacement at nozzle should be less then 1 mm in all direction.

I hope this clarifies all.

Regards,
Aslam

Hi Aslam,

Appreciate your experience and patience in giving the answers.

Please elaborate on the difference between sustained and operating load cases. Why in sustained condition the term T is not appearing while all the other ingredients are present such as the fluid which is supposed to provide temperature also along with pressure. My doubt is while pressure exerted by fluid is present why temperature is not present. I want to understand these basics clearly.

Hi Aslam
Now I understand your question.
You are thinking WNC will aleays give less load to nozzle & supports since there is no fluid inside. Yes you are right in your openion, this is valid for normal rest supports.
I told before problem will aries if there is a spring in the system & It keep unlock ( working condition)

For example
Say in a system nozzle load 100 N first spring support 3000N second rest support 5000N so total weight = 8100 N ( pipe+fluid weight)

Now say fluid weight vanish only piping weight 5000N

Load distribution could be Nozzle 5N, First support 1095 N second support 3000N . If you are using all resting support it will be like this no problem.

If your first support is spring then still it will try to keep around 4000N, nozzle say +1500n( upward load) senond support 2500N.

That unwanted upward nozzle at WNC is a problem for us. You can make the file & check the result.

Because spring take define load & that load varies with upward & download movement only. If no vertical movement there is no load variation. whearas normal support take load as you put load in piping.

When you are slection spring ( select that manually by keeping free at nozzle node that option available in caesar) then you will understand the difference better.

Please try this & check if it is clear to you or I can send you file with further explanation.


Regards

Habib

Dear Mathew,

1)Both pressure and temperature are considered to be present simultanously only difference they are treated separatelly because they falls in two different catogory of loads.i.e.primary and secondory loads.

2)Primary loads are those with which you make equations of equilibrium for solving unknown.For secondory loads equilibrium law is not applicable because stress due to secondory loading(secondory stress)is self limiting

3)The stress due to pressure loading is considered as primary stress while the stress due to temperature loading is considered as secondary stress.

4)Code has develoved seperate equations for calculating and limiting sustained(pressure effect)and displacement stress range(secondary stress-temperature effect).

Hope, I answred to the satisfaction.

Regards,
Aslam

Dear Habib,

I do not want to discuss a hypothetical problem.We can have as much garbage as we wish.But,please share your self experiend practical problem while working for a reputed client where you have actually faced this situation.Then the issue will over and I will agree with you in totality with humility.

I am unable to convince you that WNC load case is not used for nozzle evaluation.

Any other gentllmen here,could you please help to address this issue more clearly.

I have started by responding to following reply

"I think WNC case is for to address spring upward load issue. Sometime spring support is used closer to equipment nozzle to minimize operating load at nozzle. When plant is shut down or line is not in operation with no fluid inside spring may produce unacceptable upward load on nozzle"

And I do not know what wrong with this...........

"I do not know where from you draw conclusion no need for sustain load check

***3)It does make a sense to evaluate nozzle in sustained load case?????.***"



Regards,
Aslam

Hi Aslam,
Thank you for your reply. Can I summarize it as follows:
Sustained condition can be considered as a variant of operating condition. Here the effect of temperature is not considered for calculation. This is not an installed condition where the pipe is empty and temperature is ambient. Sustained loads will not be affected by the input values of ambient temperature we provide using the special execution parameters or any other temperature.
This topic must have been discussed several times here. But every time I go through the various texts, I feel sustained condition is similar to installed condition-or similar to a condition just after the erection of piping at site and additionally fluid weight and pressure considered as a "margin". Also can I take the expansion case is the algebraic difference between the operating and sustained which gives the pure thermal effect alone.

For further confirmation.
Thanks.

Hi Mathew,

Your discription is coorect.regarding "Pure thermal effect" you will have it in case you have linear system.

Regards,
Aslam

Aslam,
WNC case is not a hypothetical case and WNC case may have significant effect on nozzle loads. Consider 36" cooling water/ liquid line with spring support near nozzle to carry load in operating condition.
for 36" std sch. pipe weight of water/liquide will be 75% of weight of water + weight of pipe.
That means when line is drained out, 75% of spring load will pull the Piping system up ward and based on geomery of system it may cause enough moment and force to damage the nozzle.
Other typical case for consideration of WNC case is a sytem in which for a perticular period of time operating fluide is in gaseous form and normally it is in liquide form(example De coking system ). With WNc case you can simulate both the system in a single CAESAR file.

Regards ,
khalid

Deae Khalid,

Please read and understand carefully whole discussion,then you will be clear that I have not said WNC is a hypothetical case.

I described the problem given by Mr.Habib as hypothetical as the problem from you also falls in the same category.

Answer to my following question...please

Are you submitting nozzle evalution sheet for WNC load case with your stress analysis report as a deliverable for any of the equipment nozzle irespective of the type of restaint used to support your piping?I repeat, nozzle evaluation sheet for WNC load case

Answer to this question as straight as I asked honestly.The If and but for any justification we will discuss after this answer.

Regards,
Aslam

Hi aslam

Whatever khalid & myself try to explain is a practical area of concern when we use spring specially in large bore pipe.

You might not have encounter the problem but it's a real one ( My personal opinion).

If you ask me to answer whether I have ever made data sheet for nozzle load for WNC case I say "NO"

Still I feel as professional stress engineer it's my duty to cover all stress relater area & ensure safety even if I do not reflect it on black & white paper.

I like some expert to enter in this topic & give proper guidance.

Regards
Habib

Now we can move forward..........

Dear Khalid/Habib could you please make a sketch of your real problem (36" water filled line) you discribed, with original suppors arrangement.Mark all data required for inputting and analysing this 36 Inch water filled line which is connected to a equipment nozzle.Please also give Nozzle allowable loads.
Also attach a evaluation sheet for the following four load case after analysing this line.

Max Design Load case
Normal Operating Load case
Sustained Load case
WNC Load case(Hanger Active)
WNC Load case(Hanger Rigid)

Regards,
Aslam

Regards,
Aslam

Hi Aslam
There is no need for any special pipe routing.
Just make a normal routing for large bore line put one or two consecutive spring support from the equipment nozzle & check WNC you will find unwanted upward load at nozzle.
Since you raise concern I just check a test file , yes it behave as expected.
Please check it & come up with result.

Regards

Habib

It sound strange to hear " come out with results" without defining and knowing the problem.You mean to say that I have to device a problem to prove your point.
That is what I termed as hypothetical.If you have real problem with you to prove your point do it here now.

Regards,
Aslam

Hi Aslam,
Thank you very much for your reply.

Incidentally: spring tables give theoretical installed loads and actual installed loads for variable springs, and hot load for constant springs. In the trials I did when compared spring table with the restraint summary, the theoretical installed as well as the actual installed loads are either very close to the sustained load or these loads are in between the values of operating and sustained loads. It seems that the WNC case will be the same. So once the nozzle load is checked for the operating and sustained loads it should be safe for WNC condition also.
For further discussion.

Dear Mathew,

You are absolutelly correct with your observation.This is the way nozzle is evaluted for any equipment and piping for any support arrangement.

The original question was asked like below

"When we have to consider the WNC load case. Is there any specific considerations to use this load case.

If we consider the WNC case do we need to activate the cold loads for spring hangers. "

I advised valid and genune reasons to consider WNC load case at the same time I was surprised to know that some respected member are relating it only with nozzle loads which is unheared off.

I guess dear habib is missing something while sizing his spring near his equipment nozzle.

For further discussion.

Regards,
Aslam

Hi Aslam
have a look on attached file may be this will help.
Purpose of this test file is only to indicate spring & nozzle behavior at WNC.

Regards

Habib

Dear Habib,

I have re-analysed your line and evaluted nozzle for two design i.e.your design and my design.

I have made following changes...

1)Spring was about 3 times over designed
Your spring-Cold load=133388N/spring rate=2189 N/mm
My Spring- Cold load=56000N/Spring rate=701 N/mm

2)Bottom spring modelling was not correcct.

Compare results and have your say.

Regards,
Aslam

Hi aslam
I don't ask for good design or solution.
This is just a test file. What if vendor don't accept operating & sustain load calculated by you.

Sometime we put two consecutive spring support to match vendor accepted load, that makes the situation worse.

I never claim if there is a spring in system there will be problem.

I just try to explain from beginning that use of spring have it's
own limitation be aware of it & take care of it.

Regards

Habib

We must work to provide best design and solutions to problems.
I think you will not convince and agree that WNC load case is not used for nozzle evaluation.

Best Regards.

Aslam

I already mention that I never send WNC load at nozzle to vendor.
I say, I take care of it.
If say I do not include WNC case in my test file I even could not aware that there is still some problem in system.

I am ok with discussion but one thing I would like to say.How comfortably we change our stand is something which disturbe me.

Best Regards,
Aslam

My point was whether as stress engineer we should be worried about WNC or not?
My answer is YES.

If you take care of sustained condition,WNC get addressed by default.

The use of WNC load cases is different and for some other reasons.

I have given you some clue.Try to do some search you will be more educated on the subject.

Best Regards,
Aslam

Hi Aslam
I take your note positively.
I advice you take fresh look on "If you take care of sustained condition,WNC get addressed by default"

For me your statement is valid only for rigid support not for spring.

Regards

Habib

Dear Habib,

I have nothing to say about this statement because you have to read it in connection with ongoing discussion.

Best Regards,
Aslam

The actual installed load is calculated only when the “Calculate actual cold loads” check box in the Hanger design criteria dialogue box is marked.This load case Caesar2 automatically creates. Result comes in the hanger table. Or we can "KEEP" in load cases.
(Note: Ambient is 11deg C and direction of nozzle is X – nozzle report may be corrected!!.) When compared the results, one restraint summary have more or less uniform loads in direction and magnitude. Other one have operating load in negative direction and sustained load in positive at the nozzle node and all other nodes have uniform loads. This means already the spring is pushing the piping upwards in sustained condition (All the other parameters same, but there is no thermal effect). So naturally when the content is not there, spring will push the nozzle further to upward direction and have higher nozzle load in positive direction.
My observations for further comments.

Hello Shiny,

What my experience is when you select the spring hanger in the sustained condition it shoule be in lock state that means the spring will work as a rigid hanger. We unlock the spring only when the pipe is hot i.e. in operating condition the spring will function. So i dont think there will be any vertical movement or load for spring in sustain condition. Correct me if I am wrong.

Regards,
RK

Hi shiny
I don't understand your model properly.
But your observation is correct spring may try to push nozzle in upward direction.
Try to set spring manually so that in sustain condition nozzle take very little vertical load.
Spring is a very good toll if you properly manually select it to manage nozzle load.
Like Khalid mention before if in same line sometimes flow liquide & sometime gas flow be careful about upward load for spring design ( when gas flow in line). For nozzle also upward load could be problem.
There are some limitation for spring also it require maintenance, costly, Crete problem for vibrating equipment.

Hi Rk

Spring kept under locked condition for hydro test condition not for sustain. Sustain load we check when plant is running so can not lock spring.

Regards

Habib

Hello Habib,

Yes you are right what I want to say is we prepare one more case as per client requirement its called erection case in that case we only considered the weight of pipe and operating temperature as ambient only. This is a same when pipe is just laid down on site and there is no fluid inside it. at that time we need to lock the spring.

Regards,
RK

Selecting spring manually is fine but care has to be taken against using this haphazardly.

By selecting spring manually you are asking spring to take some load which is not real.You are asking spring to take some load, say 5000 N load in ope, so that your nozzle loads are within allowables. Then with spring rate of 100 and disp. 0f -5 mm, we get sustained load of 5500 N.
If the actual sustained load for the line at that point is more than 5500 N then the spring's ope. load will be different and will not act as we have designed it on paper.

So it is always good to let caesar decide the spring load for us. The sustained load should be such that the spring deflection in that case is as close to 0 mm as possible. In caesar and on paper we can make the
spring work as we want it to. But on the field it will work as it wants to. Hence tweeking the spring on paper, though useful, is not a great idea and enough care has to be taken.

Another advantage of having sutained def. close to 0 mm is that the installation of spring is also easy. The unlocking of spring will be easy too.

Hi All,

While considering the WNC with hanger I have considered the load case as WNC+H and what should be the stress type for this type of load case.
should i consider the HGR type or SUS type. In caesar reommended load cases it is giving this load case as HGR case.
Please suggest me in this regard

It seems spring support fundamentals are not clear to many among us.

I was stunned again to hear someone saying we need to lock the spring on site after piping has laid down.I am asking very simple question and expect a simple answer.

What does spring locking means and who is responsible for springs locking.

Regards,
Aslam

Hello,
Spring foundamentals are clear to me i think you need to brush up some knowledge if you dont know locking of spring on site. when springs are deleavered on site it is always in lock condition.

Springs are locked & unlock by the commisioning engineer at the start of the plant. while when we take out the pipe for maintenance then the maintenance engineer will lock it again.

Hi Aslam
I do not know whether you are really asking about locking or critisiging some fellow member.
Sometime if spring is used for gas line operating/sustain load means almost only the piping load.
When hydrotest is done load at that point is piping + water weight.
There could be lot of difference between that two weight.

Spring may not take that load variation.
So there is a lock pin in spring If you lock that it will work as normal hanger rod.
Site people are responsible to lock it before hydrotest.
For preparation of data sheet we need to mention hydrotest weight, vendor will design spring to take care of it at locked condition.

Regards

Habib

Hi Rk
If you like to make addition load case for erection it is absolutely fine.


Hi Bigsyis

Your suggestion is right. But sometime our aim is to make sensitive nozzle safe. So spring can be select manually. But yes one must know what he/she is doing & what is possible impact.


Hi nikhilsaiba

I think you can select type SUS


Regards

Habib

Hi habib,

Thanks for your reply. We are using this case to check the sagging of pipe also. Kindly let me know your views on the same. In this case we are only considering the pipe weight, and ambient temperature. As per my knowledge it will help us to decide the sagging of a pipe when there is no any fluid inside it. but no doubt that we are checking the sustain condition also.
What I found in both that there is not much difference but yes when it comes to spring hanger there is a considerable difference in load.

Regards,
RK

Hi RK
Yes you have to check sagging for sustain/operaing case also when spring is active in addition to what you are checking for erection/hydrotest case spring locked.

If your system is of steam line , check for permissible amount of sagging to avoid steam pocket.


Regards

Habib

Dear RK and Habib,

I intended no offence to anyone.I still have not got satisfactory reply.
Let me summurise it follows .

All springs are caliberated and pre-set by the vendor to the pipe cold load before shipment.

The spring is pre-set in vendor shop to the pipe cold load position and is locked against up or down movement until the piping is ready for operation at which time the side locking bolts are removed.

After the pipining and all springs are comletely erected and the hydrostatic test made the side locking bolts are removed from the rigid housing and spring became free to deflect.

Without further adjustment the spring will accurately supports the pipe as the pipe expands to its operating position.

In the event that the line is required to be dismantled,replace the side locking bolts in the rigid spring housing and the spring unit can be easily removed and stored for re use.

The locked spring assembly supports and holds the erected pipe at cold position and act as a rigid hanger.

With lock bolts removed the spring is free to support pipe as it expands to hot position.

Important point.
The moment you will learn to salect spring manually your nervosness/discomfort realating to spring sizing will disappear.

Best Regards,
Aslam

Hi Aslam
Yes your understanding is ok.
Additionally I think some fine adjustment is done at site to fix the spring with pipe at pre- define force ( called theoretically installed load).
Once we fix the spring it try to maintain same upward load ( unless move up or down) at that point of the piping system.
That is the problem with spring system.
1) If certain case more force at that point expected ( hydro test case) then it will disturb the whole system if spring is in operation. We need some remedy. We lock the spring.
2) In certain case if less piping force at that point expected ( WNC case) it again create problem in whole system ( just reverse in direction than hydro case). We need some remedy .
3) If you closely follow the modified caesar file ( suggested by you) vertical load at spring almost same at spring location because no much vertical movement at spring. But check the load variation at nozzle, Sustain- 64814, Operating -82204 whereas WNC only +4510. Percentage variation of Sustain to WNC very high. In this particular case may be it is still within permissible limit. But may be not in some other case.
For rigid support in any time less load come from piping system is absolutely no problem, If it a spring once you fix it less load or more load come from piping system at that point is equally bad.

Regards
Habib

Dear Habib,

I presume a competent stress engineer knows these basics and address them in his design.I may be wrong with my perception but there are engineers who are successful without knowning many of the important basics things required for doing stress analysis.

Best Regards,

Aslam