Existing Spring can 2

[Dj364]

Dear members,
I am checking the existing Spring can in order to re-use it. after modifying a the same spool by adding a new flanged valve. I asked for data of the spring, only spring rate, the and type are available informations. the cold load is missing.
Does anyone know what are the possibilities to perform such analysis without having the cold load of the spring.
NB: I ve tried to model the old system with assumptions, i couldn t get the springs size that do exist ( there are three existing one close to nozzle suction turbine).
I will appreciate your light.
thanks in advance
dj

 

[RossABQ]

Do what is right; springs are cheap. Specify them based on your analysis.

 

[Dj364]

thanks for your reply,
I have modeled the existing line as well as the spring support, i have performed the analysis with adding a valve.
I ve checked loads against the applicable code ( NEMA SM23), they still within the allowable.
the spring support could be re-used according to my analysis.
Is there anything ,they have to do in the field regarding the installation of spring can?
I would appreciate any comments.
Thanks in advance.

 

[JohnBreen]

Hello Dj364,

There are other considerations.

I am going to treat this question in a rather simplified way and I will say “up front” that anyone who wants to can “nit pick” as I will leave a lot of room for such. If I may summarize your posting, you want to “reverse engineer” this spring hanger design. The piping system has been modified by adding a valve and matching flanges. So what you are looking for is a new “cold setting” for the original hanger with the additional weight that has been added to the system. The hanger location will remain the same.

I would recommend to you that you first review the artful science of pipe hanger and support design by reading the Anvil design manual available here:

http://www.anvilintl.com/db/datasheets/Pipe_Hanger_Design-05.pdf

For reference download the Anvil-Grinnell catalog here:

http://www.anvilintl.com/literature/index.php?lit=cat

The “cold setting” for a spring hanger may be calculated by the following procedure: first do a cold weight plus pressure (W+P) analysis with a rigid hanger at the location and determine the weight on the rigid hanger (required supporting force). Then take the calculated weight and add it to the model as a constant supporting force (opposite of the weight) and do an “operating case analysis” (T+W+P). From this analysis you will calculate the vertical movement at the hanger location. If you were designing a new spring hanger you would at this point consult the hanger manufacturer’s catalog (see reference above) and choose an appropriate spring “size” for the weight and the vertical “travel”. Choosing the appropriate spring size, you would look in that column of the catalog to find the “spring rate” (pounds of force per inch of travel). You want the spring hanger to provide a supporting force exactly opposite of the calculated weight in the hot position. If the pipe is moving up when the system goes from cold to hot then the spring would be compressed when the pipe system goes from hot to cold. Multiply the calculated travel (from cold to hot) in inches by the spring rate (pounds per inch) and you will have the THEORETICAL “cold setting (in pounds - note that since the actual weight at this point would be less, the cold setting would have to be “held” by a preset bar until after the hydrostatic pressure test is finished – when the preset bar is released, the piping will “jump” to a point somewhere between the “hot” and “cold” settings). You would come back to the spring hangers after the system is at the operating temperature, and adjust all the spring hangers to their “hot” setting and adjust the system to achieve the desired “pitch” (slope) of the piping.

Your case will be different. Since you are adding weight, you will have to calculate a new “cold setting” for the existing spring hanger. The spring rate will remain the same and the travel will remain the same. The required “hot” supporting force (setting) will be greater so obviously the “cold setting” will be greater. So you use the procedure outlined above to obtain the new weight to be supported at the hanger location (do a cold weight plus pressure analysis with a rigid hanger at the location and determine the weight on the rigid hanger). The difference in weight between the original weight analysis and the new weight analysis would be added to the original “cold setting” to get the new “cold setting” (then the “hot” setting will be as required when the system goes to the operating temperature). BUT, here is where your problem arises. Since the original spring will have the same spring rate, it will have to be compressed more in the “hot” condition to supply the new (greater) “hot” supporting force. Therefore, as the system goes to the cold condition the original spring will be compressed much further than the manufacturer intended it to be compressed. It may “bottom out” the spring (all the spring coils in contact with each other) and this is not a desirable situation. Find the manufacturer’s catalog and follow the discussion below.

Look at pages 134 and 135 of the Anvil-Grinnell catalog (see address above) where you will find a chart showing the weight (supporting force) and travel for various spring sizes. You will see that at the top and the bottom of the table some rows are “shaded”. You should pick a spring for which data is shown in the “unshaded” rows. These “unshaded” rows show the data for preferred spring travel/support. If you find that the new weight falls below the “preferred” (unshaded) range you should pick a new spring of a different size. It is likely that your existing spring hanger (with the new greater weight applied) will fall outside the “preferred range” of the manufacturer’s catalog and therefore using it with the new weight (supporting force) is not recommended.

Also, since the entire increase in weight will not be supported by just one spring hanger (the weight will be distributed over several adjacent spring hangers), all the spring hangers in the area of the added valve/flanges will have to be evaluated.

It would be better for you to do a "new" hanger analysis and let the software pick "new" spring hangers for the "new" weight distribution over your piping system.

Regards. John.

 

[Dj364]

Thanks so much John for your valuable explanation, "The forum wouldn't have any value without a Gentleman like you, I learnt many things by reading your replies in the forum"

One last question, If you don 't mind.
when running (weight + pressure) analysis in order to get Hot load and calculate the new "Cold setting", one of the support lift off "Fy" positive, and the displacement is down (DY is negative) this spring is a little bit far from the new added valve. It looks to me that the system is unbalanced.
is it correct ?

 

[JohnBreen]

Hi Dj364,

Yes, I think that anytime you see (in the analysis) the pipe lifting off a +Y rigid support it indicates the the support is not in the proper location. Of course if the piping is "lifted off" such a support then there is no support at that point in the operating condition.

If the lifted off condition resulted from the placement of a new concentrated load (valve and flanges) it is an indication that the locations of all the adjacent hangers/supports should be evaluated. There is a "see-saw" (teeter-totter) condition when the support adjacent to the heavy load is acting like a fulcrum, resulting in an uplift at some support further away. This is an "unbalanced condition".

Regards, John.

 

[Dj364]

Thanks very much, I appreciate your help John .
Kind regards
Dj.

 

[Dj364]

Hello John,
Just to make sure what you wrote above.
"first do a cold weight plus pressure (W+P) analysis with a rigid hanger at the location and determine the weight on the rigid hanger (required supporting force)."

Without using computer selection, calculating the hot loads by first doing weight analysis "dead weight only" or sustained (W+P)? the weight on rigid hanger means froces from dead weight only?
Thanks in advance.
Dj.

 

[JohnBreen]

Hi DJ,

Yes, the "cold" (W+P) analysis with all rigid supports will tell you what the natural distribution of weight will be over the system of supports - make sure thought that WEIGHT is dead weight (material of construction) PLUS live weight (contents of the piping).

Then (as a check) if you use the forces calculated at the hanger locations as supporting forces (turn them around) and repeat the "cold" (W+P) run you should notice that the pipe movement will be identical to the first run - very nearly zero "Y" deflections at the supports/hangers. Equilibrium!

If you then do a T+W+P analysis with the previously calculated supporting forces in place, you will see how the piping would move if it were entirely supported by constant force spring hangers. Of course, the movements at some of the support points might not need to be supported by springs (minimal movements). The pipe designer can get a lot of information from this sort of analysis and if you have strain sensitive terminal equipment nearby a hanger you can experiment with various springs and cold settings to optimize (minimize) the loadings. For example, if you (temporarily) "detach" the "anchor" that represents the equipment nozzle you can see what weight the nearby hanger must support to take the "Y" load off the nozzle.

The iterative process that I described is essentially the same as the software will go through if you have the software design spring hangers for you. You must keep in mind that sometimes (especially around vertical risers) the iterative process will give you "approximate" spring choices and the designer must "fine-tune" these by his/her experience.

Regards, John.

 

[Dj364]

Hello John,
I am sorry I got a little bit confused.
Hanger sizing: I have spring selection procedure in CAESAR II ( section note seminar held by Dave Diehl 2004).
The procedure of sizing a spring hanger/support is outlined in this section, it is clearly mentioned with a Y restraint at the support location will produce a load (Dead Weight=gravity only) that can be used to size the spring. that DW is a hot load.
I might be mistaken, please clarify.
I appreciate it very well .
NB: Right now I am using Autopipe for stress analysis, it does the same calcs as Ceasar.
Warm regards.
Dj.

 

[JohnBreen]

Hi DJ,

I am sorry I do not understand your confusion.

You (or the software) must choose a spring hanger if there is a point in the piping system where the pipe must be supported but there is significant change in elevation when the pipe goes from the cold condition to the hot condition. You (or the software) must perform a cold W+P run to determine the weight (dead weight and live weight) of the piping at all the support/hanger locations. If it were not done as a cold run the piping might "lift off" the support in going from cold to hot and you would have no weight calculated for that point.

The WEIGHT that is calculated at the hanger locations by the cold W+P run is the weight (gravity effect) to be supported in the hot condition. The WEIGHT (dead weight plus live weight) of the pipe and its contents does not change as the system goes from its cold elevation to its hot elevation. So, in the hot condition (at the hot elevation) you want the spring supports to supply a supporting force exactly the same as the weight.

In the example where the pipe moves up when going from cold to hot (you want the supporting force to be equal to the weight when the pipe is at its hot elevation - the hot spring setting will be equal to the weight and the cold spring setting will be greater than the weight) the pipe will compress the variable spring when it goes from its hot elevation to its cold elevation. This means that as the pipe moves down, the spring is compressed it will (in theory) supply more supporting force than the weight of the pipe at that point. In the example where the pipe moves down when going from its cold elevation to its hot elevation (again, you want the supporting force to be equal to the weight when the pipe is hot - the hot spring setting will be equal to the weight and the cold spring setting will be less than the weight. In either case, the hanger will develop the exact supporting force (equal to the calculated weight) when the pipe comes to its hot condition elevation.

Regards, John.