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#74820 - 07/01/20 09:52 PM Spring Hot setting and cold setting.
NAG Offline
Member

Registered: 03/13/19
Posts: 39
Loc: INDIA
Hi experts,

Hope you all doing good and safe.

I had one query related to spring support Hot setting and cold setting. Which setting is advantages, some of the spring vendor saying always go for Hot setting method. But why not cold setting?.

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#74822 - 07/02/20 06:35 AM Re: Spring Hot setting and cold setting. [Re: NAG]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 6045
Loc: Houston, Texas, USA
From a "business viewpoint", you always want your piping system operating - right? If you agree with this assumption then we can conclude that the piping system is in the operating condition - hot.

A spring can only balance the system in one position. Continuing with the assumption above (that the system is always going to be hot), you would therefore want to balance the spring/system in the hot position.
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Richard Ay

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#74825 - 07/03/20 07:46 AM Re: Spring Hot setting and cold setting. [Re: NAG]
mariog Offline
Member

Registered: 09/29/07
Posts: 711
Loc: Romania
I accepted this explanation long time ago as well Caesar's (and other similar software) good algorithm to apply it. In 90's I made hand calculations explained in an old reference book.

About the explanation itself, what means for you "balanced"/ "not-balanced" for a piping which is essentially a statically indeterminate structure?

Thanks.

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#74828 - 07/05/20 05:50 PM Re: Spring Hot setting and cold setting. [Re: NAG]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 6045
Loc: Houston, Texas, USA
Let's start with something simpler - say a fish scale. You catch a fish and put it on the scale and it reads some number N. At that moment the spring is balanced with the weight of the fish. If you pull down on the tail of the fish (or push up), the reading on the scale will no longer be N. The spring is balanced by the weight of the fish plus or minus your change in load.

In a piping system the objective is for the spring to carry the weight load of the system (or that portion of the system associated with the hanger). But what is the weight load considering that at temperature the status of non-linear restraints can change from the installed state? This boundary condition change causes a weight load redistribution in the system and (typically) a change in the load at the hanger locations.

Therefore the load at the hanger locations, determined in the "restrained weight" load case is assumed to be the hot load, so that the spring is balanced in the hot position of the piping system. (In CAESAR II, if during the "free thermal" hanger load cases a non-linear restraint changes status the program changes that restraint and re-performs the restrained weight load case. This because the objective is to balance the spring with the load seen in the hot (operating) load case.) As soon as something else happens to the system, a snow or ice load for example, the spring is no longer balanced with the weight load of the piping (just like pulling on the fish tail).
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Richard Ay

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#74846 - 07/08/20 10:32 AM Re: Spring Hot setting and cold setting. [Re: NAG]
mariog Offline
Member

Registered: 09/29/07
Posts: 711
Loc: Romania
Dear Mr. Ay,

I cannot say I fully agree with your explanation. The fish scale example cannot be extrapolated to a statically indeterminate structure.

Let's consider a system with just two anchors an intermediate rest supports. The configuration is such that under temperature the system lift-off a number of rests. We decide to place spring supports. Only as an exercise, let's say that the decision is to replace all rests with springs.
The algorithm to calculate springs parameters starts with an internal case called Restrained Weight and Caesar includes a rigid restrain in vertical direction at every location where a hanger is. Simply said for our case, Caesar starts with the initial configuration :two anchors and intermediate rest supports just to evaluate the loads in rest supports. The load of the restrains from this analysis is considered the load that must be carried by the spring supports in HOT condition and this is called "balanced".
I have the objective to balance vs weight, however if I do not follow this objective, the system will simply respond in another way (preserving the stability) and for sure it has no memory of weight status to be able to protest.
So, unlike the fish scale example (where fish weight determines the response of scale), such piping system can be modified in hot conditions without be "balanced" for weight in the way described. What happens if I modify the algorithm such as I consider in hot condition the load carried by each spring as 110% of those calculated when there was a rest? For our piping system- a statically indeterminate structure- the effect is in anchors (may be a benefit or not) and in displacements. And by the way, as I said in another old post, I will never agree that proceeding in such way I introduced "cold springs" in my piping system.

I have also a remark: maybe, in the beginning of the design, I was not able to select the locations of rests (later hangers) supports as to be "optimum" versus the loads in anchors, because of the field constrains. Which is the reason I must fight to preserve such loads by "balancing" the system in hot condition? In practice I would "force" the algorithm selection for two or three spring supports and the system may be more "comfortable"...

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#74851 - 07/09/20 09:24 AM Re: Spring Hot setting and cold setting. [Re: NAG]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 6045
Loc: Houston, Texas, USA
Mariog,

I agree with everything you state. The "fish scale" example is indeed over-simplified, but it usually gets the point across.

Your modification requirement above is exactly why, on the hanger dialog, you can adjust the Operating weight to be carried by the hanger.
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Regards,
Richard Ay

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