Hi everybody,

I have created a model in Caesar of an existing pipework.
There are two hanger in this model.
I know that Caesar can design the hangers, but the results do not match the information that I have about the Hangers. Hence, I thought to pre-design the hangers in order to obtain a model that is more similar to the reality.

I found the catalogue of the variable spring hangers so the data that I have are reliable.

Caesar ask me 2 things to pre-design the hangers:
- Spring rate (and I have it)
- Theoretical cold (Installation) load.

I imagine that this last parameter correspond to the load that the hanger see when only the weight of the structure is considered.

So I thought to run a simulation and read the reaction of the restrain in the node of the hanger when only W is considered and use this data as input to pre-design the hanger.

Is this process correct?

(Hope to have explained myself)
Regards,
Ilario

That depends. If you assume "Cold Load Design", then, yes - the hanger is designed to balance the weight in the cold position.

Typically, though, the hanger is designed to balance the deadweight load in the operating position. In this case, the theoretical cold load is that weight load you already have plus the thermal growth at that point times the hanger stiffness. If the pipe grows up, the theoretical cold load is greater than the hot load.

Ok that is perfect. Thanks.

I actually have another question.

I have noticed that depending on the T load case that I use (T1 different from T2) the loads that the same restraint see in the case W (so the cold load) are slightly different.

This does not make sense to me. Shouldn't they be exactly the same? (In my opinion the temperature should not be considered in this case)

Thanks again.
regards,
Ilario

It sounds to me like you are using different thermal sets to design the springs. For different T's you will get different theoretical cold loads even with the same weight component.

Yes, it is exactly that. As I have different T's I decided to run more than one simulation to see which case is the worst.
But I did not expect to have a change in cold load.

Could you explain why a variation in temperature influences the weight of the structure?

Thanks.
Regards,
Ilario

The weight remains the same (unless there is liftoff due to the changing thermal set). Remember, the theo. cold load is weight PLUS the selected spring rate times the calculated thermal growth. With different T's you have differing calculated thermal growth.

Sorry to bother all of you on Monday morning.

Dear Dave, what you wrote above means that I am considering the temperature effect in the cold case. And so, the cold case becomes the hot case. Hence, I am designing the hangers for the hot case. Is it right?

So, to calculate the actual cold load (due only to the weight of the structure) I remove the hangers, I insert a +Z restraint at each hanger position and I calculate the weight.

Isn't it this the actual cold load?

(The fact is that I am not designing a new system. I am trying to model a pipework already existing and I would like the model to be as similar as possible. Hence, as I know the spring rate I need to know the actual cold load.)

Springs that are to balance the weight in the operating position will have a different load in the installed position. Your CAESAR II input should have this installed load defined. You can confirm that you have defined the correct load when the reported spring load for the operating load case listed in the output equals your design hot load.
The actual installed load is a separate calculation which considers the existing loads when springs are set (no fluid?) and considers the effect if the installed imbalance on pipe position.