Hot elastic modulus

Hi all,

I think I have a good topic for discussion:

When do we use the hot elastic modulus for stress calculation and when do we use the cold one.
The default value in Caesar II, is the cold elastic modulus.

Kind regards,

Yves

There are many posts on this topic, you might use the 'search' facility.

The B31.3 Code and others require the cold modulus to be used in stress calculations. It is common to use the hot modulus with high temperature pipework, when equipment loads or restraint reactions are required and NOT stresses.

Hi,
It is a codal requirement to use cold modulus(reference modulus of elasticity: Ea) in stress calculation(B31.3, Cl. 319.4.4 (a)).

However, hot modulus can be used for Loads/reactions.

Most stress analysis software use Cold modulus for Loads on Restraints, However options are available for user to use hot modulus instead of cold modulus. Using hot modulus especially for > 300 deg C systems will have significant reduction in Loads on Nozzle and supports.

It does seem strange though that CAESAR doesnt offer the option to correct expansion stresses back to the cold modulus when the system is solved using the hot modulus, as per B31.3 319.2.2b(4).

I got this phrase from my reference:

Self-springing occurs only when the system is subjected to higher temperatures. For the expected strain(expected expansion per unit length), if the modulus of elasticity at this high temperature is used to back calculate stress, the stress value will be lower thatn when it is calculated using modulus of elasticity value at lower temperature(cold condition). That is, calculated stress is higher when material properties in cold condition are used. This provides a built in safety in design. Hence the stresses are calculated using cold modulus of elasticity.


Hope it will help!

MPB,

If I want to eat my cake and have it too (use hot modulus for load and cold modulus for expansion stress), I can run the operating case twice - once using cold modulus for use in the L1-L2 (EXP) scheme, and a second time using the hot modulus to better estimate the operating loads.
I believe the sort of stress adjustment you want (multiplying a "hot" stress range by Ec/Eh) is a little fuzzy in the L1-L2 approach. Remember that the Codes focused on a simpler flexibility approach where one would anaylyze T1 or T1+D1 alone to get this stress range; where there is only one Eh throughout the system AND no nonlinear boundary condtions.

" Note that, if you are using B31.3, you are not permitted (see 319.5) to use Eh for any reason other than to calculate the amount of required cold spring."

CraigB - One year ago


Regards,

If we were talking of an 'unbalanced' system then its not quite the same, a hand calc would be called for.

An "unbalanced systems" (see 319.2.2(b) - Overstrained Behavior) is a different issue.