If an upset temperature occurs for very short period of time, can it be classified under occational load? If so,how the load case should be set up?


As such, the occatioanal load( wind, earthquake) is added to sustained loading and a higher allowable is permitted.

Dave,

I think the load from upset temperature can not be classified as primary load. It must be secondary loads, that is, it should be expansion stress in the ASME B31.3.
Secondary loads are self-limiting and usually displacement driven such as thermal expansion, tank settlement, imposed anchor movements, etc. Generally secondary load does not produce failure with a single application of the load and the allowable stress limit of secondary load is subject to cyclic modes.

Apart form secondary stresses there may be an impact of this upset event on the way the primary stresses are evaluated. Though the state of the primary stress will not be affected by this upset temperature (provided it is not accompanied with a pressure surge or rise in any other type of primary load and ignoring nonlinear effects), this rise in temperature may change creep behavior thus lowering the allowable stress. In such a case if primary stress ratio exceeds one, we may follow Code rules regarding occasional loads, to ensure that accumulative creep damage is within acceptable range.

B31.1 allows anchour displacments to calculated as either occasional or expansion loads. A temperature variaion has the same effect as an anchour displacment.

Thermal loads can also be of the primary nature,kindly read one of my earlier responses in this issue, which I am quotaing here for reference"If a thin tube is subjected to a length-wise temperature profile that has a sharp jump discontinuity at some point along the tube and this discontinuity moves repeatedly along a length larger than, typically, a couple of radii of the tube, then the shakedown limit is near equal to the elastic limit, i.e. 2 is replaced by 1 . These cases can be found in Fast Breeder Reactors.The thermal stress effectly acts as a primary stress instead of a secondary stress".

I don't think the case that has been discussed by Dave fits into the above category and hence it will be secondary only. However pl. check the case for "thermal shock" if any. You will find some reference to this in SEC III.

Regards