Temperature drop below design temperature

[Alejandroh]

I would like to ask a (complicated) question.
During eventual special situations (startup/shutdown), it could be possible to have a temperature drop below the minimum design temperature (i.e., -45 degC for killed steel) due to flashing of the process liquid.
However, we will not have continuous operation at this low temperature. After the eventual flashing of the liquid, the system will gradually warm up again. What will happen to the material of construction in that case (killed steel)? Will it fully recover its mechanical integrity or is it still possible to have a brittle fracture?

 

[weldstan]

The problem with the low temperature exposure is the potential for brittle fracture. Repeated temperature cycling may result in crack extension to a critical size where brittle fracture would occur.
Mechanical properties of the material at room temperature will not be adversely affected after exposure to the low temperature.

 

[Alejandroh]

Thank you for the reply weldstan!!
Do you have any reference where I could read more about it?

 

[weldstan]

Advise googling Brittle Fractures. There is a wealth of information.

I should have stated for clarity, "The problem with the low temperature exposure is the potential for brittle fracture at the low temperature."

You could readily have a small crack in a weld that when exposed to a low temperature where the toughness properties of the weld/base metal are effectively non-existent and appropriate stress applied where fracture will occur. A few years ago, a brittle fracture in a structure occurred when the real temperature delta was not effectively taken into account. Contraction of the structure occurred and if two points are fixed, added tensile forces are induced across the joint, which had a small crack. Fracture initiated at the small pre-crack to failure. Charpy V-Notch impact tests of the material adjacent to the fracture at the failure temperature gave absorbed energy values of 4 to 5 ft-lbs.

Repeated temperature cycling of pipe, when not appropriately designed, could cause a small crack or other discontinuity to grow to sufficient size where the loads applied during the next low temperature exposure results in failure.

 

[SJones]

First of all, check the MDMT and the definition thereof. -45 deg C smacks of the Charpy test temperature of certain steels which is not necessarily the same thing. After that, read up API 579-1, Part 3 (+Errata)

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[weldstan]

On gas processing plants designed during the mid '70s to early '90s for locations in the USA, Canada and Scotland on which I had direct involvement, Joules-Thompson cooling during flare/shutdown was fully utilized in selection of the MDMT for piping and vessels, initially at the direction of the Owners.