Calculating cyclic stresses due to temperature differential at mixing point?

[YungPlantEng]

I have a 4” pumped condensate line tying into a 8” valve controlled condensate header with each at different temperatures. I wanted to understand if there were any heuristics or practices that allowed one to state if the cyclic thermal stresses would need to be considered based off the deltaT between the two streams. Consider both as pressurized to where the flow is single phase. See sketch for example.

Keep in mind we’ve already observed fatigue related failures in a slightly different piping arrangement under these temperature conditions. This new arrangement would tie in such that the temperature differential is greatly reduced but I wanted to see if any standards or practices would help formalize this as a solution. Couldn’t find much on thermal stresses specific to mixing points.

 

[hvillasenor]

Hi YungPlantEng. In order for me to provide you with the right info I'd like to know what application and industry this is for. For process piping, I will refer you to ASME B31.3 (2012 or newer), Sec. 302.3.5, Sec 304.3.5, and Sec 319. Sec 319 discusses the thermal displacements, strains, and stresses. For a multi-axial stress state, which is the case in this piping, you'll use the Von Mises, or Equivalent Stress, theory. You'll then compare the equivalent stresses to the allowable stresses.

You can do this by hand calculations (slightly more tedious and involved which can lead to human error, unless you've done this before), or you can use CAESAR II software if it is available to you. Below is ASME B31.3 (2012) "Stress Range Factor vs Number of Cycles". As with all engineering matters, take caution and make sure you understand the applicability of this chart.




For condensate (assuming steam piping), see ASME B31.1-2022 (Power Piping), Sec 104.8. Specifically, Sec. 104.8.3 - "Stress Due to Displacement Load Ranges".

Hope this helps.

Best regards,
Humberto Villasenor, P.E.
VEMAC Consulting, LLC.
www.vemac-consulting.com