As per discussed,
at which point does the time imbalance in these forces become important?

ie for V=300m/s through pipe 10m long,
then duration of imbalanced force on elbow = 0.03s which is negligable,
however if the said pipe = 300m long then the duration of imbalance = 1s
which i would be worried about.

what are the norms used for when these time imbalances becomes important.?

or, at what point using simple static techniques
does the system fall out of equilibrium.?

I also assume that if t=0 then energy =0?

Best regards