Hello Everybody

I 'm trying for quite long time to figure out when and why a thrust block is required and how it is calculated. In previous discusions i could not find any clear indication of the fundamental engineering of these subjects
My opinion is that thust blocks should be designed to compensate the forces created on changes of direction due to changes in the mass flow direction as well as any pulses that may arise due to a water hammer effect.
In the relative literature that i found these forces are calculated in every type of fitting as the pressure thrust PA (for a TEE) or 2PAsin(d/2)(for a bend) etc. My question is that isn't these forces internal for a system in equilibrium and are compensated between bends or generrally fittings and equipment? The bend is not going to move whatever the pressure inside.It may explode, but it will NOT MOVE...
Finally from a energy plant that i am working now, i noticed that generrally thrust blocks are required for large GRP piping (circulating water of 3.5bar) but nothing of similar size is required for mettallic piping that according to the assumption that pressure is the defining variable the feed-water piping that is working under 180bar should be blown to space! Why is that?
I Know that this is fundemental engineering that i should know but it seems i am not the only one. No one so-far has gave me a pure scientific answer of what forces and why they derive on a fitting that require such a thrust block, but spesifically why dont the same laws exist for mettalic piping.

I would aprecciate any contribution
Thank you for your time.