Whilst applying out of balance forces to elbows on a relief system, we add the force in one direction at a time in the axial direction.

But, For a 45deg elbow, i assume we split the force into the two components of X*Sin45.

A simple and maybe daft question...............
But am i right?

Based on Hydrodynamic theory, force vector components exerted on a  deg bend in x,y plane, due to change in fluid momentum are calculated as follows:

Fx = m(V2cos – V1) – p1A1 + p2A2cos
Fy = mV2sin + p2A2sin

where 1 is inlet caracteristic (fluid and geometry) and 2 is outlet caracteristic, m is mass flow rate.
The force exerted by the bend acting on fluid is F (pointing towards convex side of bend, opposite direction to the velocity vector).
Opposite force is exerted by the fluid onto the bend, -F.
Hope this is clear, otherwise check for the source of this equation in Perry's Chemical Engineers' Handbook
Ranka