In PSV's the first step is to define the forces first at the PSV nozzle and then at each bend. Process Engineering can define the value of the forces as it changes with change in diameter on the outlet side (Bernoulli's Equation). The forces are applied tangentially to each elbow’s as two components. On an open system you may provide just one reaction force, unless the opening has a 45 deg. slot cut then resolve it into components. Process can go further and provide you the exact forces at each location.