Say your elbow (and flow) starts in the +Y and exits to the +X.

The fluid imparts a load onto the pipe (and the pipe onto the fluid) in order to force it to turn.

The force of the pipe onto the fluid is -Y and +X, and vice versa, the force of the fluid onto the pipe is -X and +Y.

Generally, though, the flow is slow enough that it doesn't amount to meaningful loads to pipe.

But if you're flowing fast enough and heavy enough, it can generate issues for finicky nozzles.

The calculation for this is found in any Fluid Mechanics textbook.