Velocity outside Specification Design Limit: This warning occurs when the user has specified velocity limits in the Design Limits section of the Specification dialog box.

If the velocity is greater than the design limit max value, the pipe will have red arrowheads.  If the velocity is less than the design limit min value, the pipe will have green arrowheads.

Direction of Flow Reversed: When the user draws a pipe, PIPE-FLO assumes the direction of flow to be the direction the pipe is drawn.  This warning will occur when PIPE-FLO calculates the flow in a pipeline going in the opposite direction of the direction the pipeline was drawn. 

Closed to Prevent Backflow:  This warning occurs when the user selects prevent backflow on a certain pipeline.  If PIPE-FLO calculates the flow going in the opposite direction, it will close the pipe to prevent flow in that direction.