Redneck in training
Elite Member
Those tables are based on velocity, not pressure drop. The suction line is limited to 5 feet per second, probably to insure the flow doesn't cavitate. The pressure side is limited to 20 feet per second, to keep the flow laminar. At higher velocities, the flow may go turbulent at obstructions or fittings, resulting in very large losses.
Those tables don't appear to consider the size or configuration of the fittings. There's a note that says you should go to a larger size if you have long lines or a large number of fittings, but that doesn't provide very useful guidance.
Any abrupt change in geometry of the flow profile will cause turbulence at any velocity. The pressure drop is proportional to velocity squared. If the flow profile changes gradually and the flow remain laminar all pressure is recovered and there is no loss (in ideal circumstances). In real world there will be some small loss. Since elbows always cause turbulence they have significant effect on pressure drop.