This depends on the type of fluid (Gas, Liquid or two phase).

In piping systems:

- Liquid pipings:

1) Water: Maximum pressure drop is: 5 psi/100 feet and maximum velocity is: 1 ft/s
2) Hydrocarbons: Maximum pressure drop: 5 psi/ 100 feet, and maximum velocity is: 15ft/s


- Vapor lines.

it depends on the operating pressure, for example: P = 50 - 150 Psig, the target pressure drop is: 0.25 - 0.75 Psi/100 feet.
For vapor lines, you need to take into account the erosional velocity as well as the maximum velocity to avoid vibration and noise (70 feet/s).


Thanks