Customary maximum fluid velocities in pipes are 15 ft/s for liquids and 150 ft/s for gases. Do not generally exceed these limits. For single phase gas lines, I use a maximum of 60 ft/s so that in-plant noise does not become a problem. Try to avoid too small a velocity (use > 15 ft/s) so that liquid accumulation is avoided. For liquid lines, use up to the maximum of 15 ft/s, if the pressure drop is not a problem, and apply a minimum of 3 ft/s to avoid possible solid or liquid accumulation in pipe bottoms. For lines such as cross-country pipelines the velocity you use would be an economic balance between acceptable pressure drops, flow rates desired etc. For two phase flow, flow velocity must not exceed the erosional velocity as given by Ve = C/(sq root dm), where Ve is the erosional velocity ft/s, C is a constant and equal to 100 for continuous service, dm is the density of the gas & liquid mixture at operating conditions, and an equation too long for entering here. For steam lines, use 175 ft/s for 50# to 150# steam, 130 ft/s for 150# to 400# steam, and 100 ft/s for 400# to 600# steam.