Yea, currently my basis for line sizing is should be within 10-20 m/s. But if evaluate using parameter 6-9 m/s, I think existing pipe design is undersize. based on this figure, most of existing design is ok. Based on my company technical standard, gas should be 10-20 m/s. That’s why I afraid that 6” has been chosen due to wet air (as plant air didn’t go through drier) and 4” chosen due to dry air.Ħ-9 m/s for me is too stringent. Based on user demand, plant air is much lesser than instrument air. I just question the existing design for the piping (existing design is since 1994) that goes to plant air is 6” and piping that goes to instrument air is 4” (from inlet to drier up to main instrument air header). Refer this famous article on compressed air piping: The limits for a well designed compressed air piping system is 6-9 m/s (20-30 ft/s). What are you evaluating currently? Is it compressed air piping? If yes, then velocity limits are much lower for compressed air. If you want my personal opinion, I would rarely design any continuous service gas line for velocities above 20 m/sec (~65 ft/sec), regardless if the gas is dry or wet (= contains condensable components). At higher velocities, inhibitor becomes ineffective according to many field and experimental studies. In some other cases, when wet gas contains corrosive components (H 2S, CO 2), velocities are reduced below 60 ft/sec so that inhibitor injection does not lose its effectiveness. This can create significant erosion and damage. One reason I can think of for reduced velocities in a wet gas system, is due to (possible) condensation of liquid droplets from gas and carrying of condensed liquids at high velocities towards elbows, bends, and fittings/valves. At around 25-30 m/sec, noise becomes substantial and it is normally not tolerated in a continuously manned facility. You will rarely see continuous service gas lines designed for velocities at the high-end of the recommended ranges. Like the books says, it is just a rule of thumb.