What are you trying to achieve? Are we talking lean phase, or dense phase transportation?

With flowmeters for liquids the issue is to get a steady flow at the meter. Turbulence can cause false readings, so the approach length is intended to homogenise the eddies caused by the bend in the liquid flow path.

With pneumatic conveying you have a similar situation, but it depends on how you are going to measure flow. Are you happy with static pressure, do you want velocity pressure, or do you need density as well? Talk to your Instrument Vendor. They will give you calibration length needed. ■