When calculating velocity for a pipe section, the source system pressure is considered.
While the volume of a section is considered at atmospheric pressure, the gas is assumed to be compressed.
AdjustedFlowRate = Flowrate x 101325 ÷ (101325 + SystemPressure ÷ 100) ÷ 1000
Velocity = 4000000 × (AdjustedFlowRate) ÷ pi ÷ dia ÷ dia