API 14E AGA 3 ISO 5167
API 14E methods, discharge coefficients, critical flow, and blowdown calculations
Cd = discharge coefficient,
Ev = velocity approach = 1/√(1-β⁴),
Y = expansion factor
Flow becomes sonic when pressure ratio drops below critical—further ΔP increase won't increase flow.
This calculator sizes restriction orifices for equipment drains and depressuring systems. It uses API 14E erosional velocity limits and AGA Report No. 3 orifice flow equations to determine the required orifice diameter.
Drain orifice sizing follows API 14E for erosional velocity limits and AGA Report No. 3 (API 14.3) for orifice flow measurement calculations. The calculator applies these standards to determine safe orifice bore diameters.
Higher upstream pressure increases the pressure differential across the orifice, allowing a smaller bore to pass the same flow rate. The calculator accounts for critical (choked) and subcritical flow regimes.
API 14E defines the erosional velocity as C/√ρ, where C is an empirical constant (typically 100–150 for continuous service) and ρ is the fluid mixture density. Exceeding this velocity risks pipe wall erosion.