Required orifice area for atmospheric storage tank breathing per API 2000 / ISO 4126 subcritical compressible gas flow formula
Formula (subcritical, P2/P1 > critical pressure ratio): G = P1 · √( (2k/((k−1)·R·T·Z)) · ((P2/P1)2/k − (P2/P1)(k+1)/k) ) where R = 8314/M. Then Ac = Q / (G · 3.6e−3 · Kdr · Kp) (mm²). Breather valves operate at low pressure (kPa gauge), so flow is subcritical — the P2/P1 pressure-ratio term is essential, unlike safety-valve critical-flow formulas. API 2000 covers thermal inbreathing/outbreathing and liquid movement for atmospheric storage tanks.