Size a breather (pressure/vacuum relief) valve for atmospheric storage tanks per API 2000 / ISO 4126. Uses the subcritical (subsonic) gas flow formula with the P2/P1 pressure-ratio term — essential at low tank pressures. Formula: G = P1·√((2k/((k−1)·R·T·Z))·((P2/P1)2/k−(P2/P1)(k+1)/k)).
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Includes the P2/P1 pressure-ratio term — at low tank pressures (kPa gauge) flow stays subsonic, unlike safety-valve critical formulas.
G = P1·√((2k/((k−1)·R·T·Z))·((P2/P1)2/k−(P2/P1)(k+1)/k)) per ISO 4126, with R = 8314/M.
Breather-valve discharge coefficient (between liquid 0.4 and gas 0.8 safety valves).
Kp accounts for the allowable overpressure at the set point (21% typical).
Covers thermal inbreathing/outbreathing and liquid movement for atmospheric tanks.
Set pressure in kPa gauge — far below safety-valve MPa range; pressure ratio P2/P1 close to 1.
Input required relief capacity Q (kg/h) and pressures P1, P2 (kPa abs).
Enter molecular weight M, compressibility Z, temperature T and k.
Set Kdr (0.65), Kp overpressure correction and end configuration.
Read mass flux G, required area Ac, diameter do and verified capacity W.
Enter your parameters and get instant sizing and pass/fail results.
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