Actual Flow to Normal Flow ,Normal Flow to Actual Flow and Important Useful Calculations

Actual Volume to Normal Volume and Vice Versa

The actual (measured) flow rate is normalized to the flow rate at a certain reference pressure (P) and temperature (T).    For Metric system, the "normal" condition is

Pref = 1 atm = 1.01325 bar = 101325 Pa

Tref = 0 °C = 273.15 k    For Standard (English) system, the "standard" condition is

Pref = 1 atm = 14.6959 psi

Tref = 70 °F = 529.67 °RIdeal gas assumes that volume is proportional to temperature and the reciprocal of pressure. The normalized volume flow rate becomes

Q = QN × (Pref / Pa) × (T / Tref)where Pa, the actual pressure, is the sum of surrounding atmospheric pressure and the gage pressure (Pa = Patm + Pg). The unit of flow temperature T must be either Kelvin (k) or Rankine degrees (°R).

(A) Convert Air Volumetric Flow (@ Std) to Vapor Volumetric Flow (@ Std)Vapor Volumetric Flow @ STD = Air Volumetric Flow @ STD × Vapor Temperature Correction Factor / Vapor Specific Gravity Correction Factor

[Eq.1]QStd,Vap = QStd,Air × √[(TStd,Vap+460) ⁄ (TActual,Vap+460)] / √[MWVap / MWAir]

whereQStd,Vap = Vapor Volumetric Flow @ STD (SCFH)QStd,Air = Air Volumetric Flow @ STD (SCFH)TStd,Vap = Vapor Temperature @ STD = 60 degFTActual,Vap = Actual Vapor Temperature (degF)MWVap = Vapor Molecular

WeightMWAir = Air Molecular Weight (28.96)

(B) Convert Vapor Volumetric Flow (@ Std) to Vapor Volumetric Flow (@ Actual)

Vapor Volumetric Flow @ PT = Vapor Volumetric Flow @ STD × [Actual Temperature / Standard Temperature] × [Std Pressure / Actual Pressure ]

[Eq.2]QActual,Vap = QStd,Vap × [(TActual,Vap+460) ⁄ (TStd,Vap+460)] ×[PStd,Vap/PActual,Vap]

whereQActual,Vap = Actual Vapor Flow (SCF)PStd,Vap = Vapor Pressure @ STD (14.7 psia)PActual,Vap = Actual Vapor Pressure (psia)

Example 1 :Nitrogen (MW=28) blanketing a tank. The calculated air flow @ std is 50,000 SCFH. The tank is at 1 psig and 68 degF when venting via PVRV occurred. Calculate the equivalent Nitrogen relief load at actual flow condition.

From [Eq.1]QStd,N2 = QStd,Air × √[(TStd,N2+460) ⁄ (TActual,N2+460)] / √[MWN2 / MWAir]

QStd,N2 = 50000 × √[(60+460) ⁄ (68+460)] / √[28 / 28.96]

QStd,N2 = 50463.22 SCFH

From [Eq.2]QActual,N2 = QStd,N2 × [(TActual,N2+460) ⁄ (TStd,N2+460)] ×[PStd,N2/PActual,N2]

QActual,N2 = 50463.22 × [(68+460) ⁄ (60+460)] × [14.7/(14.7+1)]

QActual,N2 = 47975.91 CFH

Example 2 :A tank feed from condensate stablizer. In case of gas blowby, flash vapor generated plus displaced vapor (MW = 35) is 60,000 CFH and tank is at 1.5 psig and 122 degF when venting via PVRV occurred. Calculate the equivalent Air flow at STD condition.

From [Eq.2]QAct,Vap = QStd,Vap × [(TAct,Vap+460) ⁄ (TStd,Vap+460)] ×[PStd,Vap/PAct,Vap]QStd,Vap = QAct,Vap × [(TStd,Vap+460) ⁄ (TAct,Vap+460)] ×[PAct,Vap/PStd,Vap]QStd,Vap = 60000 × [(60+460) ⁄ (122+460)] × [(1.5+14.7)/14.7]

QStd,Vap = 59078.48 SCFH

From [Eq.1]QStd,Vap = QStd,Air × √[(TStd,Vap+460) ⁄ (TActual,Vap+460)] / √[MWVap / MWAir]

QStd,Air = QStd,Vap × √[(TActual,Vap+460) ⁄ (TStd,Vap+460)] / √[MWAir/ MWVap]

QStd,Air = 59078.48 × √[(122+460) ⁄ (60+460)] / √[28.96 / 35]

QStd,Air = 68710.62 SCFH

Concluding RemarkAbove equations are pretty useful for conversion of air - vapor and actual-standard condition