Blowdown Calc Formulae
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Sizing Calculations The sizing that can be done for a blowdown valve usually involves calculating either: • The total time required to blowdown a fixed volume of gas from one pressure to another • Calculating the valve flow coefficient (Cg) in order to blowdown a given volume in a fixed period Both of the calculations use the same formula it is just rearranged to solve for the required variable. The formulae are as follows: Calculations A. Critical Flow (P f > Pc) _____ C g = (93V/t) (√G-TZ ) Ln (P i /P f ) V = actual enclosed volume of piping or vessel (ft 3 ) t = time to blowdown (min) G = specific gravity T = temperature degrees R (°F + 460) Z = compressibility factor P i = initial pressure (psia) P f = final pressure of enclosed volume Pc = pressure below which letdown to flare system is not sonic B. Subcritical Flow (P f < Pc) If P f < P c , the following equation for subcritical flow must be used in addition to the one above: __ _ __ C g = (3.133VC 1 ÷ t) (√G-TZ ) Ln [(√P c + √P c - P a ) ÷ (√P f + √P f - P a )] V = actual volume (ft 3 ) t = time to blowdown (min) G = specific gravity T = temperature degrees R (°F + 460) P i = initial pressure (psia) P f = final pressure (psia) of enclosed volume P c = critical pressure (psia) P a = atmospheric pressure C 1 = valve Cg/Cv Critical pressure can be calculated by: Pc = P a / (1-(C1 / 59.64) 2 P a = atmospheric pressure C 1 = valve Cg/Cv
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Blowdown Calculations
Transcript of Blowdown Calc Formulae
Sizing Calculations The sizing that can be done for a blowdown valve usually involves calculating either:
• The total time required to blowdown a fixed volume of gas from one pressure to another
• Calculating the valve flow coefficient (Cg) in order to blowdown a given volume in a fixed period
Both of the calculations use the same formula it is just rearranged to solve for the required variable. The formulae are as follows:
Calculations A. Critical Flow (Pf > Pc)
_____
Cg ==== (93V/t) (√√√√G-TZ ) Ln (Pi/Pf)
V = actual enclosed volume of piping or vessel (ft3)
t = time to blowdown (min) G = specific gravity T = temperature degrees R (°F + 460) Z = compressibility factor Pi = initial pressure (psia)
Pf = final pressure of enclosed volume
Pc = pressure below which letdown to flare system is not sonic B. Subcritical Flow (Pf < Pc)
If Pf < Pc, the following equation for subcritical flow must be used in addition to the one above:
__ _ __
Cg ==== (3.133VC1 ÷÷÷÷ t) (√√√√G-TZ ) Ln [(√√√√Pc + √√√√Pc - Pa) ÷÷÷÷ (√√√√Pf + √√√√Pf - Pa)]
V = actual volume (ft3)
t = time to blowdown (min) G = specific gravity T = temperature degrees R (°F + 460) Pi = initial pressure (psia)
Pf = final pressure (psia) of enclosed volume
Pc = critical pressure (psia)
Pa = atmospheric pressure
C1 = valve Cg/Cv
Critical pressure can be calculated by:
Pc = Pa / (1-(C1 / 59.64)2
Pa = atmospheric pressure
C1 = valve Cg/Cv
