Gas blowdown time calculation

Gas blowdown time calculation is used to estimate the time required to evacuate gas from a section of gas pipeline. In emergencies or during routine testing in oil and gas industry gas blowdown time is critical part pf safety consideration. objectively the gas pipeline is isolated by closing the mainline gas pipeline.

The pipe size required to blowdown a section of pipe will depend on the gas gravity, pipe diameter, length of pipe section, the pressure in the pipeline, and the blowdown time. The equations below are recommended by American gas association (AGA).

Gas blowdown time equation

In US customary units

$T\ =\ \frac{0.0588\cdot P_{^i}^{\frac{1}{3}}\cdot G^{\frac{1}{2}}\cdot D^2\cdot L\cdot F_c}{d^2}$

Where;

T = blowdown time, min
Pi = initial pressure, psia
G = gas gravity, dimensionless (for air = 1)
D = Main pipe inside diameter, in
d = blowdown pipe inside diameter, in
L = pipe section length, mile
Fc = chock factor (See table below) 

In SI units

$T\ =\ \frac{0.0886\cdot P_{^i}^{\frac{1}{3}}\cdot G^{\frac{1}{2}}\cdot D^2\cdot L\cdot F_c}{d^2}$

Where;

T = blowdown time, min
Pi = initial pressure, kg/cm2
G = gas gravity, dimensionless (for air = 1)
D = Main pipe inside diameter, mm
d = blowdown pipe inside diameter, mm
L = pipe section length, km
Fc = chock factor (See table below)