Here'* a math problem for people that like math. Say you have a pipe that is 12 points 9 inches inside diameter... 3000 feet of it is filled with liquid and needs to be drained through two 3/4 quarter inch lines. what is the total volume of liquid that will be drained and how long will it take using only gravity.
(12.9/2/12)**2 x 3.14159 x 3000 = 2722.9 cubic feet
x 7.48 gallons per cubic foot = 20,367 gallons
The time is just a number I made up. It can't be calculated since you don't know the slope of the pipe. If it is sitting perfectly flat it will never completely drain. If you stand it straight up it will drain in 2 minutes.