To calculate the leakage in MMcf (Million Cubic Feet) per year at a base pressure of 14.4 psia and a temperature of 60°F, take as a basis a one mile section of gas line under one hour test.

Use the formula:

Ly = 9 * D2 * [P1 / (460 + t1) - P2 / (460 + t2)]

Where:
D is the inside diameter of the pipe in inches.
P1 and t1 are the pressure and temperature at the beginning of the test in psia and degree Fahrenheit respectively.
P2 and t2 are the pressure and temperature at end of the test.

Note: The error in using this formula can be about 4%.

Example: A section of a 10-inch pipe line one mile in length is tested using the pressure drop method. Inside diameter of 10-inch nominal pipe is 10.136 inches. Pressure and temperature at the start of the test are 190.5 psig and 52°F. After one hour the readings are 190.0 psi and 52°F. Calculate the leakage in MMcf/year.

The atmospheric pressure for the area is 14.4 psia. Thus

P1 = 190.5 + 14.4 = 204.9 psia
P2 = 190.0 + 14.4 = 204.4 psia

Ly = 9 * (10.136)2 * [204.9 / (460 + 52) - 204.4 / (460 + 52)] = 0.903 MMcf per mile per year