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:

L_{y} = 9 * D^{2} * [P_{1} / (460 + t_{1}) - P_{2} / (460 + t_{2})]

Where:

D is the inside diameter of the pipe in inches.

P_{1} and t_{1} are the pressure and temperature at the beginning of the test in psia and degree Fahrenheit respectively.

P_{2} and t_{2} 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

P_{1} = 190.5 + 14.4 = 204.9 psia

P_{2} = 190.0 + 14.4 = 204.4 psia

L_{y} = 9 * (10.136)^{2} * [204.9 / (460 + 52) - 204.4 / (460 + 52)] = 0.903 MMcf per mile per year