Calculate the required time to get a shut-in test on gas transmission lines, and approximate a maximum acceptable pressure loss for new gas lines

These two rules may prove helpful for air or gas testing of gas transmission lines. They are not applicable for hydrostatic tests. Values of the constants used by different transmission companies may vary with economic considerations, line conditions and throughput.

To determine the minimum time required to achieve a good shut-in test after the line has been charged and becomes stabilized, use the following formula:

Hm = (3 * D2 * L) / P1

Hm: Minimum time necessary to achieve an accurate test in hours
D: Internal diameter of pipe in inches
L: Length of section under test in miles
P1: Initial test pressure in pounds per square inch gauge

Example: how long should a 14-mile section of 26-inch (25.375-inch ID) pipe line be shut-in under a 1050 psig test pressure to get a good test?

Hm = {3 * (25.375)2 * 14} / 1050 = 25.75 hours or 25 hours and 45 minutes

When a new line is being shut in for a test of this duration or longer, the following formula may be used to evaluate whether or not the line is "tight":

Pad = (Ht * P1) / (D * 949)

Pad : Acceptable pressure drop in pounds per square inch
Ht : Shut-in test time in hours
D : Internal diameter of pipe in inches
P1 : Initial test pressure in pounds per square inch gauge

Example: What would be the maximum acceptable pressure drop for a new gas transmission line under air or gas test when a 44-mile section of 30-inch (29.25-inch ID) pipe line has been shut in for 100 hours at an initial pressure of 1100 psig?

Pad = (100 * 1100) / (29.25 * 949) = 4 pounds per square inch

If the observed pressure drop after stabilization is less than 4 psig, the section of the transmission line would be considered "tight."

Note: Corrections must be made for the effect of temperature variations upon pressure during the test.