What is the procedure to test the vapor tightness of a cargo tank ?

First you need to know the conditions under which a tank is considered to be vapor tight.

You need to calculate the Maximum Allowable Pressure Drop (PM).

This is done through the formula : PM=0.861 x Pi x (L/V) where :

 P  is pressure drop between initial and final pressure (inch/Wg)                                   P=Pi-Pf                                                                                                                   Pi is the initial pressure                                           (inch/Wg)                                      Pf is the final pressure                                             (inch/Wg)                                 LV is the maximum permitted loading rate                  (bbls/hr)                                      V is the volume of tanks in 100%                              (bbls)

If your system uses other units for the above formula you should convert it to inch/Wg

for a tank to be considered as vapor tight always should PM>P

Now, you should pressurize the tanks using the inert gas system to a level (below pressure relief is activated of course) and wait for 90 minutes.

You should record the initial pressure (when stopped introducing inert gas) and the pressure after passing 90 minutes after you stopped.

If you use the above formula and your result is satisfying the condition of PM>P than your vessel is considered Vapor Tight, otherwise not.