A fatality occurred when a service company was performing a two stage nitrogen foam frac treatment. The rig-up required four nitrogen pumps manifolded together. In the process of beginning the second stage, a nitrogen pump failed to maintain prime on the boost pump. While attempting to regain prime, gas from the remaining pumps back-flowed into the nitrogen pump’s storage vessel. The relief system on the vessel was not capable of venting the volume of gas entering the vessel resulting in a catastrophic failure due to overpressurization.