scope:

Description of Driving Event:

The present method of annual calibration of fuel and oxidizer storage area relief valves involves removal of the valves, thus opening the tank to the atmosphere. Large amounts of toxic vapor may be released. During one operation for the N2O4 storage tank relief valve, a greater than expected quantity of N2O4 vapor released (28 gallons compared to the one gallon expected). The extensive dispersion of the vapor cloud exceeded the predesignated clear area of 700 ft. Downwind. Also, several personnel were exposed to the vapor due to unrelated circumstances.