scope:

Description of Driving Event:

On January 4, 1995, at approximately 8:50 a.m., two technicians at WSTF were overcome and collapsed from breathing in an oxygen deficient atmosphere that formed during shutdown operations following a transfer of liquid nitrogen (LN2) from a vendor supply tanker to a WSTF storage tanker. This transfer operation was normally controlled by monitoring a tanker vent line. Flow from this vent line indicated the tank was full, and the operation was suspended. There is also a liquid level indicator and a pressure gage on the WSTF tanker for monitoring purposes.

On the morning of the mishap, conditions (high humidity and still air) existed which caused a larger than normal water vapor cloud to form during continuous venting of cold gaseous nitrogen (GN2) during the filling operation. This venting is required to keep the WSTF tanker at a lower pressure than the vendor tanker during the fill operation. The cloud prevented the normal visual monitoring of the vent line, the liquid level indicator, and the pressure gauge. The technicians, instead of positioning themselves to be able to monitor the vent line and gauges, located approximately 30 feet away from the tanker to both remove themselves from the heavy vapor cloud and also to remain out of the rain. During the operation, the technicians would periodically approach the tankers to monitor the liquid level indicator and the pressure gauge. The vent line was never observed due to being obscured by the vapor cloud. After about 50 minutes, the weather got worse and the technicians decided to suspend the operation due to weather and because they thought the tanker was nearly full. Data collected later showed that the tanker was completely full, and that LN2 was being released through the vent line instead of GN2. As the first technician attempted to close the vent and transfer valves, he was overcome and collapsed due to the lack of oxygen. The second technician, who was monitoring the activity, also was overcome and collapsed due to the lack of oxygen. A few moments later, vendor personnel approached and eventually remove both technicians from the area. Later investigation indicated that LN2 flow from the vent line coupled with adverse weather conditions created an oxygen deficient atmosphere in the area.

A hazardous atmosphere can result in some environmental conditions when using overflow indications to monitor and control transfer operations of cryogenic fluids.