A waste pretreatment tank operator was performing surveillance rounds on a tank and found the Composite Lower Flammability Limit (CLFL) Analyzer sample flow reading 1.4 cubic feet per hour (CFH). The Operational Safety Requirements (OSR) document required flow range is 1.5 CFH to 2.5 CFH. The Limiting Condition for Operation was immediately entered and the tank operator adjusted the flow into the required surveillance range.

The hydrogen and CLFL monitors are used to detect the presence of flammable gases in waste tanks vapor space. Maintaining the concentration of flammable vapors in the tank vapor space below flammability limits maintains tank integrity by preventing the possibility of tank deflagrations. The hydrogen monitors provide an automatic means to monitor flammable gas concentrations in waste tanks. They indicate % LFL locally in the respective tank valve houses, remotely in the control room Distributed Control System (DCS) console, and have alarm capability for certain conditions.

Later that day, the tank operator found the tank CLFL analyzer reading out of limits and the shift manager immediately entered a LCO, which included terminating the tank to waste transfer and allowing the tank operator to adjust sample flow. However, sample flow could not be adjusted within roundsheet limits. At 0945 hours, the tank operator obtained tank vapor space sample results using the alternate monitor (sample results reading was zero % LFL).

As a result of the investigation, the direct and root causes were attributed to "Equipment/Material Problem; Contaminant" - moisture in the sample line. Moisture comes from the waste tank vapor space and affects the operability of the CLFL analyzer. As moisture accumulates in the sample line, the operators are directed by the surveillance roundsheets to blow down the monitoring system. The tank to tank transfer that was in progress at the time of the occurrence contributed to the moisture in the tank vapor space, and the subsequent moisture loading of the tank CLFL sample filter.

Incident Date
Sep 20, 1999
  • Safety Systems
  • Measurement / Sensing Device
Damage and Injuries
Probable Cause
When Incident Discovered
Lessons Learned

This incident occurred due to moisture in the sample line monitoring system. Removing moisture from the sample line increases the reliability of the equipment. Further evaluations are being considered for improving system reliability.