A deficiency was discovered in the application of a hydrogen sensor in the Rotary Mode Core Sampling (RMCS) portable exhauster. The sensor is installed in the flow stream of the exhauster designed to be used with a RMCS truck for core sampling of watch list tanks, and is part of the flammable gas detector system. During the previous week, a quarterly calibration of the sensor, per maintenance procedure, was attempted by Characterization Project Operations (CPO) technicians. Ambient temperatures during the sensor calibration were approximately 20 to 30 degrees F. Inconsistencies in calibration results and the failure of the sensor to meet the response-time calibration requirement lead to the conclusion that the unit could not reliably perform its safety function at low ambient temperatures. The hydrogen sensor is an electrochemical cell used to initiate alarms and shut down the RMCS system in the case of a hydrogen release in the tank being sampled. The sensor is from the same vendor and category of sensor used in the tank farm Standard Hydrogen Monitoring System (SHMS) cabinets. However, the SHMS cabinets are climate-controlled, whereas the RMCS application does not provide a climate-controlled enclosure for the sensor. The RMCS exhauster flammable gas detector is required to operate in outdoor temperatures down to -20 degrees F.
The hydrogen sensor is one of two independent sensors in the exhauster flammable gas detector. The other sensor has not experienced these calibration problems. The flammable gas detector is not required for push-mode core sampling. During further investigation of the RMCS exhauster hydrogen detector system, a deficiency was discovered in the application of low-pressure differential transmitters. The low-pressure differential transmitters are qualified for use in a Class 1, Division 1, Group B environment. The transmitters are rated for input voltages of 12 to 36 volts per the vendor's catalog. However, during the process of reviewing the test reports, it was discovered that to be qualified for use in a Class 1, Division 1, Group B environment, the transducer must be configured in a specific fashion. This configuration is defined in the control drawing, which requires that the "associated apparatus" must provide no more than 20 volts. However, the intrinsic safety transmitter supply unit used in the exhauster hydrogen detector system could allow a maximum amount of 28 volts open circuit. Characterization Equipment Engineering concluded that this condition could result in a non-compliance with the intrinsic safety requirements of the National Electrical Code, Article 504, and ANSI/ISA-RP12.6 "Installation of Intrinsically Safe Systems for Hazardous (Classified) Locations."
The direct cause of the deficiencies is an inadequate design. The functional design criteria require that the system operate in a -20° F to 115° F environment. Although the vendor specifications for the cell require operating temperatures of 70° F to 120° F, the system design did not provide thermal protection for the cell.
- Safety Systems
- Measurement / Sensing Device
Parallel-path design activities require an increased level of management oversight and control to mitigate the risks inherent in this process.
Schedule pressure cannot be allowed to compromise the integrity of the design process.
Turnover of personnel responsibilities that may be needed during the design process must be formal and controlled.