As a prerequisite to a storage tank slurry pump run, a tank operator identified a Lower Flammability Limit (LFL) Analyzer surveillance reading to the control room that was out of limits low. The reading was a negative zero % LFL indication (-0 % LFL). The tank operator roundsheet limits are 0 to 10% LFL. The "null" value (value read on analyzer when air with 0% LFL is drawn through the analyzer) as directed by the LFL Analyzer loop calibration procedure is set between 0 and 4% LFL.
To alert personnel to the buildup of potentially dangerous levels of explosive gases in the tank, a Combustible Gas Detection System is used to monitor and analyze sample air drawn from the tank vapor space. This system consists of a sensing element, a 4-20 milliAmp direct current (mADC) output transmitter and a single channel controller. The indicating range of this unit is 0-100% LFL indication which corresponds to 0-100% of span and 4-20 mADC transmitter output.
A review of vendor documentation found that the tank analyzer transmitter output can be affected by ambient temperature changes. The manufacturer specifies a zero shift of 0.02% LFL indication per degree celsius change. Vendor documents also stated that a transmitter output signal of less than four milliamps (+/- 2%) is displayed as a negative % LFL reading. (Whenever the transmitter output is less than 4.05 mADC, it is displayed as a negative % LFL indication.)
The tank LFL Analyzer sensor and transmitter are located in an insulated cabinet in which the temperature is controlled at a nominal 37.8 degrees celsius. When the analyzer "null" value was adjusted to correspond to a 4.091 mADC transmitter output, the cabinet doors were open. With the cabinet doors open, the transmitter was subjected to the ambient outside temperature, which at that time was approximately 22.8 degrees celsius. When the negative zero indication was reported, the cabinet doors were closed and the cabinet temperature was being controlled at 37.8 degrees celsius. Due to the specified zero shift per degree celsius change, the 15 degree change in temperature resulted in the transmitter milliamp output decreasing enough to cause a negative zero meter indication.
This incident highlights the need to ensure that the performance of special procedures does not place facility equipment in a condition that could lead to entry into a LCO.