A control room received a tank lower flammability limit (LFL) analyzer low sample flow alarm. The control room operator initiated the appropriate alarm response procedure and the facility entered limiting conditions of operation. At the time of the alarm, the facility was experiencing severe weather and the field operator was unable to investigate the alarm in the field. After the severe weather cleared, the field operator investigated the alarm and found the sample flow to be low and out-of-limits.
At the given facility, composite lower flammability limit (CLFL) monitors are used to detect the presence of hydrogen and other flammable gases in waste tank vapor spaces. Maintaining the concentration of flammable vapors in tank vapor spaces below CLFL levels maintains tank integrity by preventing the possibility of fires, deflagrations and explosions. The CLFL point of a combustible gas is defined as the minimum concentration of that particular vapor that is combustible in a dry air atmosphere. In gas detection systems, the amount of gas present is specified in units of % CLFL and is usually displayed on a meter in these units. The flammable gas of primary concern for the tank is hydrogen.
The flammable range of hydrogen is from 4.0% to 74.2% by volume in air. A hydrogen concentration of 4.0% in air is equal to 100% LFL. Hydrogen concentrations less than 4.0% will not burn, assuming it is the only source of fuel. 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 gas drawn from the tank vapor space. The indicating range of this unit is 0-100% LFL indication which corresponds to 0-100% of span and 4-20 mADC transmitter output.
The rotameter is a variable output device for measuring and controlling flow consisting of an orifice type valve seat and needle valve assembly. The valve seat is made of a resilient material and is machined to match the surface of the needle valve. Flow is controlled by moving the needle valve in and out of the valve seat. After flow moves through the valve seat, it enters a cone shaped chamber that is wider at the top. Graduations on the side of the chamber indicate flow (typically in scfm, standard cubic feet per minute, or cfh, cubic feet per hour). A weighted float rides on the cushion of gas or fluid flowing through the chamber. During this event, it was determined that the valve seat had worn down so that the shape of the orifice did not match the shape of the needle valve which caused irregular and uncontrolled flow.
Cause and effect can sometimes be predicted by observing abnormal behavior even when the behavior is within specifications. Operators log equipment data and inform shift management when specifications are exceeded or when unusual equipment behavior is noted. Engineering establishes trending when patterned behavior is noted. Rotameter sample flow failures are too erratic to establish a cost effective preventive maintenance program. Predictive maintenance establishes a method to predict imminent failure based on symptoms that may be displayed during normal (within specification) operation. Operations and Engineering will continue to assess abnormal equipment behavior that may be within specification, and implement prediction methods, like trending, when applicable. Maintenance will implement annual rotameter inspection to aide in rotameter operational assessment.