Pipeline Monitor (PLM) alarm threshold line

Even in the absence of a leak, the volume difference and linepack change are not always exactly the same because of inaccuracies related to AVEVA Enterprise SCADA and pipeline system operations, errors in measurements, commodity properties, and linepack calculations. As shown in the figure below, the signature line that represents the normal region of pipeline operation is denoted as a dotted diagonal line through the origin of the X-Y coordinate. The tolerable region for errors in volume difference and linepack change is shown as a shaded area around the diagonal line. Errors can be interpreted as a volume imbalance, and the width of the shaded area in the signature plot indicates the magnitude of the imbalance that can be tolerated before an alarm is issued.

Note: Alarms can be generated for errors that exceed the tolerance band on both the lower-left side and the upper-right side of the signature plot. However, pipeline leaks are normally associated with errors on the lower-left side of the normal signature plot line. Because this section focuses on using the signature plot to detect leaks, a leak alarm threshold will be drawn only on the lower-left side of the tolerance region in all figures.

The distance of the leak alarm threshold from the normal signature plot line is chosen to avoid or reduce false alarms caused by errors in measurements and calculations, or problems related to operations.

The figure above shows a typical signature plot superimposed over the tolerable region. This signature plot is based on normal operations including packing and unpacking operations.