Understand signature plot generalizations

The Line Balance Signature Plot trends a combination of the Linepack/Tankage Delta Volume change (X-axis) and the Meterin/Meterout Delta Volume (Y-axis). Additionally, the plots are color-coded by time. For example, the last 20 minutes are plotted in red, the last 20 to 40 minutes in green, and the last 40 to 60 minutes in light blue.

The middle horizontal line represents "zero" loss/gain in volume. Any point (or line) above the middle line indicates that more volume has been metered "out" of the system than has been metered "in." Any point below the middle line indicates more volume has been metered "in" to the system than has been metered "out."

The middle vertical line represents "zero" loss/gain of Linepack. Any point to the right of the middle line indicates a gain in Linepack within the system. Any point to the left of the middle line indicates a loss in Linepack within the system.

If we split the plot into four quadrants—Upper Left, Upper Right, Lower Left, and Lower Right—we can make some generalizations about each quadrant that may simplify the task of reading the plot.

Upper Left – Line Pack

Represents a gain in Delta Volume and a loss in Linepack within the system. We can reasonably assume that, if the line is draining down, there will be a gain in the volume metered out of the system.

Upper Right - Error

Represents a gain in Delta Volume and a gain in Linepack within the system. This is generally improbable for most liquid products. There may be a meter or instrument failure contributing to erroneous readings.

Lower Left – Leak

Represents a loss in Delta Volume and a loss in Linepack. Since this instance resembles a leak, it should be thoroughly investigated.

Lower Right – Pack

Represents a loss in Delta Volume and a gain in Linepack. We can reasonably assume that, if the line is packing up, there is more volume metered in to the system than metered out.