Pipeline Monitor (PLM) leak patterns

The following sections describe various leak signatures and the conditions under which they occur.

Upstream Pressure / Downstream Flow Control

When a leak occurs in a steady state, the leak signature is similar to the one shown in the figure below, assuming that the leak occurs away from either of the flow measurement points, and that the upstream pressure and downstream flow are controlled. The signature line traces from point O, at the onset of a leak, to point F, at a new steady state where the leak does not cause additional transient effects. It is assumed that the leak is small enough that the pipeline is still controllable.

Point O, plotted at the origin, indicates that the section is in a steady state before the leak occurs. As described previously, in a steady state, the volume difference between the flows at the inlets and outlets is at or near zero. The linepack change in the pipeline section is also at or near zero. When a leak occurs, the pressure in the section drops and the line is unpacked before the volume difference is noticed, unless the leak occurs close to a flow measurement point. As a result, the signature line moves from point O to point A. As the transient pressure induced by the leak propagates from the leak site to both upstream and downstream locations, pressure drops further until it reaches a flow measurement point. At the flow measurement point, the upstream flow increases, causing the signature line to move to point B. The section is still unpacking, but the rate may be reduced. After reaching point C, the inlet flow compared to the outlet flow continues to increase and the unpacking rate decreases. The signature line settles at point F, where more volume flows into the section than out of it, and the unpacking stops. In other words, the pipeline reaches a new steady state at point F, with an excess of volume flowing into the section.

Upstream / Downstream Pressure Control Only

If the upstream and downstream pressures are kept constant, then in the event of a leak, the upstream flow increases and the downstream flow decreases. The linepack effect may be less significant compared to the flow difference, unless there are pressure measurements between the two controlling points. As a result, the leak signature is as shown in the figure below.

Leak Occurs during a Packing Operation

If a leak occurs during a packing operation, the leak signature looks like the one shown in the figure below. The signature line starts from the packing zone in the fourth quadrant and settles at point F when the line reaches a steady state.

If a leak occurs during an unpacking operation, the leak signature looks like the figure shown below. The signature line starts from the packing zone in the second quadrant and settles at point F when the line reaches a steady state.