Slack line flow is a phenomena in which a pipeline transporting a liquid product develops vapor bubbles at points at which the pipeline pressure falls below the vapor pressure of that liquid. This flow condition is very common in liquid pipelines. Most liquid pipelines fall into one of the following categories:
Slack line flow occurs all of the time in some section of the pipeline.
Slack line flow occurs occasionally during normal pipeline operations.
Slack line flow occurs only during abnormal pipeline operations.
Experiences installing real time simulation software on a number of liquid pipelines over the last 15 years have included the following:
On one, slack line flow existed almost all of the time. However, until the real time simulation software was installed, the pipeline operators were unaware of its occurrence. In this case, the total slack volume (volume of vapor bubble) was only 50 to a few hundred barrels in size.
On another, slack line flow occurred only during some upsets. Again, the pipeline operators were unaware of this condition until real time simulation so&are was installed. Since the simulation software was not designed to handle the slack line flow, simulations during those periods were inaccurate.
On others, slack line flow was expected as a normal part of the operation. On one large pipeline, slack volumes are in the tens of thousands of barrels.
Slack line flow behavior is of interest to pipeline operators for a number of reasons:
Frictional head loss in slack regions is much greater than in tight line regions.
Slack line regions significantly change the hydraulics of the pipeline operations, particularly the behavior of transients.
The collapse of slack line regions may cause pressure surges that must be considered when developing operational procedures to avoid damaging the pipeline.
Slack line flow behavior poses a significant challenge to developers and users of liquid pipeline simulators. On one hand, it greatly affects the transient behavior of the pipeline; on the other hand, simulation of this behavior is much more difficult than the simulation of tight line flow and is subject to many more uncertainties than is tight line simulation.
It is useful to consider what is occurring during slack line flow. From a thermodynamic perspective, when the pressure falls below the vapor pressure, a vapor bubble is formed and the fluid pressure does not drop below the vapor pressure. From a mechanical point of view, slack line flow approximates open channel flow. An open channel is one in which the fluid is not completely enclosed by solid boundaries and therefore has a free surface whose pressure remains at atmospheric pressure (or vapor pressure in this case). The fact that the pressure on the free surface remains constant requires, at steady flow, that the rate of change of potential energy of the fluid is exactly offset by the frictional work of the "falling" fluid against the fluid surroundings.