What is drag reduction?
Frictional pressure drop(or drag) restricts the flow of liquid in a pipeline, limiting throughput and requiring greater amounts of energy for pumping. Flow Improvers are commonly referred to in the industry as Drag Reduction Agents, or DRA.
In most petroleum pipelines, the liquid flows through the pipeline in a turbulent manner. This is where DRA is performed. In this flow regime, the hydrocarbon molecules move in a random manner causing much of the energy applied to them to be wasted as eddy currents and other indiscriminate motion. Drag Reduction occurs by an interaction of the polymer molecules of the drag reduction chemicals with the turbulence of the flowing fluid. In order to understand how drag reducers decrease the turbulence, it is necessary to describe the structure of flow in a pipeline.
In the very center of the pipe is a turbulent core. It is the largest region and includes most of the fluid in the pipe. This is the zone of the eddy currents and random motions of turbulent flow. Nearest to the pipeline wall is the laminar sublayer. In this zone, the fluid moves laterally in “sheets”. Between the laminar layer and the turbulent core lies the buffer zone.
There is still much to be learned about polymeric drag reduction, as there is still much to be learned about the complex phenomenon of turbulence. The most recent research into this area tells us that the buffer zone is very important because this area is where turbulence is first formed. A portion of the laminar sublayer called a streak will occasionally move up to the butter zone. There the streak begins to vortex and oscillate, moving faster as it gets closer to the turbulent core. Finally the streak becomes unstable and breaks up as it throws fluid the core of the flow. This ejection of fluid in the core is called a burst. The burst creates the turbulence in the core and energy is wasted in different directions.
Drag reducing polymers interfere with the bursting process and prevent the turbulence from being formed, or at least reduce the degree of turbulence. The polymers seem to somehow stretch in the flow, absorb the energy in the streak and thereby prevent bursts. As such, drag reducing polymers are most active in the buffer zone.