Drag reducers -- also known as Drag Reducing Agents, DRAs and Flow Improvers -- are any material that reduces frictional pressure during fluid flow in a conduit or pipeline. Using DRAs allows increased flow using the same amount of energy or decreased pressure drop for the same flow rate of fluid in pipelines. The current class of DRAs, such as CSPI’s range of LiquidPower™ Flow Improver products consist of long-chain hydrocarbon polymers that act as buffers along the pipe wall to decrease the amount of energy lost in turbulent formation.

Theory of Drag Reduction

Frictional pressure drop, or drag, is a result of the resistance encountered by flowing fluid coming into contact with a solid surface, such as a pipe wall. There are generally two types of flow; laminar and turbulent. The friction pressures observed in laminar flow cannot be changed unless the physical properties of the fluid are changed. The current class of DRAs does not change fluid properties and hence they are effective only in turbulent flow. In most petroleum pipelines, the liquid flows through the pipeline in a turbulent regime. Therefore, current DRAs can perform very well in most pipelines.

In a turbulent flow regime, the fluid molecules move in a random manner, causing much of the energy applied to them to be wasted as eddy currents and other indiscriminate motion. DRAs work by an interaction of the polymer molecules 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 turbulent flow in a pipeline. The image below shows a typical turbulent flow in a pipeline that has three parts to the flow. In the very center of the pipe is a turbulent core. It is the largest region and includes most of the fluid in the pipeline. This is the zone of the eddy currents and random motions of turbulent flow. Nearest to the pipeline wall is the laminar sub layer. In this zone, the fluid moves laterally in sheets. Between the laminar layer and the turbulent core lies the buffer zone.

Drag Reduction occurs due to suppression of the energy dissipation by turbulent eddy currents near the pipe wall during turbulent flow.

There still is much to be learned about polymeric drag reduction, as there still is much to be learned about the complex phenomenon of turbulence. Recent research into this area tells us that the buffer zone is very important because this is where turbulence is formed first. A portion of the laminar sub layer, called a “streak”, will occasionally move to the buffer region. 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 into the core of the flow. This ejection of fluid into the turbulent core is called a turbulent burst. This bursting motion and growth of the bursts in the turbulence core results in wasted energy.

Drag reducing polymers interfere with the bursting process and reduce the turbulence in the core. The polymers absorb the energy in the streak, like a shock absorber, thereby reducing subsequent turbulent bursts. As such, drag reducing polymers are most active in the buffer zone.

For an evaluation of whether CSPI’s drag reducing products can aid in your application, please contact your CSPI area representative.