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What Is Flow Stratification? | Process Engineering Glossary
What Is Flow Stratification?
Flow stratification is a two-phase flow condition in horizontal or near-horizontal piping where the liquid and gas phases separate due to gravity, with the liquid settling to the bottom of the pipe and the gas flowing along the top, creating two distinct layers rather than a well-mixed flow. This stratified flow pattern occurs when gas and liquid velocities are both relatively low, providing insufficient turbulent mixing energy to keep the phases dispersed, and is one of several two-phase flow regimes that can develop depending on flow rates, fluid properties, and pipe orientation.
Flow stratification matters to process and piping design because the separated phases create uneven loading on the pipe, can cause inaccurate flow measurement, and can lead to water hammer or slug flow transitions if conditions change, making it a condition that warrants deliberate design attention rather than simple neglect.
Applications of Flow Stratification
Two-Phase Piping Design
Recognizing stratified flow conditions in horizontal two-phase piping is essential for correct line sizing, support design, and routing decisions that account for the uneven phase distribution.
Flow Measurement Location Selection
Flow transmitter locations in two-phase piping must account for flow stratification, since conventional single-phase flow measurement techniques can give inaccurate readings when phases are stratified.
Separator and Knockout Drum Inlet Design
Inlet piping to separators and knockout drums must manage the transition from stratified pipe flow to effective phase distribution inside the vessel for proper separation.
Benefits of Knowing Flow Stratification
Prevents design errors from single-phase assumptions. Recognizing that flow stratification creates fundamentally different conditions than single-phase flow prevents applying inappropriate design methods to two-phase piping systems.
Supports correct flow measurement approach. Understanding stratification effects on flow measurement supports selecting appropriate measurement technology and installation locations for two-phase service.
Enables prediction of potential slug flow transitions. Stratified flow can transition to slug flow if velocities increase, and anticipating this transition supports designing piping and supports for the more severe loading slug flow imposes.
Limitations to Consider
Flow regime prediction involves inherent uncertainty. Predicting the exact flow regime in a given piping system involves empirical correlations with some uncertainty, particularly near regime transition boundaries.
Stratification can cause uneven corrosion patterns. The liquid layer in stratified flow contacts only the bottom of the pipe, potentially causing localized bottom-of-pipe corrosion that wall thickness measurement programs should specifically target.
Drain and vent locations must account for phase separation. In stratified flow, drains must be at the pipe bottom and vents at the top to effectively remove the intended phase from the separated flow.
Flow Stratification FAQ
How does flow stratification relate to other flow regimes?
Stratification is one of several flow regimes in horizontal two-phase flow, with transitions to slug, annular, or dispersed patterns occurring as gas and liquid velocities increase.
How does flow stratification relate to Darcy-Weisbach pressure drop calculations?
Standard single-phase Darcy-Weisbach pressure drop calculations don’t apply to stratified two-phase flow, requiring specialized two-phase pressure drop correlations that account for the separated phase geometry.
How does flow stratification affect piping stress and support design?
The uneven liquid distribution in stratified flow creates asymmetric pipe loading that support spacing and primary stress calculations should account for in the affected piping sections.
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