Two-Phase Pipe Hydraulics and Pipe Sizing

This particular course entitled “Two-Phase Pipe Hydraulics & Pipe Sizing” under the specialization entitled “Design of Industrial Piping Systems” is mainly aimed at predicting the two-phase total static pressure drop in a given piping system when both gas and liquid flow through it concurrently. Pressure drops including heat transfer coefficients depend on two-phase flow regimes since two-phase patterns and local internal structure are different for different flow regimes. Therefore, the formation of various two-phase flow regimes in horizontal and vertical pipes is to be known to the designer, and at the same time, the influence of bend on the formation of two-phase flow regimes in upstream and downstream pipes should also be known. The presence of a bend is inevitable in the piping systems of a plant and its presence restricts the formation of certain two-phase flow regimes commonly found in individual horizontal and vertical pipes for the given flow rates of gas and liquid and pipe diameter. Surprisingly, bend allows the formation of slug flow regimes in both horizontal and vertical pipe runs of a piping system. This is a nerve-wracking issue for the designer since the slug flow regime harms the piping system and in some situations, the slug flow regime becomes the main cause of the failure of the piping system. Therefore, the designer should be cautious during the design of two-phase piping systems and avoid the slug flow regime formation at any cost while designing the two-phase piping system.

Looking into the severity of two-phase flow on the piping system integrity, the present course focuses on the formation of two-phase flow regimes in horizontal and vertical pipes and their identification based on gas and liquid flow rates using two-phase flow pattern maps. Next, the course focuses on the effect of bends on two-phase flow regime formation in both upstream and downstream pipelines as piping systems are made of connecting straight pipe runs using bends. From this discussion, the learner gets a fair idea about the formation of a certain type of two-phase flow regime, when it happens, and why it happens. Next, the two-phase terminologies are covered as these are frequently used in two-phase piping system design. The relationship among them is equally important in the design and hence, covered in the present course. These terminologies and their relations assist the learner in understanding, analyzing, and applying the various two-phase models to design the two-phase piping system.

Certain idealizations are to be made while dealing with the gas and liquid two-phase flow through the pipe. Single-phase is well-established, not two-phase. To take advantage of suggested single-phase correlations by the investigators, the two-phase models are developed by assuming liquid alone flows through the pipe with the two-phase mixture flow rate. This assumption introduces the error as it does not appeal the reality. Therefore, while developing the models a term called two-phase multiplier is introduced and made as a multiplication factor to the single-phase pressure drop, to predict the two-phase frictional pressure drop within the acceptable range. The developed models are popularly known as the Homogeneous Equilibrium Model, Separated Flow Model, and Drift Flux Model, and the present course is focused on these models. Various two-phase multipliers, methods, techniques, and void fraction correlations are covered in detail in this course. Finally, in this course, practical two-phase problems are considered to demonstrate the prediction of total static pressure drop which is a sum of two-phase frictional, accelerational, and gravitational pressure drops using the two-phase well-known models, methods, techniques, two-phase multipliers, and void fraction correlations and how closely they predict so that learner cannot face any hiccup while he/she designing the two-phase piping systems including single path and multi-path piping systems known as piping networks.