2Phase flow.pdf

ME 437PIPELINE ENGINEERING

CHAPTER VIII

TWO PHASE FLOW & SOLID TRANSPORTATION BY

PIPELINES

Prof.Dr.O.Cahit ERALP

Pipeline Engineering-ME 437Prof. Dr. O. Cahit ERALP

VIII.1. Two Phase Flow

All of the discussion so far has been confined to single phase flow i. e. flow of either gas or liquid.

There are situations in which are economically advantageous to transport both a gas and a liquid in a single pipeline simultaneously.

This is called 2-phase flow.



An example 2-phase flow involves certain off-shore oil operations, where it is extremely expensive to separate the liquid and gas phases in deep water. As the technology advances two-phase flow is likely to become common place in situations such as this. Two-phase flows may occur over a wide range of liquid to gas ratios and manifest a variety of flow patterns (Fig.IX.1).



Figure VIII.1 2-Phase Flow Patterns



Two phase flow technologies is much more complex than single-phase flow. However research suggest that the design of such systems is possible.

Experimental studies have revealed that 2-phase -flow shows larger drops in pressure compared to single-phase flows.



Two components of pressure drop are suggested in literature for 2-phase flows :

1. The pressure drop due to friction which increases with increase in gas flow rate. This is the only component in horizontal lines.

2. The pressure drop due to the head of liquid in inclined lines. Practically all of this pressure drop occurs in the uphill sectionof the pipeline.


VIII.2 Transportation of Solids by Pipelines

Another form of two phase pipeline flow involves the transportation of solids in a liquid or gas medium. Solids pipeline transportation has been confined to manufacturing, mining, and construction activities. Solids pipeline have most often been used for loading and unloading operations, in plant movement of materials and transportation of construction materials for short distances. A coal transportation pipeline of 170 km from mines to a power station exists in Ohio.



In order for solids to be transported successfully and economically by pipeline, the following conditions must generally be satisfied:

1. The solid material should not react in any undesirable way with the carrying of fluid or become otherwise contaminated within pipeline system.

2. Attrition (abrasion,erosion,wear) during transport should either be beneficial or its effect be negligible on subsequent operations.



3. The top particle size should be such that it can be handled in commercially available pumps, pipes and preparation equipment.

4. The solid material should mix easily with, and separate easily from, the carrying fluid at the feeder and discharge terminals respectively.

5. The solid material should not be corrosive, or become so, in the carrying fluid.



Solid pipeline flow may occur in at least four flow regimes:

1. Flow as a homogenous suspension.

For velocities experienced in most pipeline applications, this regime involves suspension of particles of diameter less than 30 microns.Provided the flow is turbulent, the suspension flows like a homogenous fluid. In this case, head losses may be computed as previously described using density and viscosity values corresponding to the suspension.



2. Flow as a heterogenous suspensionIn a heterogenous suspension, the concentration of particles at a cross section is not uniform. This type of flow involves transportation of particles slightly larger than those which flow in a homogenous suspension. In heterogenous transport, the solid particles travel with a velocity slightly less than that of the fluid.



3. Flow by SaltationIn solids pipeline flow involving relatively large particles and flow velocities, particles collect at the bottom of pipe and form a stationary bed. There is virtually no movement of solids in this case. At higher velocities, movement of particles at the interface occurs, called Saltation.



4. Flow with a moving BedAt high liquid velocities particles of large diameter may slide forward along the bottom of the pipe in a single mass or as a moving bed. This movement occurs at a much lower velocity than that of the fluid.



Which of these flow regimes occurs in a given instance will depend on the mean velocity, particle size, pipe diameter, and specific gravity of the material.

The effect of velocity and particle size is illustrated in Figure VIII.2.



Figure VIII.2 The effect of velocity and particle size



The Location of boundaries separating various regimes for a given case will depend on the nature of the solids-fluid mixture.

Basically, the problem of designing a solids pipeline is to find a flow velocity which will prevent setting, yet minimise the friction loss.

At the same time the percentage of solids and diameter of pipe must be sufficiently large to transport the required tons per day.



The minimum of critical carrying velocity and friction losses in a pipeline transporting fluid-solids mixtures are dependent on the following factors. 1. The density and /or percentage solids by

weight of mixture 2. The specific gravity of solids relative to that

of the transporting fluid3. The gradient of the pipeline4. The presence of slimes and/or colloidal

materialFriction losses are influenced by interior pipe surface (type of pipe) and gradation (distribution of various diameter particles) of the solids being transported.


VIII.2.1 Definition of Slurry and Sludge

The terms sludge and slurry are used for systems which generally consist of more than one phase such as a solid suspended in a liquid or two immiscible liquids of different densities

“Slurry” refers to a relatively thin or watery, hence dilute suspension.

“Sludge” denotes a heavy phase concentrated suspension, or simply a very viscous fluid.


VIII.2.1 Definition of Slurry and Sludge

Either word implies the existence of a substance which can be made to flow through a piping system but which is not a simple, homogeneous, Newtonian fluid such as water.


VIII.2.2 Slurry Characteristics

Slurry pipeline systems are efficient and reliable transportation modes. Flow of mixture of solids and liquids in pipes differs from flow of homogenous liquids in several important ways. With liquids, the complete range of velocities is possible, and the nature of the flow (laminar, transition and turbulent) is defined by the physical properties of the fluid and system.With slurries, two additional distinct flow

regimes and several more physical properties are superimposed on the liquid system.



The two regimes of slurry flow involve:

1. Homogeneous slurries.2. Heterogeneous slurries.



Design of equipment for handling slurries is based on the type of slurry involved.

The four basic kinds of slurries are:

Settling slurries (heterogeneous),Nonsettling slurries behave as homogeneous non-Newtonian fluids (homogeneous),Stabilized slurries,Slurries that show thixotropic properties(generally nonsettling).



For, the minimum transport velocity and settling (heterogeneous) slurries the pressure gradient is found by empirical correlations.

For slurries that behave as homogenous non-Newtonian fluids, again empirically derived correlations predict the laminar/turbulent transition and the pressure gradient.



Stabilized slurries contain large particles to be conveyed. These particles are supported by a dense or heavy medium which consists of much finer particles and which impart non-Newtonian shear thining flow behavior to the heavy medium.Shear-thinning media is highly suitable for transporting coarse particles. In low shear region near the center of the pipe, the apparent viscosity is high and the settling velocity of the suspended particles is either low or zero if the medium has a sufficiently large yield stress. Near the pipe wall, shear rates are high, apparent viscosities are low, and in consequence, pressure gradients are not huge.



When the solid particles are homogenously distributed in the liquid media, slurries often exhibit a non-Newtonian rheology (i.e., the effective viscosity is not constant, but varies with the applied rate of shear strain.)

Concentration gradients exist along the vertical axis of a horizontal pipe even at high flow rates; i.e. the fluid phase and solid phase retain their separate identities. Thus, the designer will often be faced with defining the dominant characteristic of a slurry.


VIII.2.3 Design Considerations

VIII.2.3.1 Critical Velocity

The depositional critical velocity is directly related to the settling velocity of the coarser particles in a heterogeneous slurry and the degree of turbulence in the pipe

Homogeneous and heterogeneous slurries have entirely different critical-velocity characteristics



Figure VIII.2 The effect of velocity and particle size



On the chart, top curve illustrates the deposition critical velocity typical of heterogeneous slurries.. Therefore, it increases with increasing particle size or specific gravity, and with increasing slurry concentration or viscosity.

Deposition velocity generally exhibits an increase proportional to the square root of pipe diameter.



Bottom curve shows viscous-transition critical velocity, which is a characteristic of homogeneous slurries.The design of a system for operation below the transition critical velocity is acceptable for truly homogeneous slurries. Here, no turbulent flow exists to suspend even small amounts of heterogeneous particles.Slurries are broadly classified as being in the homogeneous or heterogeneous regime, each regime having distinctive characteristics.



There are many correlations found in literature about determination of homogeneous or heterogeneous flow.Two new velocities are described as a function of

solid concentration, pipe diameter, and one or two particle parameters.

(Durand correlation)where

Vc: Critical velocityFL: Durand and Condolios factor D: Inside diameter of pipeS: Ratio of solid to liquid density

( ) 2/1)1.(..2. −= SDgFV Lc



If the design velocity is less than the critical velocity, the flow is choked in the pipe.

(Newitt correlation)where VT: Transition velocity

w: Settling velocityThis Equation describes the transition from heterogeneous to homogeneous flow.

3/1)...97.50( wDgVT =



If transport velocity of slurry is greater than or equal to the transition velocity, the flow is homogeneous. Otherwise, the flow is heterogeneous.


VIII.2.4 Settling Slurries

When handling settling slurries, the main design problem is predicting a design velocity high enough so that there is no possibility of blockage, but not so high that the pressure gradient and wear rates are excessive.The flow of settling slurries in horizontal pipes can be classified into various flow regimes.These flow regimes refer to the in-situ vertical solid concentration profile and whether all solids are suspended or a proportion is conveyed as a sliding bed along the bottom of the pipe. At high mean velocities it may be possible to convey some coarse slurries, but it is usually not economic to operate pipelines carrying high settling velocities



In order to predict the design velocities for horizontal pipe flow, there are a number of definitions for design velocity.

Sliding-bed velocity: This is the velocity at which the shearing forces in liquid are just sufficient to move particles that lie on the floor of the pipe. This is normally an inefficient method of transporting solids, but it may be the mechanism by which solids are carried at high concentrations.

Saltating velocity: At this velocity, particles are repeatly picked up by the liquid, from the bed and deposited further along the pipe.



Suspending velocity: This is the lowest velocity at which all the particles are picked up and remain in suspension. This velocity is used for designing most solid transportation pipelines.

Deposit velocity: The velocity at which particles start to settle out as the flow velocity is lowered. Here, particles may settle to a static or a sliding bed. This velocity is not the same as the suspending velocity.

Velocity for homogeneous flow: This is the velocity at which the particles become evenly distributed throughout the pipeline.


VIII.2.5 Non Settling Slurries

Solid-liquid mixtures that settle can be treated as nonsettling, if the settling rate is low. These slurries can be treated as single-phase homogeneous fluids.


VIII.2.6 Stabilized Slurries

For the hydraulic transportation of settling slurries containing large particles, relatively high flow velocities are required to prevent pipe blockage. In addition, pipe wall wear rates are high and specific energy requirements (the energy consumption per unit mass of solid transported per unit distance) are usually much in excess of those for the hydraulic transport of nonsettling slurries. There are a number of ways to reduce this specific energy requirement. Two important methods are “dense phase conveying” or alternatively, by both increasing the density of carrier fluid and imparting the non-Newtonian shear thinning property to the suspending medium through the presence of fine particles.



Dense Phase Conveying: This is done by increasing the solid concentration in the pipe to such an extent that the flow within the pipe resembles that of a sliding bed occupying practically the entire pipe cross-section.

The mean slurry velocity can be over a wide range because the coarse particles are not being supported by numerous particle to particle contact throughout the slurry.



Use of a Heavy Medium: Large particles, which would normally settle out rapidly in low viscosity carrier liquid such as water can be incorporated into a ‘heavy medium’ which is usually a homogeneous suspension of collodial particles in liquid. These collodial particles may often be flocculated (yumaklaşmış), resulting in the carrier medium possesing shear-thinning non-Newtonian flow property. Such a property can reduce or almost eliminate the settling of large particles as a result of the medium viscosity. At the relevant low shear rates, while, at the high shear rates close the pipe wall, the viscosity of the carrier liquid and of the slurry as a whole (including the large particles) is reduced because of shear-thinning behavior.


VIII.2.7 Thixotropic Slurries

These are the slurries that possess strong interparticle attraction and form a gel when left to stand in a pipeline.The start-up pressures are very much greaterthan normal operation pressure. A method for predicting start-up pressure and the time taken for the gelled material to be expelled from the pipeline has been developed according to the generalized Bingham fluid.

Documents

2Phase flow.pdf