Accurate slurry modeling begins with a single decisive step — correctly classifying the slurry — and that decision shapes everything that follows, from friction loss and deposition velocity to pump sizing. Solid particles interact with the carrier fluid and pipe wall in ways that shift with velocity, concentration, and orientation, which is why the dependable shortcuts of single-phase flow no longer apply. FluidFlow meets this challenge with a dedicated framework of correlations and two-phase definitions built specifically for solid-laden flows. This guide serves as the practical companion to that toolset, helping engineers make sound modeling decisions at every stage and arrive at results they can confidently design and operate against.

Slurries are liquid-solid mixtures where solid particles are dissolved, suspended, or transported within a carrier fluid. They appear across a wide range of industries:

Unlike single-phase liquids such as water, slurries cannot be accurately modeled using standard Newtonian fluid assumptions. The solid content fundamentally alters the way the fluid behaves inside a pipe — affecting pressure drop, flow regime, and the risk of pipe blockage. FluidFlow provides a dedicated slurry modeling framework that handles these complexities through specialist correlations, two-phase definitions, and solids-aware inlet boundary conditions.

Slurry systems present several modeling challenges that do not exist for standard single-phase flows:

All of these factors must be addressed simultaneously to produce a reliable slurry system model.

The first and most critical step in any slurry modeling workflow is correct classification of the slurry type. FluidFlow applies a fundamentally different modeling approach to each class.

Non-settling slurries contain fine solid particles that remain uniformly suspended throughout the carrier fluid at industry-relevant flow rates. Their key characteristics are:

Settling slurries contain coarser solid particles that will settle to the pipe bottom within approximately one hour of becoming stagnant. Their key characteristics are:

When laboratory testing data is unavailable, industry practice applies a 75-micron threshold:

FluidFlow provides two distinct modeling approaches for slurries, each suited to a different class of slurry behavior. Selecting the correct approach is the most important decision in any slurry modeling workflow, as it determines which correlations, input data, and calculation methods are applied throughout the analysis.

In the liquid-only flow approach, the slurry is treated as a single-phase fluid with a modified viscosity definition. The solid particles are not tracked individually — instead, they are assumed to be fully integrated into the carrier fluid, forming a pseudo-homogeneous mixture.

This approach is used for non-settling slurries, where fine particles remain uniformly distributed at all operating velocities. It is configured identically to a standard liquid model in FluidFlow, with the key difference being that the fluid viscosity is defined using a non-Newtonian rheology model (Power Law, Bingham Plastic, Herschel-Bulkley, or Casson) rather than a constant Newtonian viscosity.

Key characteristics:

In the liquid-solid flow approach, the slurry is treated as a true two-phase mixture — the carrier fluid and the solid particles are modeled separately and their combined effects on pressure loss, deposition, and pump performance are evaluated using empirical correlations.

This approach is used for settling slurries, where coarser particles are prone to gravitational settling. It requires additional solids data at the inlet boundary and explicit configuration of deposition velocity and friction loss correlations.

Key characteristics:

💡 The choice of modeling approach is not always determined solely by particle size. Laboratory rheology testing may confirm non-Newtonian behavior for slurries with particles above 75 microns, in which case the liquid-only approach is more appropriate. See Section 13 for guidance.

Deposition velocity is the most important operating criterion for settling slurry systems. It defines the minimum flow velocity required to prevent solid particles from forming a stationary bed at the pipe bottom.

In practice, slurry pipelines are designed to operate at a velocity of at least 130% of Vₛₘ to maintain an adequate safety margin against deposition.

FluidFlow calculates Vₛₘ using empirically-derived correlations developed from large-scale loop testing. The available methods are:

Applicable to stratified and heterogeneous flows with d50 > 150 microns. Uses the Moody friction factor at V = Vₛₘ.

Also applicable to stratified and heterogeneous flows with d50 > 150 microns. Explicitly accounts for particle diameter.

GIW VSCALC is not a single correlation but a systematic application of multiple models, with selection governed by particle size:

The Vₛₘ result from GIW VSCALC is capped at the Wilson 1992 value to prevent physically unrealistic predictions at extreme particle sizes or concentrations.

Correlation selection depends primarily on particle size and carrier fluid type:

By default, FluidFlow applies GIW VSCALC as the deposition velocity method. This is the recommended starting point for most settling slurry applications, as it automatically selects the appropriate underlying model based on particle size.

Pipe inclination significantly affects the velocity required to prevent deposition:

FluidFlow applies the Wilson-Tse 1984 correlation to correct the horizontal deposition velocity for pipe inclination angle. The correction uses the Durand deposition parameter FL,M and a correction term 𝚫D:

The correction is valid for inclination angles from -20° to +80° from horizontal (extrapolation is used between 50° and 80°).

The 2019 extension accounts for the particle size to pipe diameter ratio (d50/D):

Both the 1984 and extended 2019 models are available in the FluidFlow calculation options. The default is the Wilson-Tse 1984 chart.

FluidFlow calculates settling slurry friction loss using the additive pressure drop principle:

The carrier fluid component is calculated using the Darcy-Weisbach equation. The solids contribution is determined by a selected empirical correlation. This approach is expressed through the dimensionless solids effect term ɸ :

Pipe inclination affects friction loss results because it changes the forces acting on suspended particles:

FluidFlow also provides dedicated vertical pipe friction loss correlations for fully vertical pipe segments, which follow different physical principles from horizontal or mildly inclined flow.

The following correlations are available in FluidFlow for calculating settling slurry friction loss in horizontal pipes:

A generalized correlation that uses only d50 to approximate particle size, assuming uniform particle distribution. The additional pressure drop from solids is:

Where Ω is the Durand constant (default value = 82, configurable in calculation options).

A more sophisticated method developed by Edward J. Wasp that incorporates particle size distribution data to separately calculate friction losses in the stratified and homogeneous concentration layers:

Layer breakdown is determined through an iterative calculation involving terminal settling velocity, head loss, and slurry physical properties.

Similar in approach to Wasp, but with two key enhancements:

Designed for fully stratified flow or as a preliminary estimate when particle size data is unavailable:

This correlation does not use particle size as an input, so particle size input fields are disabled when it is selected. Results represent a conservative upper limit for friction loss.

Effective for slurries with minimal or no stratified components, or when scale-up test data is available. It is the previous default method used in FluidFlow versions prior to V3.54.

The most comprehensive settling slurry correlation available. The 4CM accounts for all flow regimes by computing the additive contributions of different particle fractions — pseudo-homogeneous, heterogeneous, and stratified components. It is the recommended default method for settling slurry analysis in FluidFlow.

Use the following guidance to select the most appropriate horizontal friction loss correlation:

Vertical pipe segments in settling slurry systems require dedicated friction loss correlations, as the flow physics differ significantly from horizontal transport. In vertical upward flow, turbulent eddies are primarily responsible for keeping particles suspended, and there is no gravitational component working across the pipe cross-section.

FluidFlow provides the following vertical pipe friction loss correlations:

When a horizontal friction loss correlation from prior to FluidFlow V3.54 is selected (such as Durand, Wasp, Liu-Dezhong, or WASC), FluidFlow automatically reverts the vertical pipe friction loss method to Vertical Pipe WASC Loss — the method used in earlier versions of the software. The Vₛₘ correlation also triggers this automatic selection, since both 4CM Vertical and the Spelay et al. model require particle size data as input, which is not applicable when Vₛₘ is used as the horizontal method.

When using the Reset to Defaults option in the Calculation Options, FluidFlow automatically assigns a consistent and compatible vertical pipe friction loss method to match the selected horizontal correlation.

A settling slurry classification does not automatically mean the liquid-solid flow approach must be used. In some cases, a slurry is better represented as a non-Newtonian liquid using the liquid-only flow approach. This is appropriate when:

FluidFlow supports four non-Newtonian rheology models. Each describes a different relationship between shear stress 𝜏 s and shear rate 𝛾:

A non-linear model with no yield stress (curve passes through the origin). Behavior depends on the flow exponent n:

Requires a minimum yield stress 𝜏BP before flow initiates. Above yield, behavior is linear (Newtonian-like). Common in concentrated slurries, toothpaste, ketchup, and mustard.

A generalized yield-stress power law model. Combines the yield stress of the Bingham model with the non-linear shear behavior of the Power Law model. Widely applied to drilling fluids, mining streams, and food processing applications.

A yield pseudoplastic model derived empirically from pigment-oil suspensions. The International Office of Cocoa and Chocolate has adopted this model as a standard for chocolate flow analysis. Also applied to blood, printing ink, and food products such as yogurt and tomato puree.

When raw shear rate vs. shear stress data is available from viscometry testing, FluidFlow can automatically fit the data to any of the four models using a curve fitting routine. The quality of fit is evaluated using the coefficient of determination (R²), where a value approaching 1.0 indicates excellent correlation. Multiple fluid entries can be created for different candidate models to support sensitivity analysis.

Friction loss for non-Newtonian flow is calculated using a modified Darcy-Weisbach equation:

The key difference from Newtonian flow is the non-Newtonian friction factor fₙₙ which is not obtained from the standard Moody chart but from model-specific correlations:

Centrifugal pump performance curves supplied by manufacturers are developed using water as the test fluid. When pumping slurries, performance is degraded by two mechanisms:

For non-Newtonian liquids, the shear rate — and therefore the apparent viscosity — varies through the pump internals. Because these shear rates cannot be directly modeled, viscosity correction factors must be obtained from laboratory testing or manufacturer specifications. In FluidFlow, this is handled using the Fixed Reduction Ratio method.

Coarse solid particles create additional internal friction loss within the pump, reducing both throughput and efficiency — a phenomenon known as pump derating. This is quantified using:

For most practical cases, it is assumed that HR ⩰ ER, simplifying calculations. However, this assumption may be invalid for:

To address uncertainty, engineers typically apply a 15–25% margin to installed driver power plus an additional 10–15% margin for anticipated wear.

FluidFlow provides five methods for calculating the head and efficiency ratios:

Important: All derating methods except the Fixed Reduction Ratio require particle size data. When using the liquid-only modeling approach (non-settling slurries), FluidFlow will display a warning if a particle-size-dependent derating method is selected.
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When the Vₛₘ friction loss correlation is applied (no particle size input), FluidFlow restricts derating options to Fixed Reduction Ratio or ANSI Monosize only.