Hydrotransport 17 Effect of comminuted flint on pumping chalk slurry in the 92 km Kensworth – Rugby pipeline N.J. Alderman1 N.I.Heywood1 and D. J. Clowes2 1: BHR Group Ltd, The Fluid Engineering Centre, Cranfield, Bedfordshire, UK. 2: Cemex UK Ltd, Rugby, Warwickshire, UK. Hydrotransport 17 – May 2007 © BHR Group Limited 2007 Slide 1

Background Plans in 2005 to open a new area for excavating chalk from Benches 1, 2 and 3 (top three benches) that contain significant pockets of flint, clay and other materials. to introduce a flint regrinding system to chalk slurry make-up plant to ensure flint particle size in slurry <45 m prior to being pumped in the 92-km pipeline. Require pipeline to be operated at a velocity just above that for laminar flow breakdown to ensure comminuted flint always remain suspended for different operational conditions. Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 2

Slurry samples Samples collated from These samples were combined with three locations within Bench 1 (quarry top) three locations within Bench 2 one location within Bench 11 (quarry bottom) These samples were combined with three flint concentrations of 0, 5 and 10% by weight flint (at a fixed flint psd of 80% < 45m) three additive concentrations of 0.7, 0.8 and 1.0% 63 slurry samples with moisture content fixed at 35%w/w prepared Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 3

Flow curve measurements relevant to pipeflow Contraves Rheomat 30 coaxial cylinder viscometer used Sample under test subjected to two shear ramps of bob rotational speed from 0.0647 to 350 rpm. Average torque reading obtained from two up and down curves taken Target volumetric flow rate through 92-km pipeline = 245 m3/h  8V/D for D = 247.6mm = 45.6 s-1  90 s-1  8V/D for D = 254.4mm = 42.1 s-1  85 s-1  8V/D for D = 279.0mm = 31.8 s-1  65 s-1  Restrict analysis of average flow curve over 0 to 100 s-1 Without exception, flow curves for 63 slurry formulations best described by Bingham plastic model Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 4

Variation of yB with flint and additive concentration Bench 1 Sample 1 Bench 11 Sample 1 Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 5

Variation of P with flint and additive concentration Bench 1 Sample 1 Bench 11 Sample 1 Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 6

Use of viscosity at 60 s-1 Slurry viscosity Wall shear rate for pipeflow calculated for each D in 92 km pipeline With n values, minimum and maximum values of wall shear rate were then obtained for each of 63 slurry formulations To encapsulate shear rates for all slurry samples within each bench and from to bench to bench, a reference shear rate of 60 s-1 was taken Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 7

Use of viscosity at 60 s-1 Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 8

Determination of flow regime for slurry pipeline flow Four approaches: Hanks (1963) method used in a previous study on this pipeline* Intersection of curves for estimation of pipe wall shear stress for laminar and turbulent flow as a function of pipe flow velocity* Slatter (1999) method Wilson-Thomas (2006) approach Little guidance given on which of these approaches is most reliable Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 9

Hanks (1963) approach Reynolds number for pipeflow of a Bingham plastic fluid Critical value of ratio of Bingham yield stress to wall shear stress Critical Bingham Reynolds number for laminar flow breakdown Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 10

Hanks (1963) approach Turbulent Turbulent Laminar Laminar Turbulent Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 11

Hanks (1963) approach Bench 1 chalk slurries Sample 2 Samples 1 & 3 ReB/(ReB)c  1.1 for 9 different formulations Pipeline would operate in turbulent flow regime for the range of slurries tested and for the three pipe IDs Samples 1 & 3 Behaved differently to Sample 2 since ReB/(ReB)c < 1 for slurries containing 0.8 and 1.0% additive with either 0 or 5 % flint content indicating pipeline would operate in undesirable laminar flow regime Bench 11 chalk slurries ReB/(ReB)c > 1.1 for 9 different formulations Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 12

Critical values for laminar/turbulent flow with viscosity at 60 s-1 Actual values of ReB/(ReB)c for a given slurry formulation vary significantly from sample to sample and from bench to bench. Very difficult to identify flow regime with any degree of certainty To assist in flow regime identification, ReB/(ReB)c versus plotted as a function of pipe ID for all 63 chalk slurry formulations Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 13

Critical values for laminar/turbulent flow with viscosity at 60 s-1 Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 14

Critical values for laminar/turbulent flow with viscosity at 60 s-1 These plots provide a possible tool for identifying flow regime in each of three pipe IDs by measuring of chalk slurry taken from any bench lying between Bench 1 and Bench 11 provided: flint concentration does not exceed 10% at a fixed flint psd of 80% < 45m additive concentrations lie between 0.7 and 1.0% volumetric flow rate is 245 m3/h in the pipeline Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 15

Intersection of wall shear stress curves Pipe wall shear stress for laminar flow - Buckingham equation Pipe wall shear stress for turbulent flow - Wilson-Thomas method Point of laminar flow breakdown is the pipe velocity at which the two wall shear stresses calculated for laminar and turbulent flow are equal Found use of this method for defining laminar/turbulent transition led to ~15% lower critical velocities than those obtained in Hanks method For conservative design, it was decided to adopt Hanks method Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 16

Conclusions Chalk slurry samples prepared using ground flint, additive (Alcosperse/soda ash/process water), and additional process water. Slurry flow curve measurements showed that they were described well by Bingham plastic model over relevant shear rate range for pipeflow (0 to 100 s-1). Viscosity fixed at a reference shear rate of 60 s-1 used to assist in identification of rheological trends. Actual viscosity values varied from sample to sample and bench to bench for nine different formulations (3 flint concentrations, 3 additive concentrations). Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 17

Conclusions Flow regime in pipeline dependent on chalk slurry formulation, type of chalk sampled pipe diameter For Bench 11 slurries, the pipeline would always operate in turbulent flow regime for nine different formulations and for three pipe IDs. For Bench 1 chalk slurries, flow regime depended on formulation and pipe ID. Using viscosity measurements at a shear rate of 60 s-1, predictions of laminar to turbulent transition could be made for all chalk samples and pipe sizes. Hydrotransport 17 - May 2007 © BHR Group Limited 2007 Slide 18