Hydrotransport 15 Conference, Effects of Slurry and Operational Variables on Flow Conditions in the Rugby Cement 92-km Chalk Slurry Pipeline Nigel Heywood, Neil Alderman, Hyprotech UK Ltd, UK David Clowes, Rugby Cement, UK Hydrotransport 15 Conference, 3 to 5 June 2002, Banff, Canada © 2002 AEA Technology OHT serial no 1 1

Background to Study Chalk (with variable amounts of limestone, flint and clay) is extracted from Kensworth Quarry near Dunstable in the UK, and slurried and pumped 92 km to the Rugby Cement Works. In Year 2000, new chalk processing equipment installed at the quarry led to a larger fraction of >0.3mm particles entering the pipeline than had previously occurred.

Background to Study Analysis was undertaken to investigate if the pipeline was continuing to operate in the turbulent flow regime during any expected variation in slurry and operational properties. In addition, analysis important as part of a study to determine the feasibility of reducing the slurry moisture content from the current average value of 36% by weight.

Slurry Characterisation Particle Size Distribution pH Rheological Tests co-axial cylinder viscometer both “upper bound” and “average” data analysed Bingham plastic model used, with yield stress and plastic viscosity correlated with moisture content

Particle Size Distribution of Chalk Slurry 2

Typical Chalk Slurry Flow Curve for (35% by Weight Moisture)

Plastic Viscosity as Function of Moisture Content 4

Yield Stress as Function of Moisture Content 5

Bingham Plastic Model 6

Bingham Reynolds Number and Hedstrom Number

Critical Reynolds Number for Laminar Flow Breakdown

Effects of Parameters on Pipe Flow Regime Flow Regime determined using Reynolds Number Ratio, ReB/(ReB)C Following Parameters Investigated: Pipe Diameter (247.6mm, 254.4mm, 278.4mm) Slurry Volume Flowrate (90 to 280 m3/h) Slurry Moisture Content (33% to 36% by weight) Sediment Height in Pipe (25mm, 50mm, 100mm)

Dependence of Reynolds Number Ratio on Pipe Size (“Average” Rheological Data) 8

Dependence of Reynolds Number Ratio on Slurry Volume Flowrate 9

Dependence of Re Ratio on Moisture Content 10

Sediment Height Effect: Hydraulic Diameter and Flow Velocity 11

Reynolds Number Ratio for Different Sediment Heights D = 247 Reynolds Number Ratio for Different Sediment Heights D = 247.6mm (“Average” Rheological Data) 13

Reynolds Number Ratio for Different Sediment Heights D = 278 Reynolds Number Ratio for Different Sediment Heights D = 278.4mm (“Average” Rheological Data)

Reynolds Number Ratio as function of Volume Flowrate and Sediment Height (D = 247.6mm, 36% Moisture) 15

Reynolds Number Ratio as function of Volume Flowrate and Sediment Height (D = 278.4mm, 36% Moisture) 16

Reynolds Number Ratio as function of Moisture Content and Sediment Height (D = 247.6mm, Q = 208 m3/h) 17

Reynolds Number Ratio as function of Moisture Content and Sediment Height (D = 278.4mm, Q = 208 m3/h)

CONCLUSIONS (I) Pipeline operation in Year 2000 operated at ReB approximately 1.4 to 2.1 times (ReB)c Inadequate safety margin does not ensure : flow remains turbulent under changing conditions sufficient turbulence intensity to support coarse limestone and flint particles in chalk slurry Safety margin falls rapidly as D rises, so greatest potential threat when D = 278.4mm, with least when D = 247.6mm If slurry moisture content reduced, flow becomes laminar at 34% moisture content; Q = 208 m3/h, D = 278.4mm At 32% moisture content, flow is predicted to be laminar for all combinations of D, and Q = 208 or 235 m3/h

CONCLUSIONS (2) Sediment is probably present in the pipeline owing to changes in quarry processing conditions Sediment formed will : reduce the pipe cross-sectional area for flow increase the average slurry velocity, V (for a fixed Q) Effects of rising V and reducing hydraulic diameter are increase in ReB for all conditions investigated, so moving it further away from the critical value for laminar flow breakdown progressively larger difference between V and limit deposit velocity

CONCLUSIONS (3) self-regulating effect likely to limit progressive build-up of sediment to level which may be approached asymptotically But, pump output is controlled by discharge pressure (Pd), and increase in Pd may cause reduced pump stroke rate Could result in accelerated sediment laydown, with further rise in Pd The analysis has shown how progressive reductions in Q as sediment is formed could cause the flow to move from turbulent to laminar