frank salt - osd - the possible effects of negative pressure on slurries
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THE POSSIBLE EFFECTS OF NEGATIVE PRESSURE ON SLURRIES11 NOVEMBER 2015
Below atmospheric pressure, carrier fluid’s boiling point and dynamic viscosity lowers
This changes the carrier fluid’s molecular activity level and its ability to hold materials (e.g. Ca) in solution
The same occurs when fluid temperature increased and hence its level of molecular activity
SYNOPSIS
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In 2010, a series of 5 failures in 120km pyritic tailings pipeline in PNG were investigated
All failures occurred in slack flow/negative pressure zones
This system had been operated in gravity flow mode for long periods and outside its design parameters
CASE STUDY
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Ok Tedi PipelinePicture at valve station (6km & 800m below pump station)
CASE STUDY
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Hydraulic profile of Ok Tedi Pipeline
Drops over 1300m in 40km length, pockets of negative pressure are generated
Fluid separation occurs when slurry velocity downhill is higher than uphill
For pipelines carrying – Water only, 9.0m vertical separation where vacuum
causes massive change in water
– Denser slurry, same effects with 4.5m vertical separation
Partial vacuum affects fluid viscosity & its ability to hold material → LINE FAILURE!!!
PIPE FAILURES
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Degree of negative pressure depends on depth of fall & fluid SG
Limited by liquid vapour point
Water vapour balances negative pressure generated by the fall
Water boiling point related to atmospheric pressure
PIPE FAILURES
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Top of pipe showed little/no sign of wear
Floor of pipe showed abrasive wear, due to sliding bed of solid material
Side walls showed buildup of Ca based material & heavy scalloping
Indication to 3 different conditions in the line plus a failure point
Of greatest interest was the massively active zone above the bottom bed
All pipe failures had occurred and calcium layer deposited at this zone
In looking for mechanism of such conditions, one needs to speculate on occurrence to flowing slurries under negative pressures
PIPE FAILURES
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THE MECHANICS OF THE PROBLEM
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Process of turning liquid to vapours requires liquid molecules to massively increase their movement rate
Boiling point of liquid reduces with falling pressure
Drop in pressure triggers same increase in molecular activity as heat application
THE MECHANICS OF THE PROBLEM
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Temp. °C Density, kg/m3 Viscosity, µPa.s
1020303540506070
999.73998.23995.68994.06992.25988.07983.24977.81
1306.91002.0797.5719.5653.5547.1466.6403.9
Viscosity of liquid water (Kestin, Sokolov and Wakeham 1978)
THE MECHANICS OF THE PROBLEM
Increasing pressure raises liquid boiling point by suppressing molecular movement
Degree of molecular activity sets the water viscosity
In normal conditions, water density fall by 3% between 0-70 deg C
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Similar viscosity changes occur in limestone slurries if water content increases in temperature
THE MECHANICS OF THE PROBLEM
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Changes in limestone slurries viscosity in relation to slurry temperature (Senapati, Panda and Parida 2009)
THE MECHANICS OF THE PROBLEM
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Pressure drop below atmospheric pressure affects both vapour point and fluid viscosity
Such lowering of viscosity is bound to affect slurry’s ability to hold material in solution
For saturated vapour pressure to balance any degree of partial vacuum, the water vapour must separate from the liquid
This is achieved through bubbles where small pocket of vapour exceed the atmospheric pressure around them after initiating around a nucleus
These could occur through the full slurry depth as well as points on pipe walls
PIPELINE AND CALCIUM BUILD-UP
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Hydraulic profile of Ok Tedi Pipeline
Undulating profile result in pockets of negative & positive pressure in slack flow mode
Liquid viscosity oscillates between supportive & unsupportive during these changes
In unsupportive phase, fluid viscosity reduction cause coarser particles to fall from suspension
If viscosity changes, rheology also changes with its ability to support particles
Heavy Ca buildup in failure zones in tailings pipeline also related to zones of negative pressure
pH levels of slurry was held in excess of pH12 by addition of lime
Examples of scale build up in kettles & boilers
Ca dropout of solution from imposing a vacuum, similar to the application of heat
PIPELINE AND CALCIUM BUILD-UP
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Temperature effect on relative viscosity of limestone slurry at different solids concentration
EFFECTS OF NEGATIVE PRESSURE & TEST WORK
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Example of which saturated vapour pressure balances a partial vacuum can be found in height of a water column that can be supported by the vacuum, as in a syphon
At sea level, a syphon has a theoretical lift of 10m
Under ideal conditions, a syphon reaches its operational lifting limit at 9m
At this point, the vacuum is balanced by the water vapour pressure
This gives an approximate absolute 1.5 psi/10 kPa in a syphon, at which point the boiling point would be 10 deg C
EFFECTS OF NEGATIVE PRESSURE & TEST WORK
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Difficult to measure changes in slurry viscosity under negative pressure, short of putting a viscometer inside a vacuum chamber
To overcome this, the slurry settling rate has been used as an indicative measurement of viscosity
To this end, the settling curves have been plotted for a single batch of slurry under a range of temperatures and with one test subject to a negative pressure
Data obtained from a 1000cc sample of magnetite slurry
Measurements at 10, 25 and 65 deg C
Tests gave similar results to the changes previously noted in limestone slurries at different temperatures
Verified use of settling curve as a crude viscometer
Test repeated under partial vacuum to get measurable effect from small level of applied vacuum
EFFECTS OF NEGATIVE PRESSURE & TEST WORK
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CONCLUSIONS
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Failures with the PNG tailings pipeline stem from long & repeated operation under negative pressure/ slack flow conditions
In looking at pipeline hydraulic profile, it is possible to see a number of locations where such conditions could exist
Whilst failures in 2010 have been used as an example, other operations such as Bougainville and Freeport Copper Pipelines have suffered similar failures
This suggests that effects of slack flow operation are generally underestimated