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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